1 Science Issues
1.1 Changes Since CoDR
The science drivers for NIFS were discussed extensively at the CoDR. The science topics considered were:
· Massive Black Holes in Nearby Galactic Nuclei
· Nearby Active Galactic Nuclei
· Brown Dwarfs and Low Mass Stars
· Late Stages of Stellar Evolution
· Nuclear Stellar Populations in Local Group Galaxies
· Old Stellar Populations in Nearby Galaxies
· Nearby Starburst Galaxies and Starburst Regions
· Ultra-Luminous Infrared Galaxies
· Dynamical Evolution of High Redshift Galaxies
The ability of NIFS to successfully address these science topics depends on both its performance and the availability of guide stars suitable for ALTAIR and the NIFS OIWFS for key objects relevant to the specific science goals. Predictions of the statistical performance of NIFS under certain assumptions about the instrumental parameters were presented at the CoDR. These are revised in §2.2 below. Questions were raised at the CoDR about how these predictions compared with the performance of operating near-infrared spectrographs on large telescopes. These comparisons are made in §2.3 below. They suggest that the NIFS performance predictions are as accurate as it is possible to make, given that many operational parameters are still uncertain.
The availability of guide stars was only superficially addressed at the CoDR where it was flagged as a concern. First generation Digitised Sky Survey images were inspected for a small number of objects. These suggested that ALTAIR natural guide stars will be available for only ~ 4% of possible science targets, and the ALTAIR laser guide star system will be usable for only ~ 9% of possible science targets. These statistics severely restrict the scope of science possible with NIFS. However, the first generation Digitised Sky Survey is based on coarsely sampled blue images so it is not ideal for identifying faint guide stars, especially against the bright background of a large galaxy. The question of guide star availability for specific science programs has been revisited using on-line data from the red second generation Digitised Sky Survey, 2MASS K band images (available for only about half of the sky to date), and archival HST WFPC2 and NICMOS preview images. The results of these investigations are presented in §2.4 below. They show that the number of objects available to NIFS with ALTAIR will be extremely limited.
1.2 Performance Predictions
The statistical performance of NIFS was estimated for the CoDR using a detailed simulation program, NIFSSIM, and a web-based calculator. Both of these programs assumed a mean detector dark current of ~ 0.01 e- s-1 pix-1 (NIFSSIM used a dark current distribution) on the assumption that the NIFS science detector would be a CdZnTe/MBE device. In fact, the NIFS science detector will be a conventional HAWAII-2 PACE device which is likely to have higher dark current; the fabrication contract for this detector specifies a dark current < 1 e- s-1 pix-1 with a goal of < 0.01 e- s-1 pix-1. Therefore, it is prudent to consider the performance that will result with a detector dark current of, say, 0.05 e- s-1 pix-1. Both programs also assumed that detector pixels map to 0.1″ in the dispersion direction and 0.05″ in the spatial direction. Anamorphic magnification of the grating actually means that detector pixels will map to 0.04″ in the spatial direction. This lowers the background noise contribution relative to dark current noise. New point source sensitivity predictions have been calculated using these revised parameters and the web-based calculator which is known to give the same result as NIFSSIM at the central wavelength of each grating band in a 0.1″×0.1″ square aperture. The results are listed in Appendix A (§10). They differ only slightly from those presented at the CoDR.
Based on the new predictions, NIFS should achieve signal-to-noise ratios of 10 per spectral pixel in a 0.1″×0.1″ aperture with median seeing and the expected Strehl ratios of 0.2 at Z and J, 0.4 at H, and 0.6 at K in a single 1800 s exposure on point sources with Z = 18.5, J = 18.1, H = 18.5, and K = 18.1 mag using the Z, J, H, and K gratings, respectively. These compare with the values of Z = 18.8, J = 18.4, H = 18.8, and K = 17.8 mag quoted at the CoDR.
The equivalent surface brightnesses for uniform, extended, continuum sources are Z = 15.1, J = 14.7, H = 14.4, and K = 13.6 mag arcsec-2 using the Z, J, H, and K gratings, respectively. These compare with the values of Z = 15.4, J = 15.0, H = 14.8, and K = 13.5 mag arcsec-2 quoted at the CoDR.
The emission line surface brightnesses required to make a 10σ per spectral pixel measurement of an extended emission line with FWHM = 100 km s-1 in a 0.1″×0.1″ aperture and 1800 s integration time are listed in Table 1.
