|
|
AUSTRALIAN
NATIONAL UNIVERSITY System Design Note 3.01 Created: 2 April 2000 Last modified: 21 March 2001 |
OPERATIONAL CONCEPT DEFINITION DOCUMENT
Peter J. McGregor
Research School of Astronomy
and Astrophysics
Institute of Advanced
Studies
Australian National
University
Revision History
|
Revision No. |
Author & Date |
Approval & Date |
Description |
|
Revision 1 |
Peter J. McGregor 06 October 1999 |
Jan van Harmelen 14 October 1999 |
Original document. |
|
Revision 2 |
Peter J. McGregor 09 February 2000 |
Jan van Harmelen 14 February 2000 |
Initial OCDD submitted to CoDR. |
|
Revision 3 |
Peter J. McGregor 15 December 2000 |
Jan van Harmelen 08 February 2001 |
Revised OCDD submitted to CDR. |
|
|
|
|
|
Contents
4.1 Basic
Instrument Parameters
5.1.1 Massive
Black Holes in Nearby Galactic Nuclei
5.1.2 Nearby
Active Galactic Nuclei
5.2.1 Brown Dwarfs
and Low Mass Stars
5.2.3 YSO Jet
Driving Mechanisms
5.2.4 YSO
Jet-Cloud Interactions
5.2.5 Late Stages
of Stellar Evolution
5.2.7 Old Stellar
Populations in Nearby Galaxies
5.2.8 Nearby
Starburst Galaxies and Starburst Regions
5.2.9
Ultra-Luminous Infrared Galaxies
5.2.10 Dynamical
Evolution of High Redshift Galaxies
6.1 Standard Setup
and Calibration
6.1.2 Daytime
Setup and Calibration
6.2 Molecular
Hydrogen Emission For OMC-1 “Bullets”.
6.2.2 Planning the
Observation
6.2.4 Setup Prior
to Observation
6.2.5 Science
Observation Sequence
6.3 Stellar
Populations in The Galactic Center
6.3.2 Planning the
Observation
6.3.4 Setup Prior
to Observation
6.3.5 Science
Observation Sequence
6.4 Massive Black
Holes in Nearby Galaxies
6.4.2 Planning the
Observation
6.4.4 Setup Prior
to Observation
6.4.5 Science
Observation Sequence
6.5 Inner
Narrow-Line Regions in Seyfert Galaxies
6.5.2 Planning the
Observation
6.5.4 Setup Prior
to Observation
6.5.5 Science
Observation Sequence
6.6 Jet Outflows
From Young Stellar Objects
6.6.2 Planning the
Observation
6.6.4 Setup Prior
to Observation
6.6.5 Science
Observation Sequence
6.7
Spectropolarimetry of Cygnus A
6.7.2 Planning the
Observation
6.7.4 Setup Prior
to Observation
6.7.5 Science
Observation Sequence
7 Summary of
Scientific Requirements
1 Purpose
This document describes the operational concept model for the Gemini Near-infrared Integral-Field Spectrograph (NIFS). The document summarizes the science cases for which the instrument has been designed, relates these to the design requirements, and discusses the key functional and performance requirements that the instrument must meet. Key operational scenarios of the NIFS instrument are identified and discussed, especially in terms of the requirements the instrument places on other parts of the Gemini system. These scenarios are described in sufficient detail for technically and scientifically skilled, but non-expert, readers to understand.
2 Applicable Documents
|
Document
ID |
Source |
Title |
|
SDN0004.01 |
RSAA |
NIFS Performance Model |
|
|
|
|
3 Introduction
The Gemini 8-m telescopes are designed to achieve unprecedented ground-based image quality using adaptive optics techniques. Diffraction-limited images have been demonstrated at 2.2 mm and will be achievable over most of the 1.0‑2.5 mm wavelength range in good observing conditions. A near diffraction-limited, moderate spectral resolution, near-infrared, integral-field spectrograph was identified as a desirable complement to GNIRS in order to realize the scientific potential of both Gemini telescopes at high spatial resolution. NIFS is a fast-tracked instrument which is intended to provide this capability on the shortest possible timescale and at low cost.
NIFS is used with ALTAIR, the facility adaptive optics system on Gemini North. At the heart of NIFS is a reflective integral-field unit (IFU) which divides its 3.0²´3.0² field-of-view on the sky into 29 slitlets each 0.103² wide and 3.0² long. Spectra are obtained simultaneously for each 0.04² pixel along each slitlet. Four reflection gratings are used with a fixed focal length camera to obtain spectra with two-pixel resolving powers of R ~ 5300 in any one of the Z, J, H, or K bands. This spectral resolution is sufficient to work between the OH airglow emission lines shortward of 2.2 mm, thus greatly reducing the detected sky background at these wavelengths. The velocity resolution of ~ 60 km s‑1 is well-matched to minimum rotational velocities expected in galaxies at high redshift. The 0.1² slitlet width slightly under-samples the <0.06² spatial resolution in the wavelength range between 1.0 and 2.5 mm expected with ALTAIR in good observing conditions, but maintains a modest field-of-view. Many NIFS programs will require good observing conditions to perform high spatial resolution observations. These programs will benefit from queue scheduling.
4 Instrument Description
4.1 Basic Instrument Parameters
· Wavelength range: 0.94‑2.5 mm.
· Pixel size: 0.04²´0.10² on sky.
· Spectral resolving power: ~ 5300 (two pixels).
· Focal plane occulting masks: 0.1², 0.2², 0.5² diameter circular.
· Neutral density filter.
· Gratings: Z, J, H, K.
· Order blocking filters: Z, J, H, K.
· Polarimetry: Wire grid with K filter only.
· Direct view mirror for target acquisition.
· Detector: Rockwell 2048´2048 HgCdTe HAWAII‑2, 18 mm pixels.
· Near-infrared on-instrument wavefront sensor (OIWFS).
- Wavelength range: 0.95‑2.5 mm.
- Steerable over 180² FOV (120² used with ALTAIR).
- Image scale: 0.170²/pixel.
- Filters: J, H, K.
- Instantaneous field-of-view: 2² diameter.
4.2 Instrument Overview
NIFS incorporates duplicates of the NIRI cryostat and OIWFS in order to fast-track its design, construction and commissioning. The cryostat has a 180 mm diameter window which defines the extent of the OIWFS field. The central 3.0²´3.0² field is reflected into the science instrument by a cold pick-off mirror. The f/16.2 telescope beam from the 7.891 m primary mirror effective diameter (due to the undersized telescope secondary mirror) then focuses at the Focal Plane Mask Wheel. This wheel contains a field mask to reduce scattered light during normal observations, occulting disks for masking the diffraction-limited image of a bright object such as the primary of a binary star or planetary system or a bright galactic nucleus, a neutral density filter for observing bright objects by attenuating the whole field-of-view, and a calibration slit mask for determining the spatial scale and spectral distortion. The proposed contents of the Focal Plane Mask Wheel are listed in Table 1. The f/16.2 focal plane is reimaged on the image slicer in the science instrument at f/256 by a single concave mirror. This mirror also reimages the telescope exit pupil onto a cold stop which baffles the system. A filter wheel located after the cold stop contains four order blocking filters matched to each of the four gratings. A second order blocking filter for the K grating is included along with a wire grid polarization analyzer. This allows NIFS to perform spectropolarimetry in the K band in conjunction with the GPOL facility polarimetry unit. GPOL is mounted at the bottom of the Instrument Support Structure (ISS), below ALTAIR. This may compromise spectropolarimetry with NIFS because the instrumental polarization due to ALTAIR is imprinted on the data. The contents of the NIFS Filter Wheel are listed in Table 2. The image slicer consists of a stack of 29 tilted curved mirrors each 1.024 mm thick and 29.718 mm long, corresponding to one 0.103²´3.0² slitlet on the sky. The individual image slicer elements act as field mirrors forming separate f/256 pupils on the pupil mirror array. These are then reimaged by the pupil mirrors at f/16.22 onto the elements of the field mirror array. Diffraction at the image slicer mirrors causes elongation of the pupil images perpendicular to the pupil mirror array. This is accommodated using pupil mirrors that are over-sized in this direction. The 53.864 mm long reformatted slit on the field mirror array has a staircase appearance due to the spatial offsets between individual slitlets on the sky. The field mirrors redirect rays from each slitlet into the spectrograph.
Table 1: NIFS Focal Plane Mask Wheel Contents
|
Position |
Content |
|
|
|
|
1 |
Clear |
|
2 |
0.1² occulting disk |
|
3 |
0.2² occulting disk |
|
4 |
0.5² occulting disk |
|
5 |
Neutral density filter |
|
6 |
Calibration slit mask |
|
7 |
0.1² diameter pin hole |
|
8 |
0.1² diameter slit |
|
9 |
Blocked |
|
10 |
Blocked |
|
11 |
Blocked |
|
12 |
Blocked |
Table 2: NIFS Filter Wheel Contents
|
Position |
Content |
|
|
|
|
1 |
Z grating band pass filter |
|
2 |
J grating band pass filter |
|
3 |
H grating band pass filter |
|
4 |
K grating band pass filter |
|
5 |
K
grating band pass filter + wire grid analyzer |
|
6 |
Blocked |
|
7 |
Blocked |
|
8 |
Blocked |
The spectrograph accepts the input from the 53.864 mm long reformatted “staircase” slit. A folded Bouwers collimator with 418.320 mm focal length produces a 25.788 mm diameter (geometrical) collimated beam on one of four reflection gratings. The gratings are oversized in order to accommodate the larger pupil image produced by diffraction at the image slicer. The gratings are mounted on a grating turret which is rotated to set the grating angle and to interchange gratings. Parameters for the four gratings are listed in Table 3. The gratings can be rotated to a set of predefined, fixed grating angles which provide full coverage of the standard J, H, and K photometric passbands as well as the inter-band regions. The dispersed beams are then imaged onto the detector using a fixed format refractive camera with an Ebert angle (i.e., collimator-camera angle) of 30° and a camera focal length of 286.0 mm.
Table 3: NIFS Grating Parameters
|
Grating |
lc (mm) |
N (l/mm) |
qblaze (deg) |
q (deg) |
R |
Dv (km s‑1) |
Range (mm) |
|
|
|
|
|
|
|
|
|
|
Z |
1.05 |
600 |
17.5 |
19.0 |
4990 |
60.1 |
0.94‑1.15 |
|
J |
1.25 |
600 |
22.0 |
22.8 |
6040 |
49.6 |
1.15‑1.35 |
|
H |
1.65 |
400 |
18.6 |
20.0 |
5280 |
56.8 |
1.49‑1.80 |
|
K |
2.20 |
300 |
17.5 |
20.0 |
5300 |
56.6 |
2.00‑2.41 |
The NIFS science detector is a Rockwell 2048´2048 HgCdTe HAWAII‑2 with 18 mm pixels. The 0.1² wide slitlets map to two pixels in the spectral direction in order to provide fully sampled spectra in a single exposure. Anamorphic effects at the grating produce different image scales parallel and perpendicular to the slitlets. The pixels along each slitlet map to 0.04² on the sky. This slightly under-samples the diffraction cores of images delivered by ALTAIR; the diffraction FWHM is ~ 0.06² at 2.2 mm. Finer spatial sampling would resulted in an unacceptably small field-of-view.
Target acquisition is achieved either by offsetting objects to a fiducial pixel position in the facility acquisition camera, or by recording undispersed images of the sky with NIFS and centering objects in the NIFS field directly. A mirror in NIFS can be flipped into the collimated beam in front of the grating in order to record these undispersed images of the sky.
An OIWFS is needed to accurately track flexure between NIFS and ALTAIR and to maintain focus. The OIWFS in NIFS does this by sensing the position and focus of an offset guide star. This information is used to generate correction signals for the tip-tilt and fast focus telescope secondary mirror. The OIWFS in NIFS is identical to that in NIRI. It operates in the near-infrared so it is not subject to differential atmospheric refraction, and the probability of finding suitable guide stars in optically obscured regions is improved. The OIWFS uses an X-Y gimbal mirror to select guide stars from anywhere within a 180² diameter field-of-view outside the region vignetted by the NIFS pick-off mirror. In practice, OIWFS guide stars are restricted to the smaller 120² diameter unvignetted field passed by ALTAIR. The NIFS pick-off mirror and its support structure vignette a 25.4² diameter circular region centered on the science object as well as a 25.4² wide rectangular strip extending to the edge of the OIWFS field. Partially vignetted OIWFS guide stars within this region should not be used because their light may scatter into the science instrument and they do not properly illuminate the OIWFS pupil prisms. A filter wheel in the OIWFS containing Z, J, H, and K broadband filters is used to match the OIWFS wavelength range to that of the science instrument. The OIWFS filters are located at a focus and are mounted along with circular apertures which act as field stops. The contents of the OIWFS Filter Wheel are listed in Table 4.
Table 4: OIWFS Filter Wheel Contents
|
Position |
Content |
|
|
|
|
1 |
Z filter + 1.0² diameter circular aperture |
|
2 |
Z filter + 2.0² diameter circular aperture |
|
3 |
J filter + 0.5² diameter circular aperture |
|
4 |
J filter + 1.0² diameter circular aperture |
|
5 |
J
filter + 2.0² diameter circular aperture |
|
6 |
H filter + 0.5² diameter circular aperture |
|
7 |
H filter + 1.0² diameter circular aperture |
|
8 |
H filter + 2.0² diameter circular aperture |
|
9 |
K filter + 1.0² diameter circular aperture |
|
10 |
K filter + 2.0² diameter circular aperture |
|
11 |
Clear |
|
12 |
Blocked |
The AO feed mirror must be deployed to use ALTAIR. This prevents the use of the second Peripheral Wavefront Sensor (PWFS2).
Light from the facility calibration unit, GCAL, is used for flat fields and wavelength calibration. This light is injected into NIFS at the level of the science fold mirror in the Instrument Support Structure which is below ALTAIR. Consequently, calibrations obtained using GCAL do not include the effects of ALTAIR or the telescope.
4.3 Observing Modes
NIFS is largely a fixed-format instrument with the output data being determined predominantly by the grating selection. Variations on the basic operating mode are achieved by 1) using any of the pre-defined, non-standard grating angles to record a non-standard wavelength range, 2) using the neutral density filter to attenuate the full field-of-view, 3) using one of the occulting disks to block light from an object at field center, or 4) using the wire grid polarization analyzer in K band. Only the latter two of these require further comment.
The occulting disks are used to record faint structure in the vicinity of an object which would otherwise saturate the detector. Care should be exercised to ensure that the bright star is not observed directly because the detector suffers from significant remnance effects; a remnant image of the saturated spectrum will persist for many hours after exposure. Consequently, the light path through the NIFS science instrument should be blocked by setting the NIFS Filter Wheel in the Blocked position while a bright star is being positioned behind an occulting disk, and only opened after the bright star is correctly centered. This procedure is described fully in one of the observing scenarios (§6.6).
Spectropolarimetry measurements can be made in the K band using NIFS in conjunction with the GPOL facility polarimetry unit. NIFS must be mounted on the uplooking ISS port to do this. Linear polarization measurements are made using a fixed analyzer in NIFS and a rotating half wave plate in GPOL. The half wave plate rotates the source plane of polarization passed to NIFS. The fixed analyzer defines the polarization plane to which NIFS is sensitive. Linear polarization in the source is measured by measuring the modulation in the component of the source polarization in this plane as the half wave plate is stepped through 90° in increments of 22.5°. Consequently, an observation obtained in spectropolarimetry mode requires data taking with NIFS to be coordinated with control of the half wave plate in GPOL. This procedure is described fully in one of the observing scenarios (§6.7).
4.4 Operating Modes
A NIFS data frame is a 2048´2048 pixel image consisting of 29 stacked spectral bands, each 70 spatial pixels high by 2048 spectral pixels wide. Data taking with NIFS consists of 1) defining detector readout parameters, 2) positioning the science object in the NIFS science field, and 3) initiating an exposure. Two quick look displays assist in positioning objects in the NIFS science field and in assessing data quality; these are the Idle Mode Quick Look Display and the Run Mode Quick Look Display. We first consider the Idle Mode Quick Look Display.
Between science exposures, the NIFS detector operates in a continuous read-out mode defined by Idle Mode parameters. The data are automatically reformatted into a spatial/spectral data cube and both raw data and spectrally-collapsed data are displayed. For example, when positioning a continuum source in the NIFS science field, the whole data cube might be collapsed in the spectral direction and the resulting 3.0²´3.0² image displayed in the Idle Mode Quick Look Display. When positioning an emission line source in the NIFS science field, the operator might choose to display spatial data only around the emission line of interest. Only this region would be collapsed in the spectral direction and the resulting 3.0²´3.0² image displayed in the Idle Mode Quick Look Display. The operator is able to plot the integrated spectrum of any subsection of the image to assess signal-to-noise ratio or to aid in selecting the optimal spectral subsection to display. The Idle Mode Quick Look Display allows the operator to specify a pre-recorded science data frame to be subtracted from the data before quick look processing to remove sky emission or a bias frame. The Idle Mode Quick Look Display is capable of processing and displaying data frames in less than the 10 s duration of the minimum NIFS science detector exposure.
Science exposures are obtained using Run Mode readout parameters. The Run Mode Quick Look Display automatically reformats Run Mode data into a spatial/spectral data cube and provides the same display options as for Idle Mode data. Run Mode and Idle Mode data are displayed separately so that recent data can be assessed while the next object is being acquired. The operator is able to write any displayed spectrum to a scratch area on disk. The operator can then recall any prerecorded spectrum and ratio subsequent spectra with this spectrum in order to remove atmospheric absorption features.
All science data obtained in Run Mode are written to the data archive in the unprocessed 2048´2048 data frame format. Idle Mode data are not archived.
5 Science Case Overview
NIFS science is adaptive optics science, so much of the Gemini core science that has been described in the science case for ALTAIR will be realized using NIFS in combination with ALTAIR. NIFS is also the facility near-infrared spectrograph on Gemini North, so NIFS will address in the north much of the science that GNIRS will address in the south. Nevertheless, NIFS is intended to be a fast-tracked, limited capability spectrograph and so is not designed to reproduce all of the GNIRS capabilities. Rather, NIFS attempts to perform a limited range of science with high efficiency. Imaging spectroscopy at near the maximum spatial resolution attainable with Gemini has been identified as the primary role of NIFS. The application of this technique to the study of the demographics of massive black holes in nearby galactic nuclei, and the related study of the dynamics of the inner narrow-line regions of nearby Seyfert galaxies are defined to be NIFS core science.
The key features of NIFS are its high spatial resolution IFU and its moderate spectral resolution in the 0.94‑2.50 mm wavelength range. This combination offers high information gathering potential, but the combination of high spatial and moderate spectral resolution means that NIFS is limited by dark current and detector read noise in some applications. The sort of observations for which NIFS excels require high resolution data of spatially complex regions having high surface brightness in either a spatial or spectral sense; spectroscopy of compact, high surface brightness continuum sources and imaging of extended, narrow emission line regions are good examples of these two extremes. These themes feature strongly in the science programs described below in order of increasing source distance.
The science requirements of NIFS cannot be treated in isolation from the requirements imposed by ALTAIR since it is the combination of NIFS and ALTAIR that will lead to the most significant scientific outcomes. We therefore attempt to estimate AO natural guide star and performance requirements where possible. Much of this information is taken from existing ALTAIR documentation. To achieve optimal performance, the natural guide star AO system requires a visible AO guide star brighter than R ~ 15 mag to be located within ~ 20² of the science object. Strehl ratios of ~ 0.2, 0.4, and 0.6 are then expected at J, H, and K, respectively, in 10% best seeing conditions. Crudely, a Strehl ratio of 0.2 means that only ~ 20% of the flux from a point source is contained within a central 0.1²´0.1² aperture. The rest of the light forms a halo with FWHM set approximately by the uncorrected seeing size. The laser guide star upgrade to ALTAIR is expected in 2004, approximately one year after NIFS has been commissioned. The laser guide star capability will increase the number of objects for which optimum Strehl ratios will be achieved, but it will not lead to improvements in these optimal values. The ALTAIR laser guide star system will require a tip-tilt guide star brighter than R ~ 17-19 mag (depending on atmospheric stability) within ~ 30² of the science object. Tip-tilt motion can be sensed either by the AOWFS in the optical or by the OIWFS in the near-infrared.