Table 1: 10s per spectral pixel emission line sensitivities in 1800 s integrations for 100 km s-1 line width for different background continuum surface brightnesses.
|
Z grating |
J grating |
||
|
μZ |
R = 4990 |
μJ |
R = 6040 |
|
(mag arcsec-2) |
(W cm‑2 arcsec‑2) |
(mag arcsec-2) |
(W cm‑2 arcsec‑2) |
|
9.0 |
… |
9.0 |
1.3×10-21 |
|
10.0 |
1.0×10-21 |
10.0 |
8.4×10-22 |
|
11.0 |
6.7×10-22 |
11.0 |
5.4×10-22 |
|
12.0 |
4.3×10-22 |
12.0 |
3.6×10-22 |
|
13.0 |
2.9×10-22 |
13.0 |
2.5×10-22 |
|
14.0 |
2.1×10-22 |
14.0 |
1.6×10-22 |
|
15.0 |
1.7×10-22 |
15.0 |
1.5×10-22 |
|
16.0 |
1.5×10-22 |
16.0 |
1.4×10-22 |
|
|
|
||
|
H grating |
K grating |
||
|
μH |
R = 5280 |
μK |
R = 5300 |
|
(mag arcsec-2) |
(W cm‑2 arcsec‑2) |
(mag arcsec-2) |
(W cm‑2 arcsec‑2) |
|
9.0 |
7.6×10-22 |
9.0 |
4.8×10-22 |
|
10.0 |
4.9×10-22 |
10.0 |
3.1×10-22 |
|
11.0 |
3.1×10-22 |
11.0 |
2.1×10-22 |
|
12.0 |
2.1×10-22 |
12.0 |
1.5×10-22 |
|
13.0 |
1.5×10-22 |
13.0 |
1.2×10-22 |
|
14.0 |
1.2×10-22 |
14.0 |
1.0×10-22 |
|
15.0 |
1.0×10-22 |
15.0 |
9.4×10-23 |
|
16.0 |
9.3×10-23 |
|
|
1.3 Performance Comparisons
The NIFS performance predictions are entirely theoretical, and as such their validity is unproven. Indeed, it was noted at the CoDR that the NIFS performance predictions are significantly worse than early claims for the performance of NIRSPEC on Keck. This prompted the CoDR committee to recommend that more detailed comparisons be made between the predicted performance of NIFS and the actual performance of other near-infrared spectrographs on 8-10 m class telescopes. The results of these comparisons are reported below.
1.3.1 Comparison with NIRSPEC
The NIFS performance predictions were originally compared to NIRSPEC performance figures obtained from the NIRSPEC User’s Manual[1]. These are limiting magnitudes per resolution element estimated for the NIRSPEC low resolution mode (R = 2200 through 0.4″ slit) in 3600 s for a signal-to-noise ratio of 10 (Table 2).
Table 2: NIRSPEC User’s Manual Limiting Magnitudes (10:1 in 1 hr).
|
Wavelength (μm) |
R=2000(OH) |
R=2000 |
|
|
|
|
|
1.25 |
20.2 |
22.2 |
|
1.65 |
19.4 |
21.5 |
|
1.8 |
19.2 |
21.3 |
|
2.0 |
19.4 |
21.1 |
|
2.2 |
19.1 |
20.1 |
|
2.4 |
18.6 |
19.9 |
NIRSPEC performance predictions are also available on the Gemini web pages[2]. These are limiting magnitudes and line flux estimates for the NIRSPEC low resolution mode on point sources with 0.5″ seeing and a 3×3 pixel (0.57″×0.57″) slit for a signal-to-noise ratio of 3 in 30 min (Table 3).
Table 3: Gemini NIRSPEC Limiting Magnitudes (3:1 in 0.5 hr).
|
Wavelength (μm) |
Magnitude |
Line Flux (W m-2) |
|
|
|
|
|
1.00 |
19.0 |
7.5×10-20 |
|
1.25 |
19.5 |
3.3×10-20 |
|
1.65 |
18.5 |
2.4×10-20 |
|
2.2 |
18.5 |
1.8×10-20 |
A description of NIRSPEC by Ian McLean (19 August 1999) is available on the UCLA NIRSPEC web page[3]. This quotes a signal-to-noise ratio of 10 in 600 s at H = 16.5 mag for the low resolution mode which is read noise limited. If a 0.5 hr total integration time is comprised on three 600 s exposures, NIRSPEC should achieve a signal-to-noise ratio of 3 in 30 min at H = 18.4 mag based on this estimate. This agrees well with the Gemini value (Table 3). However, a limiting magnitude of H = 17.5 mag (10:1 in 1 hr) is suggested, if the 1 hr total exposure is comprised of six 600 s exposures. This is ~ 2 mag worse than claimed in the NIRSPEC User’s Manual. We therefore adopt the Gemini values as representative of the current performance of NIRSPEC.