The effect of partial AO correction on NIFS data is different for unresolved and for resolved image structures. An adaptive optics system concentrates the object light. Consequently, higher Strehl ratios lead to improved signal-to-noise ratios on unresolved structure. The signal-to-noise ratio achieved on a uniform extended source is set by the source surface brightness. At low Strehl ratio, light from a broad region of the source contributes to each pixel. Improving the Strehl ratio does not improve the signal-to-noise ratio, but it does concentrate more of the light into the diffraction-limited core. The detected surface brightness distribution then more closely follows the true source surface brightness distribution. Correcting for the residual seeing halo in partially AO-corrected NIFS data is an important part of the NIFS data reduction.
Specific performance predictions for NIFS are addressed in the document “NIFS Performance Model” (SDN0004.1) and in the NIFS Conceptual Design Review documentation.
5.1 NIFS Core Science
5.1.1 Massive Black Holes in Nearby Galactic Nuclei
One of the most profound results from the Hubble Space Telescope (HST) is the evidence for the existence of massive (107 to 109 Mʘ) black holes in the nuclei of many nearby early-type galaxies (e.g., Kormendy & Richstone 1995; Lauer et al. 1995; Faber et al. 1997). An apparent relationship exists between the black hole mass and bulge mass which suggests that either central black holes grew by accreting inner bulge stars, or else that the central black hole and the bulge formed coevally in major merger events. However, this correlation suffers from strong observational selection effects (Ford 1997), and it is yet to be determined whether elliptical galaxies and spiral galaxies follow the same or different correlations. Given the potential close link between black hole formation and galaxy evolution, it is important to define the mass distribution and frequency of occurrence of central black holes in both classes of galaxies. These are still poorly known; most particularly in late-type spiral galaxies in which the nuclear regions are obscured by dust and the bulge masses are smaller so the black hole masses may also be smaller. Observations with NIFS will help determine the demographics of massive black holes in galactic nuclei. Spatially resolved, high resolution, dynamical studies of the innermost nuclear stellar populations and LINER-like gaseous accretion disks at near-infrared wavelengths are necessary to do this. Observations of both surface brightness distributions, mean rotation, and radial velocity dispersion profiles with spatial resolutions of a few parsecs and spectral resolutions of 3000-5000 are required to model the stellar dynamics or gaseous accretion disk dynamics and infer properties of the central black hole. The high spatial resolution required dictates the use of adaptive optics correction. This, and the presence of obscuring dust in the central regions of many spiral galaxies which complicates the interpretation of optical data from HST, dictates the use of near-infrared observations. The CO (2-0) absorption bandhead at 2.3 mm is ideal for measuring velocity dispersions of cool stellar populations in the nuclei of low redshift galaxies (e.g., Gaffney, Lester, & Doppmann 1995; Shier, Rieke, & Rieke 1996). The presence of a mass concentration is indicated by a rising stellar velocity dispersion profile near the nucleus. The emission lines of H I Pb (1.281 mm), H I Brg (2.166 mm), [Fe II] 1.257 mm, and [Fe II] 1.644 mm are expected to arise in the shock-excited gas of circumnuclear accretion disks. The enclosed mass is inferred from the rotational velocity, assuming that the gas follows Keplerian orbits about the mass concentration.
The velocity dispersion profile of M32 rises from sV ~ 60 km s‑1 at 1.0² radius to sV ~ 95 km s‑1 at 0.1² radius (Bender, Kormendy, & Dehnen 1996). Stellar velocity dispersions are therefore expected to be ~ 50 km s‑1 in the outer parts and ~ 100 km s‑1 at the centers of galaxies containing low mass black holes. These velocity dispersions correspond to Gaussian FWHMs of ~ 118 km s‑1 and ~ 235 km s‑1, respectively. A FWHM velocity resolution of ~ 100 km s‑1 therefore suffices to measure stellar velocity dispersions from the CO (2-0) absorption bandheads in the K band. The LINER gas disk in the elliptical galaxy M84 has peak rotational velocities of ±400 km s‑1 (Bower et al. 1998), and the gas disk in NGC 4261 has peak rotational velocities of ±200 km s‑1 (Ferrarese, Ford, & Jaffe 1996). A velocity resolution of < 100 km s‑1 will be required to measure disk rotational velocities in a range of lower mass objects. Similarly high spectral resolving powers of R ~ 4000-5000 are required to significantly separate individual OH airglow lines in the J and H bands in order to perform sensitive measurements of the emission lines from circumnuclear LINER-like disks.
M33 does not contain a black hole, but it does possess a blue nuclear star cluster (Kormendy & McClure 1993; Lauer et al. 1998). This star cluster has a velocity dispersion of only 21 km s‑1 (Gaussian FWHM ~ 50 km s‑1). Velocity resolutions of ~ 50 km s‑1 are needed to study similar nuclear star clusters in other galaxies.
High Strehl ratios are required for these observations. Galaxy nuclei are in general too faint and diffuse to be effectively used as guide stars for ALTAIR. However, the laser guide star facility will allow this sample to be extended and will deliver near optimal Strehl ratios on most objects. It will be necessary to accurately determine the PSF by frequent measurement of a PSF star, by reconstructing the PSF from OIWFS frames, or by modeling based on the AO control loop output (Véran et al. 1997).
5.1.2 Nearby Active Galactic Nuclei
Many nearby galaxies possess active nuclei which are characterized by broad (FWHM ~ 500 km s‑1) emission lines originating in their central regions over size scales of 100 pc up to ~ 2 kpc. This is the so-called narrow-line region (NLR). The ultimate energy source is believed to be accretion onto a massive black hole in most objects, although intense starbursts in dense regions may be responsible for some LINER-like activity (Terlevich & Melnick 1985). Emission from the immediate vicinity of the accretion disk produces the broad-line region (BLR) which remains unresolved with existing telescopes. Understanding the nature of the central energy source, its interaction with the host galaxy, and the global implications for the evolution of galaxies are continuing themes in the study of Active Galactic Nuclei (AGN). High spatial resolution optical studies of AGN with HST (e.g., Winge et al. 1997; Axon et al. 1998; Capetti et al. 1998) have revealed a wealth of information about the structure and excitation of the inner NLR. While it has traditionally been believed that the NLR clouds are photoionized by the central source (Ferland & Netzer 1983; Wilson & Tsvetanov 1994), these recent high spatial resolution imaging and dynamical studies have demonstrated that NLR clouds may instead be predominantly shock excited by energetic thermal and non-thermal mass outflows from the central object. Strong dynamical interactions between the emission line gas and radio-emitting ejecta can be explained if the NLR is formed from shells of ambient interstellar medium swept up and compressed by the supersonic expansion of hot-gas heated by interactions with the advancing radio jet (Pedlar, Dyson, & Unger 1985; Taylor, Dyson, & Axon 1992; Steffen et al. 1997). The nuclear regions of Seyfert galaxies are invariably obscured by dust clouds making near-infrared observations of the inner NLR desirable. The near-infrared region also offers the best ground-based spatial resolution with adaptive optics correction. [Fe II] 1.257 mm, [Fe II] 1.644 mm, H I Pb 1.282 mm, and H I Brg at 2.166 mm emission lines are well-suited to excitation and dynamical studies of the low-excitation recombination zones associated with shocked regions. The mechanical energy flux from the jet can be estimated from the [Fe II] and H I Pb lines in less obscured regions, and H I Brg in more obscured regions. Strong coronal emission lines are the primary initial coolants of hot gas in partially radiative shocks. With NIFS, the [Si VI] 1.961 mm coronal line will become accessible at modest redshifts. H2 1-0 S(1) 2.122 mm emission in Seyfert galaxies is collisionally-excited, but generally has a smaller velocity width of ~ 300 km s‑1 suggesting that it may arise in a different emission region (Veilleux, Goodrich, & Hill 1996). Shock heating by the interaction of the radio jet with the interstellar medium and shock excitation in outflows from star formation regions may both contribute to the H2 emission in Seyfert galaxies. High spatial resolution dynamical studies may provide a means of distinguishing between these alternatives.
Seyfert activity is frequently associated with circumnuclear starbursts, often in rings, but the role these play in fueling or refueling the active nucleus is still unclear. The presence of circumnuclear starburst rings demonstrates that large quantities of gas have been channeled into the region close to the nucleus. This gas may accrete directly onto the black hole, but it must lose its remaining angular momentum to do this. It may be the stars, or their remnants, formed in the starburst ring or the central star cluster that feed the central black hole. High angular resolution spectral imaging of Seyfert galaxy cores will reveal structure interior to the starburst ring. The morphology and dynamics of the emission line regions will permit new insight into how gas is funneled into the core and the role played by stellar bars. Detailed comparison of spatially-resolved spectra with starburst models (e.g., Leitherer et al. 1999) will provide estimates of the starburst ages, masses, and star formation histories. These can then be compared to particular models for AGN fueling (e.g., Norman & Scoville 1988). The potential for AO-corrected imaging of Seyfert galaxy cores in addressing these issues is beginning to be explored (Marco, Alloin, & Beuzit 1997; Chapman, Walker, & Morris 1998; Rouan et al. 1998; Marco & Alloin 1998, 1999).
Measurement of the central black hole masses in Seyfert galaxies is also highly desirable. Stellar velocity dispersions can be measured using the CO (2-0) absorption bandhead and interpreted in the same way as for normal galaxies (§5.1.1) to place limits on the enclosed mass and hence detect or constrain black hole masses. The analysis is complicated in the case of Seyfert galaxies by the intense Seyfert core. The core of the nearest Seyfert galaxy, Circinus, has a radius of < 1.5 pc at K (Maiolino et al. 1998), corresponding to < 0.08², and is much brighter than the surrounding starlight. Minimizing contamination from the core will therefore depend on achieving high Strehl ratios. Nevertheless, approximately 43% of nearby galaxies show a detectable low level of nuclear activity (Ho, Filippenko, & Sargent 1997). These galaxies may contain either lower mass black holes or massive black holes that are currently accreting at well below their Eddington limit. The galaxies in this sample that are closer than ~ 20 Mpc, have sufficiently high K band central surface brightnesses, and have suitable guide stars will be prime candidates for NIFS.
The AO requirements on Seyfert galaxy programs are less severe than for normal galaxies. Many Seyfert nuclei are bright enough and sufficiently compact to use as AO guide objects for ALTAIR. High Strehl ratios are needed to measure black hole masses, and are desirable when studying emission from NLR clouds.
5.2 Gemini Core Science
5.2.1 Brown Dwarfs and Low Mass Stars
Recent near-infrared sky surveys have succeeded in identifying large numbers of low mass stars and brown dwarfs. However, objects in binary systems still offer the only means of empirically determining precise masses and absolute magnitudes for this class of objects. These data are basic to an understanding of the substellar mass function, and ultimately the transfer of angular momentum during star formation and the universal proportion of matter bound up in sub-stellar companions. The distances to objects in binary systems can be determined, so they provide empirical calibration of the color versus absolute magnitude relation that can be applied to field and cluster brown dwarf candidates. NIFS with occulting disks will record moderate resolution near-infrared spectra of the close companions that will provide effective temperatures and other physical parameters for the companions. The imaging capability of NIFS will be invaluable in removing the complex residual “speckle” pattern of the bright primary star.
Spectra in the J, H, and K bands with a resolving power of R ~ 1000 are sufficient to determine molecular absorption band strengths for temperature determination (e.g., Gl 229B; Geballe et al. 1996; K ~ 14.8 mag). The central star can be used as the AO guide star in all conceivable cases. Finding a nearby OIWFS star will be subject to random field statistics.
5.2.2 Young Star Clusters
A knowledge of the initial stellar mass function over the full range of masses from the Eddington limit to below the hydrogen-burning limit, and its dependence on environment, are fundamental to an understanding of the star formation process. The upper stellar mass cut-off needs to be explored in nearby regions of massive star formation in order to better understand the nature of massive star formation occurring in more extreme regions, such as starburst galaxies. Concentrations of high mass stars are found in young Galactic star clusters. These are often obscured by dust due to their youth or their large distance from Earth. High spatial resolution observations are needed to probe the cores of dense star clusters associated with Galactic giant H II regions (e.g., Blum, Damineli, & Conti 1999) and clusters in the vicinity of the Galactic center (e.g., Cotera et al. 1996). The physical parameters of embedded massive stars can be derived from high (~ 70) signal-to-noise ratio, moderate resolution (R > 1000) spectra in the H and K bands (Blum, Damineli, & Conti 1999; Hanson, Howarth, & Conti 1997; Hanson, Conti, & Rieke 1996).
Knowledge of the low mass end of the stellar initial mass function in different environments is needed to determine the amount of Galactic mass locked up in low mass stars, to understand chemical enrichment and recycling in galaxies, and to determine the impact of starbursts on galaxy evolution. Most low mass stars currently forming in the Galaxy appear to be forming in star clusters associated with giant molecular clouds (Lada et al. 1991). Stellar masses for lower mass pre-main-sequence stars (M < 5 Mʘ) cannot be determined unambiguously from broadband near-infrared photometry alone due to the indeterminate effects of interstellar extinction and the nature of their evolutionary tracks. Moderate resolution K band spectra of obscured, low mass, cluster pre-main-sequence stars are required to assign them spectroscopic temperatures (Hodapp & Deane 1993; Luhman & Rieke 1998), and hence infer their masses based on evolutionary tracks.
Source confusion and the irregular backgrounds from complex reflection and emission nebulosity associated with dense young star clusters make slit spectroscopy of faint, embedded, young stars difficult. NIFS with its IFU will allow more accurate removal of these irregular backgrounds. NIFS will be capable of measuring K band spectra with R ~ 5300 and a signal-to-noise ratio of ~ 40 on stars with K = 16 mag in ~ 1800 s.
5.2.3 YSO Jet Driving Mechanisms
The driving mechanism for outflows from young stellar objects (YSOs) has not been observationally identified. Shocked, collimated jets are seen at large distances from the star, but the properties of the winds at their origins, and even the mass loss rates, remain uncertain and model dependent. High spatial resolution spectral imaging in emission lines probing shocked gas, such as H2 1-0 S(1) 2.122 mm and [Fe II] 1.644 mm, will allow observation of the energetic, highly collimated jets as they emerge from the inner regions of the accretion disks. The high spectral resolution of NIFS, relative to narrow-band line filters, will enable better discrimination against continuum emission making NIFS the preferred Gemini instrument for near-infrared spectral imaging of faint, narrow emission line sources. High resolution spectral imaging of YSO jets with NIFS will provide simultaneous morphological, excitation, and kinematic data which, over time, will allow the evolution of features in these stellar jets to be traced as they progress along the jet and interact with the surrounding material. Such observations are crucial to understanding the role played by high energy outflows in terminating infall and determining the final stellar mass. For example, a “Herbig-Haro” emission knot located in a nearby dark cloud ~ 150 pc from Earth and moving at 100 km s‑1 traverses 0.13² in one year. Proper motions of such Herbig-Haro knots could be followed over a 2-3 yr period, allowing the acceleration mechanism to be probed as well as the interaction of these knots with the ambient cloud. Temporal variations in an extremely young Herbig-Haro flow ejected from XZ Tau have been seen in the optical with HST (Krist et al. 1999). Emission line spectroscopy of such features with NIFS will reveal details of how the flows expand and evolve.
The targeted emission lines will be the H2 lines in the K band and [Fe II] 1.644 mm in the H band. The ratio of H2 1-0 S(1) 2.122 mm to H2 1-0 Q(3) at 2.424 mm can be used to derive the interstellar extinction correction. These observations require the highest possible spatial resolution and velocity resolutions in the H and K bands of 50-100 km s‑1. Velocity centroids can be determined to Dv ~ FWHM/SNR which should be ~ 5-10 km s‑1 with typical signal-to-noise ratios. Visible T Tauri stars can be used as natural guide stars for ALTAIR, when available, but a different star will be required for the OIWFS. The near-infrared responsivity of the OIWFS will greatly increase the availability of suitable OIWFS stars in star formation regions. Many YSOs are either not visible objects or are resolved in the optical. Laser guide stars will be required for AO-corrected observations of these stars.
5.2.4 YSO Jet-Cloud Interactions
YSO outflows remove excess angular momentum from the protostar system, they contribute to the turbulent support of molecular clouds, and they may be responsible for disrupting molecular clouds and ultimately terminating star formation within them. YSO outflows generally consist of a highly collimated, high velocity bipolar jet embedded in a less well collimated low velocity bipolar molecular outflow that is detected at millimeter wavelengths The physical parameters of the highly collimated YSO jets are still poorly understood. They emit most strongly in shock excited transitions of H2 and [Fe II] in the near-infrared and low excitation emission lines typical of Herbig-Haro objects in the optical in regions where the jet material impacts the surrounding medium. A bow shock forms where shocked gas impacts quiescent material in front of a Mach disk which forms where the jet impacts previously shocked material. Observed emission line strengths can be modeled either as J-shocks, C-shocks, or a combination of the two (e.g., Buckle, Hatchell, & Fuller 1999). J-shocks occur where the magnetic field is weak and the gas properties change suddenly. C-shocks occur in the presence of a strong magnetic field. A C-shock can form at the bow shock and a J-shock can form at the Mach disk when the magnetic field is slightly weaker. Which type of shock applies in YSO jet-cloud interactions is still controversial. The relative emission line strengths give an indication of the type and speed of the shock (Smith 1995). The bow shock and Mach disk are expected to be separated by ~ 500 AU (~ 3.4² at 150 pc) in most cases (Hartigan 1989), but the curved structure of the bow shock complicates identification of this feature. High spatial resolution spectral imaging with NIFS may succeed in separating these components.
YSO jets often have a knotty appearance, possibly due to jet instabilities or episodic ejection. Emission arising from between the knots may be due to the jet being partially molecular, due to entrainment of ambient material in a mixing layer, or simply due to the existence of unresolved emission knots. YSO jets have many similarities (and differences) with relativistic jets emanating from radio galaxies. Understanding the physical processes occurring in YSO jets may also help in understanding the nature of these extragalactic jets.
Spatially-resolved NIFS spectra in the K band are needed to determine H2 emission line fluxes, flux ratios, and radial velocities. Proper motions are also needed for knots within the jet to test jet models quantitatively. Typical jet velocities are ~ 200 km s‑1. Projected velocities are correspondingly lower. So velocity resolutions of < 50 km s‑1 are required. The 5s surface brightness limit for detecting H2 1-0 S(1) line emission spread over 3 spectral pixels (~ 75 km s‑1) in 1800 sec with 0.1²´0.1² spatial resolution will be ~ 2´10‑23 W cm‑2 arcsec‑2; ~ 500 times fainter than the highest surface brightness knots in OMC-1 (Stolovy et al. 1998).
Moderate Strehl ratios are probably sufficient to resolve these extended regions. Optical AO guide stars will be scarce in dark clouds, making laser guide star observations highly beneficial. It will be possible to use the outflow source or nearby embedded stars as the near-infrared OIWFS guide star in most cases.
5.2.5 Late Stages of Stellar Evolution
In recent years, HST and deep ground-based imaging of asymptotic giant branch (AGB) stars, proto-planetary nebulae, and young planetary nebulae have revealed remarkable but previously unknown structures. These show exquisite detail of central, point symmetric, often bipolar, cavities being carved out of the centers of spherical AGB star mass-loss envelopes (Sahai et al. 1998; Sahai et al. 1999a,b; Sahai & Trauger 1998). How such non-spherical cavities can be produced inside the recent spherical AGB star mass-loss wind is an unsolved mystery. Binarity, rotation, and magnetic fields have all been suggested. Most importantly, it is the immediate post-AGB phase where the asymmetries develop, so it is here that we should look for objects beginning the aspherical mass-loss process. Samples of candidate objects are currently being examined from the ground (e.g., van der Steene & Wood 1999): they often show Ha emission with a central peak of width ~ 100 km s‑1, a P Cygni type absorption on the blue edge of the emission line, and broad wings with widths of order 1000 km s‑1. High spatial and spectral resolution observations of these objects are required in order to examine the gas dynamical processes occurring within them. In particular, K band observations in the lines of H2 1-0 S(1) 2.122 mm and H I Brg 2.166 mm are required in order to study the beginning of cavity generation (as evidenced by H2 emission from shocked gas) and to determine the location of the H I emission region: is it a wind from the AGB star remnant, is it a jet from an accretion disk around a companion star, do the broad 1000 km s‑1 and 100 km s‑1 components of the H I lines come from the same place spatially? Velocity resolution of a few tens of km s‑1 are required for this work in order to make models of the gas flows in these systems. Since the H2 emission has been detected by NICMOS, it should be measurable with NIFS. Many proto-planetary nebulae and most young planetary nebulae have central stars that can be used as reference stars for ALTAIR. OIWFS guide stars will be subject to random field statistics.