Comparison with NIFS requires scaling for the different primary mirror diameters, the different spectral resolution, and the different spatial resolution. Using NIRSPEC on Gemini would degrade signal-to-noise ratio by a factor of (10/8)2 = 1.56 because NIRSPEC is read noise limited. Similarly, operating at R = 5300 would degrade signal-to-noise ratio by a further factor of 5300/2000 = 2.65. The better image quality achieved with ALTAIR allows NIFS to use narrower slitlets than NIRSPEC. However, this has no effect on signal-to-noise ratio because NIRSPEC remains read noise limited. We therefore degrade the quoted NIRSPEC signal-to-noise ratios by a factor of 1.56*2.65 = 4.14 to permit a direct comparison with NIFS.
The direct performance comparison of NIFS and NIRSPEC is made in Table 4. This lists point source limiting magnitudes for a signal-to-noise ratio of 3:1 in 0.5 hr on Gemini with R = 5300. Based on this comparison, it is clear that the predicted performance of NIFS is much better than the actual performance of NIRSPEC if used on Gemini with R = 5300. This is as expected since the NIRSPEC detector is significantly inferior to the assumed parameters for the NIFS detector and NIFS is matched to the excellent point source images to be delivered by ALTAIR.
Table 4: Comparison of NIFS and NIRSPEC Limiting Magnitudes (3:1 in 0.5 hr).
|
Wavelength (μm) |
NIFS Magnitude |
NIRSPEC Magnitude |
|
|
|
|
|
1.00 |
20.4 |
17.5 |
|
1.25 |
19.9 |
18.0 |
|
1.65 |
20.4 |
17.0 |
|
2.2 |
19.1 |
17.0 |
1.3.2 Comparison with ISAAC
The performance of ISAAC on the VLT has been determined using the web-based ISAAC Exposure Time Calculator[4]. The medium resolution grating with a 0.6″ slit produces resolving powers of R ~ 5000. The same size of the VLT and Gemini primary mirrors and the similar spectral resolutions allow a direct performance comparison (ignoring ALTAIR). This comparison is presented in Table 5. The predicted performance of NIFS is better than the actual performance of ISAAC. This is also to be expected because ISAAC uses its gratings in high orders which are likely to be less efficient than the first order gratings planned for NIFS, and ISAAC should suffer from additional background noise in this comparison which ignores the different slit widths of NIFS and ISAAC. The ISAAC model also assumes a telescope emissivity of 25% for the VLT which exceeds the combined assumed emissivities of Gemini and ALTAIR. This accounts for the poorer actual performance at K. In fact, ISAAC and NIRSPEC appear to have similar performance at K.
Table 5: Comparison of NIFS and ISAAC Limiting Magnitudes (3:1 in 0.5 hr).
|
Wavelength (μm) |
NIFS Magnitude |
ISAAC Magnitude |
|
|
|
|
|
1.00 |
20.4 |
18.4 |
|
1.25 |
19.9 |
19.6 |
|
1.65 |
20.4 |
19.3 |
|
2.2 |
19.1 |
17.0 |
1.3.3 Comparison with CIRPASS
CIRPASS uses an integral field unit in the J and H bands that can be configured to have a lenslet diameter of 0.12″, similar to the NIFS slitlet width of 0.10″. The spectral resolving power is R = 2840 at 1.25 μm and R = 3750 at 1.65 μm. The web-based CIRPASS Exposure Time Calculator[5] has been used to quantify the CIRPASS performance. A seeing FWHM of 0.1″ was assumed to approximate the effect of ALTAIR. The performance comparison with CIRPASS is presented in Table 6. NIFS (with R = 5280) and CIRPASS (with R = 3750) have almost identical predicted performance at H. The origin of the poorer predicted performance for NIFS at J is unclear. It may be related to the larger spectral resolution difference at J; R = 6040 for NIFS and R = 2840 for CIRPASS. If CIRPASS is detector noise limited at J, doubling the spectral resolving power would brighten the limiting magnitude by 0.75 mag changing the limit only to J ~ 21.0 mag.