5.2.6 Galactic Center
The Galactic center is a unique region of the Galaxy populated by old stars forming the inner Galactic bulge as well as young clusters of massive stars indicative of recent intense star formation activity. There is now strong evidence for the existence of a central massive black hole in the Galactic center (Eckart & Genzel 1996, 1997; Genzel et al. 1997; Ghez et al. 1998). High spatial and moderate spectral resolution observations of stars in the vicinity of the black hole are required to determine their radial velocity dispersion to complement available high spatial resolution proper motion data, and to study the nature of the stellar population. Stars close to the black hole should interact with the black hole and with each other frequently and may show evidence of these interactions in their spectral or morphological properties. Understanding the star formation history of the central region of the Galaxy will lead to a clearer understanding of the formation of the Galactic bulge, the nature of star formation in an environment of extreme gas temperature, pressure, velocity dispersion, magnetic field strength, and tidal shear, and of the processes fueling the central black hole in our Galaxy and perhaps in other more active galaxies.
The Galactic center is obscured at wavelengths shorter than ~ 1.5 mm. Most of the bright stars detected in the K band are young late-type supergiant and asymptotic giant branch stars or older luminous red giant stars. Main sequence stars are intrinsically fainter in the near-infrared, and so more difficult to detect. Nevertheless, main sequence stars in the volume around the Galactic center are being found in deep photometric surveys using conventional techniques (e.g., Blum et al. 1996) and AO image correction (Davidge et al. 1997b). Near-infrared spectra are needed for significant samples of faint stars in the crowded Galactic center region in order to estimate effective temperatures, extinctions, luminosities, and hence masses and ages. NIFS will be well-suited to this task; the spectral resolution, spatial resolution, sensitivity, and ability to accurately characterize complex background emission are all essential. Techniques for spectrally classifying early-type stars based on high signal-to-noise ratio R ~ 1000 H band and K band spectra are now in placed (e.g., Ali et al. 1995; Blum et al. 1997; Hanson, Rieke, & Luhman 1998). Higher spectral resolving power of R > 3000 are required for radial velocity measurements of stars in the immediate vicinity of the central massive black hole. Present AO-corrected imaging (Davidge et al. 1997b) extends to K ~ 16 mag. Early-type main sequence stars are expected at K > 14 mag. High signal-to-noise ratio spectra of these stars can be measured with NIFS in ~ 1 hr per field. High Strehl ratios are essential to separate individual stars in this crowded region. A star with R = 13.9 mag (“star A”) is located 18.8² from Sgr A* and can be used as the AO guide star for observations of the central region. A range of near-infrared-bright OIWFS guide stars exist.
5.2.7 Old Stellar Populations in Nearby Galaxies
ALTAIR on Gemini will resolve stars in nearby galaxies and hence make their stellar populations accessible to photometric and spectroscopic study. Asymptotic giant branch stars in several nearby galaxies have been studied photometrically in the optical with WFPC2 on HST. These photometric studies have only recently been extended to the near-infrared with AO systems on ground-based 4 m telescopes (e.g., the nucleus of M31; Davidge et al. 1997a). Source confusion remains a problem with 0.15² spatial resolution in the central regions of even nearby galaxies. The smaller diffraction-limited images with ALTAIR on Gemini will alleviate much of this problem. However, the high Strehl ratios that are required will restrict these observations to the near-infrared. At infrared wavelengths, the stellar population is dominated by asymptotic giant branch and red giant branch stars. Near-infrared spectra of individual stars will provide information that will lead to a better understanding of the chemical abundances and abundance ranges of stars in Local Group galaxies. CO 2-0 absorption bands at 2.3 mm are the primary abundance indicator in late-type stars. The 1.62 mm feature, due mainly to CO 6-3 absorption, has also been used in late-type stars with [Fe/H] ³ -1.3 (Origlia et al. 1997).
Asymptotic giant branch stars extend to MK ~ -7.0 mag, or K ~ 17.5 mag at the distance of M31 and its dwarf elliptical satellite galaxy M32. These are among the brightest old stars in these galaxies. NIFS with ALTAIR will achieve a signal-to-noise ratio in the K band of ~ 10 per pixel in 1800 s with R = 5300 at the asymptotic giant branch tip. Smoothing to R = 1000, which is adequate for CO band studies, will increase the signal-to-noise ratio to ~ 23 per pixel. The central 2.8² diameter nuclear region of M31 contains ~ 20 stars with K = 14.5-16.5 mag detected by AO imaging (Davidge et al. 1997a). These stars are exceptionally bright and are either a separate population of young massive stars (as in our galaxy), or they are unresolved star clusters. The tip of the red giant branch in globular cluster-like populations similar to 47 Tuc occurs at MK ~ -5.0 mag (Frogel, Persson, & Cohen 1981), corresponding to K ~ 19.5 mag at the distance of M31 and M32. K band observations of these stars at R = 1000 will require integration times of ~ 2.5 hr to reach a signal-to-noise ratio of ~ 10 per pixel. The success of these observations will depend critically on the Strehl ratios achieved; high Strehl ratios increase signal-to-noise ratio and reduce source confusion which will be problematic at these faint magnitudes. The star count models of Ratnatunga & Bahcall (1985) predict there will be ~ 0.3 star arcmin‑2 with R < 16.5 mag along the site-line to M31 and M32, assuming a typical B-R color of 0.5, so there should be ~ 1 suitable AO guide star per 2¢ diameter ALTAIR field in this direction. Observations will be restricted to regions within ~ 15² of these AO guide stars. Spatial information obtained with the NIFS IFU will aid accurate background removal.
High signal-to-noise ratio, moderate resolution spectra are required for detailed chemical abundance studies. At the full resolution available with the K grating, a signal-to-noise ratio of 100 will be obtained in 1 hr on a star with K ~ 15.0 mag. Thus it will be possible with NIFS to obtain full resolution K band spectra of red giant branch tip stars with m-M < 20, or distances < 100 kpc. This will enable detailed chemical abundance analyses of red giant branch tip stars in local dwarf spheroidal galaxies and in the Galactic halo. These spectra will also provide dynamical information.
5.2.8 Nearby Starburst Galaxies and Starburst Regions
Nearby starburst galaxies are excellent laboratories for studying the dissipation processes that are believed to have occurred when galaxy spheroids formed. The dominant structural components of nearby starburst galaxies have now been spatially resolved in the ultra-violet and optical with HST; WFPC2 images reveal large numbers of “super star clusters” which resemble young, massive (105-106 Mʘ) globular clusters and which have properties more extreme than those of 30 Dor in the LMC, for example. Near-infrared ground-based observations (e.g., NGC 1808; Tacconi-Garman, Sternberg, & Eckart 1996) identify presumably young “super star clusters” embedded in dust with AV ~ 10-20 mag. These optically hidden “super star clusters” may contain the youngest and most massive stars. Studying individual “super star clusters” spectroscopically in the near-infrared will allow starburst models to be applied to specific coeval components of the overall starburst. The distribution of interstellar extinction in starburst regions affects our interpretation of starburst energetics. Measuring extinction indicators on spatial scales comparable to the dominate starburst structures will improve our ability to distinguish between foreground screen and mixed extinction models for these components, leading to more accurate interpretations. Comparison of the ages and star formation rates derived for different locations within the starburst will reveal how the starburst has progressed through the region.
Typical continuum and emission line surface brightnesses can be estimated from knots in the starburst ring of NGC 7552 (Schinnerer et al. 1997). The K band continuum surface brightnesses are K ~ 13.0-13.5 mag arcsec‑2, and the prominent emission line surface brightnesses are ~ 8-25´10‑23 W cm‑2 arcsec‑2. Emission lines with surface brightnesses of ~ 14´10‑23 W cm‑2 arcsec‑2 and FWHM ~ 100 km s‑1 should be measured with NIFS to a signal-to-noise ratio of 10 in 1800 s with 0.1²´0.1² spatial resolution. A mosaic of 3´3 NIFS fields would be needed to fully map the starburst ring and nucleus of NGC 7552.
5.2.9 Ultra-Luminous Infrared Galaxies
Ultraluminous infrared galaxies (ULIRGs) occur in the late phases of the merger of two or more gas-rich galaxies. They may be the merger remnants of former compact galaxy groups (Borne et al. 2000). They have massive (~ 1010 Mʘ; Solomon et al. 1997), dense (nH2 ³ 104 cm‑2; Solomon, Downes, & Radford 1992) concentrations of molecular gas and dust in their central regions and are powered by either intense, compact starbursts or AGN activity. Their luminosities (> 1012 Lʘ), space density, activity, and short evolutionary timescales (~ 108 yr) suggest that they may represent an early phase in the formation of quasars (Sanders et al. 1988). Double nuclei with subarcsecond separations are seen in several well-studied examples. These are interpreted as the remnant nuclei of the merged galaxies. Several ULIRGs have K light profiles that are well fitted with r1/4 de Vaucouleurs laws suggesting that elliptical galaxies will be the end result of the mergers (Wright et al. 1990; Doyon et al. 1994) and that their high central mass densities are comparable to those of elliptical galaxies (Kormendy & Sanders 1992); ULIRGs may be analogs of proto-elliptical galaxies formed at high redshift. They are currently undergoing dissipative collapse which may also be analogous to spheroid formation in disk galaxies.
High angular resolution spectral imaging with NIFS will probe the stellar populations and emission line characteristics on spatial scales commensurate with the nuclear separations. These data will provide information on star formation rates, starburst lifetimes, and the nature of the stellar initial mass function close to the compact luminosity source as well as information on the gas dynamics in the region where the nuclei interact. Velocity dispersion data can be used to derive mass estimates for the enclosed regions. Both emission line and stellar absorption-line diagnostics can be used for this purpose. Extending these studies to smaller radii will place tighter constraints on the compact luminosity sources. Millimeter molecular line data with 0.5² resolution for Arp 220, for example, suggest that the double nuclei are each surrounded by counter-rotating gas disks with radial extents of ~ 0.3² (Sakamoto et al. 1999). The formation of nuclear gas disks during galaxy mergers may be a natural consequence of the high dissipation rates in dense gas. However, the likely role these disks play in driving nuclear starbursts or fueling AGN activity is yet to be explored. Interaction between these disks should be apparent in near-infrared shock-excited emission lines such as [Fe II] 1.644 mm and various H2 transitions in the K band. These features will be sampled with NIFS at higher spatial resolution than currently achieved at millimeter wavelengths. Studies to date show shock-excited emission line tracers can peak between the two nuclei (e.g., NGC 6240; van der Werf et al. 1993) whereas starburst tracers such as H I Brg emission peak on the nuclei (e.g., Arp 220; Larkin et al. 1995). NIFS will have the spatial and spectral resolution, and sensitivity, to examine these interactions in detail.
We consider the example of Arp 220 because of the availability of high spatial resolution NICMOS data (Scoville et al. 1998). The two nuclei in Arp 220 are separated by ~ 200 km s‑1 in H I Brg (Larkin et al. 1995). The Brg fluxes from the eastern and western components are ~ 5.3´10‑23 W cm‑2 and ~ 6.5´10‑23 W cm‑2, respectively, summed over 200 km s‑1. We assume that the flux from the eastern and western components extend over 0.2²´0.2² and 0.5²´0.2², respectively, based on NICMOS continuum images (Scoville et al. 1998). The emission line surface brightnesses are then ~ 1.3´10‑21 W cm‑2 arcsec‑2 and ~ 6.5´10‑22 W cm‑2 arcsec‑2 for the eastern and western components, respectively. The continuum surface brightnesses through the same apertures correspond to K ~ 10.7 mag arcsec‑2 and ~ 10.5 mag arcsec‑2, respectively. A signal-to-noise ratio of 10 per spectral pixel will be achieved with the NIFS K grating in 1800 s on a 200 km s‑1 wide emission line against a 10.5 mag arcsec‑2 continuum with 0.1²´0.1² spatial resolution at an emission line surface brightness of 5.5´10‑22 W cm‑2 arcsec‑2, similar to the expected emission line surface brightnesses of the Arp 220 components.
A similar velocity offset is seen in NGC 1614 where the velocity separation of the two nuclei is ~ 150 km s‑1 with each component having a velocity dispersion of sV ~ 50-60 km s‑1 and together having a total Brg emission line flux of ~ 6 ´ 10‑21 W cm‑2 through a 2.4² ´ ~ 4² aperture (Puxley & Brand 1999). Stellar velocity dispersions in ULIRGs range up to ~ 360 km s‑1 in NGC 6240 (Doyon et al. 1994) and have been measured successfully by Shier, Rieke, & Rieke (1996) with a velocity resolution of ~ 80 km s‑1.
AO imaging of NGC 3690 (Lai et al. 1999) has revealed the presence of at least six “super star clusters” with K ~ 15.8-16.3 mag, V-K ranging from -0.4 to 1.4, and which are unresolved at 0.2² resolution. It is unclear whether this emission is stellar or is due to ionized gas in giant H II regions. However, their existence is important because of the high specific frequency of globular clusters in giant elliptical galaxies; the possible ULIRG end-products. Globular clusters appear to form during the mergers that trigger ULIRGs (e.g., Schweizer & Seitzer 1998). Near-infrared spectroscopy with NIFS of “super star clusters” in objects like NGC 3690 will be used to determine starburst ages for individual clusters which will show whether they are proto-globular clusters formed during the merger event. Measurement of their velocity dispersion relative to the main nuclei will provide information on the nuclear masses and the merger dynamics. A signal-to-noise ratio of ~ 30 per spectral pixel will be achieved with the NIFS K grating (R ~ 5300) in 1800 s with a 0.1²´0.1² aperture on continuum objects with K ~ 16 mag, similar to the NGC 3690 clusters.
There are no suitable OIWFS guide stars for either Arp 220 or NGC 1614 within the 2¢ diameter ALTAIR field-of-view. NGC 6240 can be measured with the ALTAIR Laser Guide Star system, but no AOWFS guide star exists for the ALTAIR Natural Guide Star system. The two brightest nuclei in NGC 3690 can be used as OIWFS guide objects for each other. The bright optical cores of these objects can be used as AOWFS guide objects.
5.2.10 Dynamical Evolution of High Redshift Galaxies
Using Gemini with NIFS and ALTAIR, it may be possible to probe the properties of normal disk galaxies to redshifts z ~ 1. The internal kinematics of high redshift galaxies are fundamentally related to the galaxy mass. Recent evidence suggests that the star formation rates in galaxies at z ~ 1 were up to an order of magnitude higher than in present day galaxies (Madau et al. 1996; Glazebrook et al. 1999), from which it is inferred that disk galaxies have undergone substantial evolution during the intervening period. The Tully-Fischer relation expresses the relationship between rotational velocity and disk luminosity for present day disk galaxies. If enhanced star formation in disk galaxies at z ~ 1 boosts their luminosities by a factor of ~ 10, the Tully-Fisher relation predicts that high redshift galaxies will have rotational velocities lower by a factor of ~ 2 than present day disk galaxies of similar luminosity. Measurement of disk galaxy internal dynamics can therefore be used to trace the evolution in mass-to-light ratio of the underlying stellar population, and hence directly confront theories for the evolution of galaxies over cosmic time. Whereas luminosity functions derived from redshift surveys probe the evolution of the galaxy population as a whole, studies of internal galaxy dynamics measure luminosity evolution in individual galaxies, potentially permitting direct relationships to be drawn with other galaxy properties. Vogt et al. (1996, 1997) have measured rotation curves for 16 faint field galaxies with redshifts extending to z ~ 1 based on the [O II] l3727 emission line. These data demonstrate that at least some massive disk galaxies existed at z ~ 1. The rotational velocities for these high-luminosity galaxies indicate only a modest increase in luminosity of ~ 0.4 mag in MB relative to the local Tully-Fisher relation. However, evidence is accumulating that the degree of luminosity evolution to z ~ 1 depends on galaxy mass (Koo et al. 1995; Rix et al. 1997; Simard & Pritchet 1998), with less massive, small, disk galaxies undergoing larger evolution. Spatially and spectrally resolved observations of low mass galaxies are therefore of the greatest importance; the bulk of the star formation since z ~ 1 may have occurred in these systems. Confirmation that high redshift disk galaxies undergo ordered rotation is also needed, since this assumption underpins interpretations based on the Tully-Fischer relation.
Ha is the strongest emission line in the optical spectra of disk galaxies, it is the emission line most directly related to star formation rate, and it is less affected by dust extinction than [O II] l3727. At z > 0.5, Ha is redshifted into the near-infrared region accessible with NIFS. Velocity resolutions of ~ 50 km s‑1 are needed to adequately resolve the rotational velocity structure expected in small, low mass, disk galaxies at z ~ 1 (Koo et al. 1995). Spectral resolutions of this order are also required to adequately separate terrestrial OH airglow emission lines in the J band so that complete galaxy rotation curves can be recorded for significant samples of objects without contamination by OH emission lines. Spatially resolved spectroscopy, as opposed to integrated line profiles, is essential in order to confidently interpret emission line kinematics as due to circular rotation of the galaxy disk. Large disk galaxies at z ~ 1 typically have disk scale lengths RD< 6 kpc (Schade et al. 1996; Simard et al. 1999), corresponding to < 0.8² on the sky. However, high redshift galaxies have been identified with disk scale lengths down to RD ~ 1 kpc (~ 0.13²; Schade et al. 1996; Simard et al. 1999), despite these galaxies having B band luminosities comparable to present day L* galaxies. These high surface brightness, small objects are the strongly starbursting, low mass galaxies that are likely to provide the best test of mass-dependent luminosity evolution (Broadhurst, Ellis, & Shanks 1988). Adaptive optics spatial resolution will be required to adequately measure rotation curves for these key objects. Galaxy rotation curves are characterized by solid-body rotation out to approximately RD, and typically do not reach maximum velocity until ~ 2 RD. It will be necessary to trace Ha emission line profiles to ~ 1.6² in the largest galaxies, and ~ 0.25² in the smallest objects known. Due to relativistic effects, angular scales change little beyond z ~ 1 so the same requirements apply to higher redshift galaxies.
About 25% of galaxies at z ~ 0.3 have [O II] kinematics unrelated to disk rotation and have been classified as “kinematically anomalous” (Simard & Pritchet 1998). NIFS will be well-suited to studying the complex kinematics of these possible merger systems at high redshifts. If low mass galaxies are the most actively star forming galaxies at z ~ 1, they will be more susceptible to kinematic anomalies due to supernova-driven winds and nuclear outflows than their higher mass counterparts, and present a more diverse range of kinematics when studied with NIFS.
Glazebrook et al. (1999) have detected Ha in several z ~ 1 galaxies with a flux limit of ~ 10‑23 W cm‑2. At z ~ 1, Ha is redshifted to 1.3 mm which is accessible with the NIFS J grating. Objects must be chosen to have redshifts that avoid the strong OH airglow lines in this band. Careful selection of objects with suitable AO and OIWFS reference stars will be needed for existing high redshift galaxy samples. However, it should soon be possible to select samples of high redshift galaxies near bright stars using photometric redshift criteria and Sloan Digital Sky Survey data.
Faint observations at J and H will be limited by the fixed dark current pattern of the detector. If the detector dark current is sufficiently stable, it will be possible to remove this pattern without adding further noise by subtracting the median of several long exposure dark frames obtained during day-time. This requirement places tight constraints on the required stability of the detector dark current over periods of many hours. The success of this difficult observation will depend on the detailed properties of the NIFS detector and on the distribution and flux of Ha emission in actual z ~ 1 galaxies.