Table 6: Comparison of NIFS and CIRPASS Limiting Magnitudes (3:1 in 0.5 hr).
|
Wavelength (μm) |
NIFS Magnitude |
CIRPASS Magnitude |
|
|
|
|
|
1.25 |
19.9 |
21.7 |
|
1.65 |
20.4 |
20.5 |
1.3.4 Comparison with CGS4
CGS4 sensitivities have been taken from the UKIRT CGS4 web pages[6]. The data are based on the 150 l/mm grating, the long camera, and are appropriate for point sources. The spectral resolving power with this arrangement is comparable to that achieved by NIFS. A correction of a factor of 4 (1.5 mag), with large uncertainty, has been applied to the published CGS4 sensitivities to allow for the larger Gemini collecting area; source signal will be larger by a factor of ~ 4 on Gemini, and the background might be unaffected if the CGS4 slit width is notionally reduced to match that of NIFS. After this correction, the intrinsic predicted sensitivity of NIFS is within ~ 1 mag of the measured CGS4 sensitivity.
Table 7: Comparison of NIFS and CGS4 Limiting Magnitudes (3:1 in 0.5 hr).
|
Wavelength (μm) |
NIFS Magnitude |
CGS4 Magnitude |
|
|
|
|
|
1.25 |
19.9 |
19.9 |
|
1.65 |
20.4 |
19.1 |
|
2.2 |
19.1 |
18.1 |
1.3.5 Summary of Comparisons
Within the considerable uncertainties associated with predicting the performance of a complex instrument from first principles, it appear that the performance of NIFS calculated by NIFSSIM and the web-based calculator is consistent with the measured or predicted performance of other near-infrared spectrographs on large telescopes.
1.4 Guide Star Availability
NIFS is an adaptive optics instrument so its scientific potential will be defined both by the foregoing signal-to-noise ratio considerations and by the availability of suitable guide stars. Guide star availability was considered statistically at the CoDR. However, the impact of guide star availability on the scientific scope of the instrument can only be assessed accurately by considering its consequences for observations of key science targets. The broad science cases at the CoDR are further refined in this section into specific observing programs based on real objects. The availability of guide stars for these objects is then assessed.
1.4.1 Guide Star Requirements
NIFS will be operated in three modes; with the ALTAIR natural guide star system, with the ALTAIR laser guide star upgrade, and on its own without ALTAIR. Observations which achieve the best image quality using NIFS with the ALTAIR natural guide star system will require an optical AOWFS guide star brighter than R ~ 15 mag as well as a near-infrared OIWFS guide star. The AOWFS guide star must be located within ~ 20″ of the science object. The limiting magnitude for OIWFS guide stars is not known precisely. It is expected to be significantly fainter than the AOWFS limit because the OIWFS needs only to track slow flexure changes when used with the ALTAIR natural guide star system. The OIWFS guide star must be located within the 120″ diameter circular field passed to NIFS by ALTAIR (§4.24.1) and outside the 25.4″ diameter circular region vignetted by the NIFS pick-off probe (§5.5.3.2.1). NIFS observations with the ALTAIR laser guide star upgrade will require a star within 30″ of the science object for fast tip-tilt and focus correction. This star will either be sensed in the optical by the AOWFS or in the near-infrared by the OIWFS. If the OIWFS is used, the guide star will have to be significantly brighter than the flexure correction star because much shorter integration times will be required. NIFS observations which do not use ALTAIR at all can access OIWFS guide stars over the full 180″ diameter field accepted by NIFS. The OIWFS will sense only slow flexure changes in this mode so fainter OIWFS guide stars will suffice.
1.4.2 Sky Coverage Estimates
Sky coverage issues for adaptive optics on Gemini have been discussed by Ellerbroek & Tyler (1998). They plot guide star density functions based on the Bahcall & Soneira (1980) model of the Galaxy (Figure 1). The adaptive optics corrected field of ~ 20″ diameter corresponds to ~ 2.4×10-5 deg2, so guide star densities of order ~ 4×104 deg‑2 are required for complete sky coverage. Actual guide star densities to R ~ 15 mag are closer to 102 deg-2 (Figure 1), so an AO natural guide star sky coverage of ~ 0.3% is expected. Sky coverage fractions for different Strehl ratios in the J, H, and K bands have been calculated in detail by Ellerbroek & Tyler (1998) and give a similar result for optimal performance (Figure 2).