5.2.11 Lyman Break Galaxies
One of the most significant legacies of the Hubble Space Telescope has been its observations of the Hubble Deep Field and the insights into the nature of galaxies at high redshift which have resulted from these observations. The high redshifts (z > 3) of many galaxies in the Hubble Deep Field can be deduced from their absence in ultraviolet images sampling the observed spectrum below the redshifted Lyman continuum absorption edge; radiation below the Lyman continuum edge is absorbed by intervening hydrogen clouds. These objects are known as “Lyman break” galaxies, and they are among the most distant normal galaxies known.
Estimates of the star formation rate in Lyman break galaxies are based on their rest-frame ultraviolet continuum luminosities. The global star formation rate at z > 3 is quite modest and suggests that we see these galaxies before the bulk of the star formation in the Universe had taken place (Madau et al. 1996). This result is of immense significance for models of the formation and evolution of galaxies. However, the star formation rates on which it is based are questionable due primarily to the unknown and potentially large effect of extinction by dust clouds within the galaxies on the measured rest-frame ultraviolet continuum fluxes. Many paths are currently being pursued to obtain more definitive star formations rates for these galaxies.
Hydrogen recombination lines provide the most direct measure of star formation rate, and in Lyman break galaxies Hb is redshifted into the K band. Measurement of the Hb luminosities in Lyman break galaxies will therefore provide far more direct estimates of the star formation rates for these galaxies. Observations of Hb in Lyman break galaxies have already been attempted with 4 m telescopes (Pettini et al. 1998). However, the signal-to-noise ratio obtained was barely sufficient to provide convincing detections of the line. Taken at face value, these results suggest star formation rates between a factor of ~ 0.7 and ~ 7 larger than deduced from ultraviolet continuum measurements. This result needs to be confirmed and extended using higher signal-to-noise ratio K band spectra.
Nothing is known about the dynamics or masses of these distant, young galaxies. Moderate resolution, near-infrared observations with NIFS of redshifted Hb in Lyman break galaxies have the potential to spatially resolve velocity structure in these galaxies that would indicate whether they are undergoing ordered rotation or whether they are still accumulating sub-galactic components and are yet to settle into a stable dynamical structure. Even crude velocity measurements would provide the first constraints on the masses of these galaxy building blocks.
Typical Hb emission line fluxes for Lyman break galaxies are a few times 10‑24 W cm‑2. At a redshift of z ~ 3.5, Hb is shifted to 2.19 mm in the K band. Pettini et al. (1998) used spectral resolving powers of 2000-2500 for their observations. Lyman break galaxies are known to be small, but resolved at AO resolution, with half light radii of ~ 0.2²-0.3² (e.g., Steidel et al. 1996). Our ability to detect [O III] l5007 and Hb at z > 3.1 with NIFS will depend on the degree to which this emission is clumped on scales matching the NIFS spatial resolution of ~ 0.1². In fact, many young galaxies are composed of discrete, compact emission regions (e.g., Weedman et al. 1998), making them well-suited to AO-corrected spectroscopic observations with NIFS if the line fluxes are sufficiently high. If the Lyman break galaxies detected by Pettini et al. (1998) are composed of ~ 5 clumps with sizes of 0.2²´0.2², the typical emission line surface brightness of each clump will be ~ 5´10‑24 W cm‑2 arcsec‑2. A 5s detection of this feature averaged over 0.2²´0.2² is expected to be achieved in ~ 2 hr. Significantly higher signal-to-noise ratios would be required to measure dynamical information. Raw frames recorded with the K grating will be dominated by sky noise. Limiting observations will need to determine this sky spectrum from the actual object exposure. The ultimate feasibility of these limiting observations can only be determined after NIFS is commissioned.
High Strehl ratios will be required to reduce sky contamination and to measure velocity differences between individual clumps. Laser guide stars will probably be needed for most objects. OIWFS guide stars may be problematic for these objects.
6 Observing Scenarios
A variety of actual observing scenarios are described in this section in order to elaborate on the science descriptions in §5 and to identify and illuminate the requirements NIFS places on other parts of the Gemini telescope system.
Three wavefront sensors in the telescope/instrument system are capable of defining pointing and focus corrections; PWFS1, the AOWFS in ALTAIR, and the OIWFS in NIFS. It is therefore necessary to arbitrate between these devices. This is done in the observing scenarios below by defining one of the WFSs to be the primary tracking reference. This WFS operates in a “star-to-probe” mode where the telescope tracking position is altered to center the guide star in the WFS. The other two WFSs operate in the “probe-to-star” mode where the guide star is centered in the WFS by moving the probe position. WFS probes are set to absolute positions during (long) telescope slews and their relativities are defined when they acquire their guide stars. These will be different at different telescope orientations due to flexure. The WFS probes should be repositioned using relative coordinates during subsequent telescope offsets (e.g., to sky positions) in order to maintain this positional relativity. When acquiring a new object, PWFS1 must begin active mirror correction first, then AOWFS must begin adaptive optics correction, and finally OIWFS can begin slow tracking and focus correction using a faint guide star.
6.1 Standard Setup and Calibration
6.1.1 Rationale
Most NIFS programs use a standard setup and require a basic set of calibration observations to be obtained during the afternoon prior to observing. Ideally, a full set of calibration frames should also be obtained at this time. These standard setup and calibration observations are described in detail here.
6.1.2 Daytime Setup and Calibration
Bias frames are obtained by selecting the Blocked position in the NIFS filter wheel and recording a sequence of minimum duration (5 s) exposures. Typically 10-15 such exposures consisting of 12 coadds each (1 min of data) will be obtained so that cosmic ray events can be removed by median filtering during data reduction.
· Close the NIFS Environmental Cover.
· Set NIFS Focal Plane Mask Wheel to the Blocked position.
· Set NIFS Filter Wheel to the Blocked position.
· Set NIFS science detector readout method to Linear Fitting and load appropriate timing file.
· Set Idle Mode NDR period to 5 s (the minimum), number of NDRs to 1, and number of coadds to 1.
· Set Run Mode NDR period to 5 s, number of NDRs to 1, and number of coadds to 12.
· Record a Run Mode bias frame.
· Bias frame is automatically displayed in Run Mode Quick Look Display.
· Load bias frame as Idle Mode subtraction file.
· Load bias frame as Run Mode subtraction file.
· Set Run Mode repeats to 14.
· Record a Run Mode sequence.
· Bias-subtracted bias frames are automatically displayed in Run Mode Quick Look Display.
Dark frames can be obtained by repeating the above sequence for each exposure time used for science observations. Measurement of accurate dark frames is especially important for short wavelength observations in which dark current and read noise are dominant noise sources. Minimum noise will be achieved using the Linear Fitting readout method with ~ 60-70 NDRs. Limiting science exposures are typically of > 1 hr duration, requiring NDR periods of > 50 s. Obtaining sufficient dark frames for these long exposures is time consuming. Automated procedures will be available to do this.
· Close the NIFS Environmental Cover.
· Set NIFS Focal Plane Mask Wheel to the Blocked position.
· Set NIFS Filter Wheel to the Blocked position.
· Set NIFS science detector readout method to Linear Fitting and load appropriate timing file.
· Set Idle Mode NDR period to 10 s, number of NDRs to 1, and number of coadds to 1.
· Load previous bias frame as Idle Mode subtraction file.
· Set Run Mode NDR period to 50 s, number of NDRs to 72, and number of coadds to 1.
· Load previous bias frame as Run Mode subtraction file.
· Set Run Mode repeats to 5.
· Record a Run Mode sequence.
· Bias-subtracted dark frames are automatically displayed in Run Mode Quick Look Display.
Flat fielding is based on exposures of the “Black Body” lamp in the Gemini Calibration Unit (GCAL) for each grating setting used. Spectra of the “Black Body” lamp are recorded with appropriate ND filters and the NIR diffuser in a lamp-on/lamp-off sequence defined in the “Cal Unit (Advanced)” window of the Gemini Observing Tool. The minimum exposure time of 5 s will typically be used with typically 12 coadds for a total integration time of 60 s. These flat field exposures should be used to remove pixel-to-pixel response variations in the spectral direction. The GCAL beam does not pass through ALTAIR, so GCAL exposures do not include spatial response variations introduced by ALTAIR and the telescope. Any variations over the 3.0²´3.0² field of NIFS are expected to be small. However, a library of twilight sky exposures will be available for assessing and correcting this variation. These will be obtained periodically during engineering time.
· Open the NIFS Environmental Cover.
· Set NIFS Focal Plane Mask Wheel to the Clear position.
· Set NIFS Filter Wheel to the K grating order blocking filter position.
· Set NIFS Grating Wheel to K grating position.
· Set NIFS science detector readout method to Linear Fitting and load appropriate timing file.
· Set Idle Mode NDR period to 5 s (the minimum), number of NDRs to 1, and number of coadds to 1.
· Load previous bias frame as Idle Mode subtraction file.
· Set OIWFS Filter Wheel to the Blocked position to avoid detector saturation by flat field lamp light.
· Close ALTAIR shutters to protect AOWFS.
· Rotate ISS science fold mirror to point to GCAL.
· Select GCAL Black Body lamp, appropriate ND filter, an NIR diffuser.
· Set Run Mode NDR period to 5 s, number of NDRs to 1, and number of coadds to 12.
· Set Run Mode repeats to 5.
· Load previous bias frame as Run Mode subtraction file.
· Switch GCAL lamp OFF.
· Record a Run Mode sequence.
· Bias-subtracted lamp-OFF frames are automatically displayed in Run Mode Quick Look Display.
· Load lamp-OFF frame as Run Mode subtraction file.
· Switch GCAL lamp ON.
· Record a Run Mode sequence.
· Lamp-OFF-subtracted lamp-ON frames are automatically displayed in Run Mode Quick Look Display.