The 120″ diameter ALTAIR field over which OIWFS guide stars are accessible corresponds to 8.7×10‑4 deg2, so guide star densities of ~ 103 deg-2 are required for complete sky coverage. Full sky coverage at 30° Galactic latitude would require an OIWFS guide star limit of R ~ 18 mag.
The OIWFS field for NIFS observations without ALTAIR corresponding to 3.5×10-3 deg2. Guide star densities of ~300 deg-2 are required for complete sky coverage, so an OIWFS flux limit of R ~ 18 mag would provide nearly complete sky coverage to 90° Galactic latitude.
Figure 1: Guide star density functions for 30° Galactic latitude and 90° Galactic latitude (Ellerbroek & Tyler 1998).
Figure 2: Sky coverage probabilities at 90° Galactic latitude for natural guide stars (lower curves) and laser guide stars (upper curves) in median seeing, 0° zenith distance, and typical windshake (Ellerbroek & Tyler 1998).
1.4.3 Guide Star Selection
Selection of specific guide stars requires detailed knowledge of stars in the vicinity of a science target. The US Naval Observatory (USNO) Catalog is one of the largest star catalogs in existence. However, tests show that it is incomplete, at least in the vicinity of external galaxies which are prime NIFS targets. Digitized Sky Survey images show faint stars, but only in clear sky regions away from other objects and where the original photographic plates were not saturated. The 2MASS near-infrared sky survey is good for identifying near-infrared OIWFS stars. However, this is currently only ~50% complete and, as with all ground-based surveys, its relatively coarse spatial sampling (~ 1″) limits its ability to reveal faint stars against extended objects. HST WFPC2 and NICMOS images, where available, provide the best data for selecting faint guide stars, especially near bright extended objects such as galaxies. In practice, a combination of all four images is required to confidently predict the subarcsecond structure of an object and select AOWFS and OIWFS guide stars.
A Perl script (gs_search.pl) has been written to automate the process of obtaining and inspecting second generation Digitized Sky Survey red images, 2MASS K band images, HST WFPC2 and NICMOS preview images, and overlaying USNO Catalog stars. A file containing a list of object names is input to gs_search.pl. The script then uses the name resolver function of the NASA Extragalactic Database (NED) to obtain object coordinates, retrieves the various images from on-line sources, and displays them along with overlays of USNO Catalog stars and the NIFS science field on a workstation screen (Figure 3). The display can be recentered at any position within the field, object coordinates and intensities can be printed, and suitable AOWFS and OIWFS guide stars can be marked.
Figure 3: Display of gs_search.pl output for the Seyfert galaxy NGC 1275. The displays show (clock-wise from top-left) the DSS-2 red image, the 2MASS K band image, a 1.6 μm NICMOS image, and a 0.70 μm WFPC2 image. USNO Catalog stars are marked with squares. The 3.0″×3.0″ NIFS field-of-view is marked with a small square at center. The three light concentric circles correspond to the 3′ diameter NIFS window, the 2′ diameter field passed by ALTAIR, and the 25.4″ diameter region vignetted by the NIFS pick-off probe. The inner heavy circle marks the 20″ radius region within which a natural guide star should be located for the ALTAIR natural guide star system. The outer heavy circle marks the 30″ radius region within which a tip-tilt star should be located for the ALTAIR laser guide star system. The lower two frames have been zoomed.
The gs_search.pl tool has been used to visually determine whether the objects discussed below are suitable for measurement with NIFS and whether they have the required guide stars for either the ALTAIR natural guide star system or the laser guide star upgrade to ALTAIR. Objects with insufficient data to make the assessment are classified as uncertain.
1.4.4 Massive Black Hole Galaxies
One of the main science drivers for NIFS is the detection of massive black holes in the cores of nearby spiral galaxies. The availability of guide stars for this program has been gauged by inspecting on-line images for 392 galaxies in Tully’s Nearby Galaxy Catalog that are closer than 20 Mpc, north of declination –30°, and have orderly spiral structure. The distance limit is required to resolve the sphere of influence of the black hole. The declination limit is required to achieve good image quality from Mauna Kea. Small irregular galaxies lacking spiral structure were deemed to be unlikely to contain detectable nuclear black holes. The results of this search are shown in Table 8. Very few nearby galaxies can be studied with the ALTAIR natural guide star system because the nuclei of most galaxies are too extended in WFPC2 images to be used as the ALTAIR guide object. Of the 24 galaxies that can be measured in this way (Appendix B, §11.1), nuclear mass limits or black hole detections are already known for at least five of these. Only 16 galaxies are reasonable massive black hole candidates. The larger sample of ALTAIR laser guide star system targets (Appendix B, §11.2) reflects the larger field-of-view over which tip-tilt stars can be accessed and their fainter limiting magnitude for tip-tilt-only correction.