Wavelength calibration is based on exposures of arc lamps in GCAL. Arc lamp frames should be obtained after the flat field frames to avoid remnant arc spectra appearing in the flat field images. Spectra of the Lpg (Ar), Lpg (Xe), and Lpg (Kr) lamps are recorded with appropriate ND filters and the NIR diffuser in a lamp-on/lamp-off sequence defined in the “Cal Unit (Advanced)” window of the Gemini Observing Tool. The minimum exposure time of 5 s will typically be used with typically 12 coadds for a total integration time of 60 s. The lamp-off spectrum can be subtracted from the lamp-on spectrum using the Run Mode Quick Look Display.
· Open the NIFS Environmental Cover.
· Set NIFS Focal Plane Mask Wheel to the Clear position.
· Set NIFS Filter Wheel to the K grating order blocking filter position.
· Set NIFS Grating Wheel to K grating position.
· Set NIFS science detector readout method to Linear Fitting and load appropriate timing file.
· Set Idle Mode NDR period to 5 s (the minimum), number of NDRs to 1, and number of coadds to 1.
· Load previous bias frame as Idle Mode subtraction file.
· Set OIWFS Filter Wheel to the Blocked position to avoid detector saturation by arc lamp light.
· Close ALTAIR shutters to protect AOWFS.
· Rotate ISS science fold mirror to point to GCAL.
· Select GCAL Lpg (Ar) lamp, appropriate ND filter, an NIR diffuser.
· Set Run Mode NDR period to 5 s, number of NDRs to 1, and number of coadds to 12.
· Set Run Mode repeats to 1.
· Load previous bias frame as Run Mode subtraction file.
· Switch GCAL lamp OFF.
· Record a Run Mode exposure.
· Bias-subtracted lamp-OFF frames are automatically displayed in Run Mode Quick Look Display.
· Load lamp-OFF frame as Run Mode subtraction file.
· Switch GCAL lamp ON.
· Record a Run Mode exposure.
· Lamp-OFF-subtracted lamp-ON frames are automatically displayed in Run Mode Quick Look Display.
Reformatting NIFS spatial data requires a knowledge of the upper and lower bounds of each NIFS slitlet and a knowledge of any curvature and variation in scale in the spatial direction along each spectrum. The upper and lower bounds of each slitlet can be determined from the signal recorded for each slitlet in a flat field frame. Curvature and stretch can in principle be determined from measurements of a star at several positions along each slitlet, but this is not a practical approach. Instead, a mask of horizontal slits is provided in the NIFS Focal Plane Mask Wheel. This is used with the flat field lamp in GCAL to simulate the measurement of multiple star images. The mask produces a series of narrow illumination bands along the length of each slitlet. Curvature is determined from the absolute position of the middle illumination band in the spatial direction, while stretch in the spatial direction is determined from variations in the separations of any two illumination bands. Configure NIFS as for a flat field exposures and then select the “Calibration Slit Mask” in the Focal Plane Mask Wheel and record exposures in a lamp-on/lamp-off sequence defined in the “Cal Unit (Advanced)” window of the Gemini Observing Tool.
· Open the NIFS Environmental Cover.
· Set NIFS Focal Plane Mask Wheel to the Calibration Slit Mask position.
· Set NIFS Filter Wheel to the K grating order blocking filter position.
· Set NIFS Grating Wheel to K grating position.
· Set NIFS science detector readout method to Linear Fitting and load appropriate timing file.
· Set Idle Mode NDR period to 5 s (the minimum), number of NDRs to 1, and number of coadds to 1.
· Load previous bias frame as Idle Mode subtraction file.
· Set OIWFS Filter Wheel to the Blocked position to avoid detector saturation by flat field lamp light.
· Close ALTAIR shutters to protect AOWFS.
· Rotate ISS science fold mirror to point to GCAL.
· Select GCAL Black Body lamp, appropriate ND filter, an NIR diffuser.
· Set Run Mode NDR period to 5 s, number of NDRs to 1, and number of coadds to 12.
· Set Run Mode repeats to 1.
· Load previous bias frame as Run Mode subtraction file.
· Switch GCAL lamp OFF.
· Record a Run Mode exposure.
· Bias-subtracted lamp-OFF frames are automatically displayed in Run Mode Quick Look Display.
· Load lamp-OFF frame as Run Mode subtraction file.
· Switch GCAL lamp ON.
· Record a Run Mode exposure.
· Lamp-OFF-subtracted lamp-ON frames are automatically displayed in Run Mode Quick Look Display.
6.2 Molecular Hydrogen Emission For OMC-1 “Bullets”
6.2.1 Scientific Background
The Orion Molecular Cloud (OMC-1) is the closest Giant Molecular Cloud and the closest region of massive star formation. The cloud behind the Orion Nebula is a source of powerful mass outflows seen in molecular emission lines in the radio and in shock-excited H2 emission in the near-infrared. Several infrared objects are embedded in this dense molecular cloud, some of which must be the driving sources of the outflows. The H2 emission from the OMC-1 outflows has provided much of the data underpinning our present understanding of shock waves in molecular clouds. This is due to the closeness of the emission region and its high intrinsic brightness which allow the emission to be studied at both high spatial and spectral resolution. Images of the region with 0.5² resolution have revealed a system of hollow “fingers” of H2 emission behind a score of compact, high velocity Herbig-Haro “bullets” (Allen & Burton 1993). These “bullets” are at considerable distances from the driving sources and may be indicative of an explosive event in the core region ~ 103 yr ago (Allen & Burton 1993) or they may be generated in Rayleigh-Taylor instabilities as a fast wind interacts with a moving shell of gas, perhaps created by an earlier outflow phase or by another source (Stone et al. 1995; McCaughrean & MacLow 1997). The OMC-1 core has also been imaged in the 2 mm continuum and in the H2 1-0 S(1) 2.122 mm emission line by NICMOS on HST (Stolovy et al. 1998). They identified other “bullets” located within the core region close to the Becklin-Neugebauer object (BN) and IRc2, the most luminous infrared objects in the region. At NICMOS resolution (FWHM ~ 0.2²) several of these “bullets” have a clumpy arc-like appearance resembling bow shocks. The inner bullets emit in H2 only, presumably due to their lower velocity. Thus H2 line ratios, which are sensitive to the details of the excitation conditions as well as the class of shock at work, would be expected to change with distance down a wake. Fabry-Perot scans with 1.5² spatial resolution of the outer “bullets” (Chrysostomou et al. 1997) show complex H2 1-0 S(1) line profiles with FWZI ~ 150 km s‑1.
The science goal of this observation is to gain an understanding of the spatial and spectral structure within the bow shock region of one OMC-1 “bullet” in order to confront existing shock models. Clear, and different, excitation gradients between the head of the bow shock and its wake are predicted for the two classes of shock model (i.e., dissociating J-shocks and magnetic C-shocks). The broad H2 emission line widths seen in star formation regions suggest shock velocities in excess of the H2 dissociation velocity, and so favor C-shocks. However, recent ISO observations of shocked H2 emission lines in OMC-1 arising from energy levels up to 40,000 K above the ground state (Bertoldi et al., in prep), are inexplicable with C-shock models, but are consistent with unpalatable (at least to our understanding of the underlying physics) J-shock models. Resolution of this quandary has therefore gained new urgency. We will measure the spatial distribution of the H2 emission, its dynamics, and its excitation with a spatial resolution of 0.1² in one “bullet”, 12²W,9.4²S (see Fig. 4 of Stolovy et al. 1998). K band spectra are required to record a range of H2 emission lines of different excitation.
Similar observations have been made using CGS4 on UKIRT and 3D on the AAT with 1² spatial resolution (Tedds, Brand, & Burton 1999) to determine both the change in emission line profile and excitation along the bow shock. Excitation changes are indeed apparent. Superimposed on a thermal spectrum is a faint signature due to fluorescence. This is strongest off the wake (i.e., it is background cloud emission), but is also present in the wake where it is attributable to UV-excitation by radiation from the bullet head. No clear excitation changes were seen in the thermal line ratios, contrary to what is predicted for C-shocks with varying shock speed. However, these data do not permit this statement to be made with great certainty due to the relatively coarse 1² spatial resolution; contributions from on- and off-wake fluorescent and thermal components may not be correctly separated.
NIFS provides an ideal tool with which to examine this issue further. The pixel size samples the bullets well, and the spectral resolution and coverage allow key lines from the v=1 to v=4 energy levels to be observed simultaneously. The relative strengths of these lines are particularly sensitive to the mode of excitation (Brand et al. 1988; Burton & Haas, 1997). Importantly, there will be no uncertainty in the relative pointings of adjacent pixels so that line ratio variations can be clearly followed along a bullet-wake. Furthermore, the inner bullets of Orion emit entirely in H2, so there is no uncertainty in interpreting the excitation conditions through different species. Gemini and ALTAIR are needed to observe at the spatial resolution required to resolve the inner bullets. The spatial integrity of NIFS makes these measurements superior to spectra obtained by orienting long slits along the bullet wakes.
6.2.2 Planning the Observation
Celestial coordinates for the science field center and AOWFS, OIWFS and PWFS1 guide stars must be selected using the Gemini Observing Tool. Coordinates for the OMC-1 “bullets” are obtained from the HST/NICMOS images of Stolovy et al. (1998). These only need to be accurate to ~ ±0.3² since the precise location of the object within the NIFS field-of-view is not critical. We use the 12²W,9.4²S “bullet” (Stolovy et al. 1998, Fig. 4) as an example. This object has a spatial extent of ~ 2² so is well matched to the NIFS field-of-view. The 12²W,9.4²S “bullet” is located 15² from the bright BN source. This is beyond the 12.7² vignetted radius of the NIFS pick-off mirror, so we can use BN as the OIWFS guide star. GSC0477400935 (mag = 8.18) is chosen as the PWFS1 guide star.
A visible star brighter than R ~ 15 mag and within ~ 20² of the science field center is need as a guide star for ALTAIR in order to achieve a Strehl ratio of ³ 0.4 in the K band in median seeing (Morris et al. 1996). A star of R £ 16.5 mag within ~ 40² is sufficient in 10% best seeing. No such star is available in the HST Guide Star Catalog so we select this star from the astrometric study of the Orion Nebula Cluster by Jones & Walker (1988). JW432 = P1822 (I = 12.8 mag) is 11.3² from the science field center. We use this as the ALTAIR guide star. Although JW432 is embedded in nebulosity, the 3