Table 8: Guide Star Availability for Massive Black Hole Galaxies.
|
|
ALTAIR NGS System |
ALTAIR LGS System |
||||
|
Y |
? |
N |
Y |
? |
N |
|
|
# |
24 |
106 |
262 |
123 |
109 |
160 |
|
% |
6% |
27% |
67% |
31% |
28% |
41% |
A central issue of this study is whether spiral and elliptical galaxies follow the same or different relations between black hole mass and bulge luminosity (the Kormendy-Richstone relation). If spiral galaxies follow the Kormendy-Richstone relations derived for elliptical galaxies, then we expect spiral galaxies with smaller bulge luminosities to have smaller black hole masses. The problem of distinguishing these smaller black hole masses from stellar mass contributions in nearby spiral galaxies was alluded to at the NIFS CoDR. We can quantify this better now that we have a specific target list.
The Kormendy-Richstone relation for known nuclear black hole galaxies (Ho 1998, Magorrian et al. 1998) is shown in Figure 4. Enclosed stellar mass limits in a 0.2″×0.2″ aperture are also shown for the subset of the ALTAIR natural and laser guide star samples that have sufficient data available. Nuclear stellar masses are estimated from central H band magnitudes measured from archival NICMOS images. The absolute calibration of these HST preview images remains slightly uncertain, but we proceed anyway. K band magnitudes have been inferred assuming H-K = 0.2, typical of late-type stars. The stellar luminosity was then converted to a mass using a maximal M/LK = 5. As noted at the NIFS CoDR, this is a factor of two higher than the maximum value attained by an old stellar population; this inferred mass is the minimum mass that could unambiguously be ascribed to a nuclear black hole. Bulge luminosities are from Ho, Filippenko, & Sargent (1997).
The open symbols in Figure 4 show that NIFS will only be able to unambiguously establish consistency for spiral galaxies with the Kormendy-Richstone relation derived for elliptical galaxies.
Figure 4: Kormendy-Richstone relation for known nuclear black hole galaxies from Ho (1998; filled circles) and Magorrian et al. (1998; filled squares). Open symbols show the stellar mass in a 0.2″×0.2″ aperture inferred from archival NICMOS images assuming a maximal M/LK = 5 for the ALTAIR natural guide star sample (open circles) and the ALTAIR laser guide star sample (open squares).
At face value, this is a disappointing result. However, there are
two mitigating factors. The first is that the nuclear magnitudes used above are
based on surface brightnesses integrated along the whole line-of-sight through
the galaxy nucleus. Detailed dynamical models will deproject the light
distribution so actual mass limits will be based on the slightly smaller luminosity
of the central three-dimensional volume. Tests performed using a sky brightness
taken immediately adjacent to the nucleus (to crudely subtract foreground and
background light) show that this leads to a reduction in log M
of only 0.1-0.3 for the centrally peaked light distributions typical of the
ALTAIR natural guide star sample. A larger margin may arise by asking whether M/LK = 5 is a realistic upper limit for an enclosed stellar population. The
assumed value is a factor of two larger than the predicted value for a 1010
yr old solar abundance stellar population with a exponentially declining star
formation rate having a time constant of 109 yr (Thatte et al.
1997). In fact, Moriondo, Giovanardi, & Hunt (1998) measure a mean bulge M/LK = 0.6±0.2 and a mean disk M/LK = 1.0±0.4 for nine early-type spiral
galaxies. This suggests that the actual enclosed stellar mass in any of our
target galaxies may be up to one order of magnitude lower than the limits
depicted in Figure 4. This would lead to an ambiguous black hole
detection. To be sensitive to the implied enclosed masses of ~ 106 Mʘ, NIFS must be able to measure a
velocity dispersion of ~ 30 km s‑1 (FWHM ~ 70 km s‑1)
at a radius of 0.1″ from the nucleus. This should be possible with good
signal-to-noise ratio spectra, given that the two-pixel velocity resolution
will be ~ 60 km s-1.
In summary, guide star
availability for this core science project restricts the sample of good target
objects with the ALTAIR natural guide star system to just 16 galaxies. The 0.1″
width of the NIFS slitlets means that unambiguous black hole detections in
these galaxies are likely only if they possess black holes with masses in
excess of the Kormendy-Richstone relation. It will not be possible to
unambiguously separate black hole and stellar mass contributions at lower
enclosed masses, making the interpretation of these results subjective. The
ALTAIR laser guide system will be needed to significantly extend the sample
size. However, the same interpretation ambiguities will apply to the laser
guide star sample.
1.4.5 Seyfert Galaxies
Determining the structures of the inner Narrow-Line Regions (NLRs) in nearby Seyfert galaxies is the second main science driver for NIFS. We specifically address the study of nearby Seyfert galaxies having radio structure that is resolved on arcsecond scales so that details of the interaction of the radio jet with the host galaxy interstellar medium can be studied with NIFS. Radio surveys of nearby Seyfert galaxies have been performed by Ulvestad & Wilson (1984a,b, 1989), Kukula et al. (1995), Nagar et al. (1999), Thean et al. (2000), and Schmitt et al. (2000), among others. We have selected our target objects to be those Seyfert galaxies from these papers having linear double or triple radio structure on scales larger than ~ 0.3″. The results of this selection are listed in Table 9 (and in more detail in Appendix B, §11.3). It is noteworthy that some classic Seyfert galaxies, such as NGC 2110, Mrk 3, NGC 2992, NGC 3227, NGC 4051, NGC 4388, and NGC 5506, are not able to be measured with NIFS and ALTAIR. The prototype Seyfert 1 galaxy, NGC 4151, and the prototype Seyfert 2 galaxy, NGC 1068, will be difficult to measure with long integration times; NGC 4151 lacks an OIWFS guide star (Figure 5) and NGC 1068 only has one OIWFS guide star at the extreme edge of the unvignetted ALTAIR field (Figure 6). It may be possible to measure these bright objects with short exposures which do not require an OIWFS star. The classic “ionization cone” Seyfert galaxy, Mrk 573, is used as an example of this observation in the NIFS OCDD. However, it is unclear whether ALTAIR will be able to use the slightly resolved nucleus seen in the WFPC2 image (Figure 7) for high-order adaptive optics correction.
Table 9: Guide Star Availability for Radio-Resolved Seyfert Galaxies.
|
|
ALTAIR NGS System |
ALTAIR LGS System |
||||
|
Y |
? |
N |
Y |
? |
N |
|
|
# |
22 |
17 |
30 |
32 |
15 |
22 |
|
% |
32% |
25% |
43% |
46% |
22% |
32% |
Figure 5: Display of gs_search.pl output for the prototype Seyfert 1 galaxy, NGC 4151. Note that there are no suitable OIWFS guide stars within the ALTAIR field (second large circle).
Figure 6: Display of gs_search.pl output for the prototype Seyfert 2 galaxy, NGC 1068. Note that the WFPC2 image is saturated so it is not possible to tell if the nucleus is unresolved (although we expect that it is) and that the only OIWFS guide star (marked by a small circle at lower left) is at the extreme edge of the ALTAIR field (second large circle).
Figure 7: Display of gs_search.pl output for the “ionization cone” Seyfert galaxy, Mrk 573. It is unclear whether ALTAIR will be able to use the resolved nucleus for high-order adaptive optics correction.
We can now use our sample of 22 radio-resolved Seyfert galaxies with suitable ALTAIR guide stars to ask whether these objects have detectable near-infrared spectra. We aim to define the morphology and kinematics of the [Fe II] 1.257 μm or [Fe II] 1.644 μm emission which are shock diagnostics in these objects. Seven radio-extended Seyfert galaxies with ALTAIR natural guide stars had their near-infrared spectra measured by Veilleux, Goodrich, & Hill (1997). Estimates of the average emission line surface brightnesses for these galaxies were presented at the CoDR and are repeated in Table 10. The [Fe II] 1.257 μm line is typically brighter than 2×10-22 W cm-2 arcsec-2 and will be seen against a J band continuum of ~ 14.3 mag arcsec-2. The performance estimates in §2.2 above suggest that a signal-to-noise ratio of 10 per spectral pixel will be achieved in ~ 8 hr of on-source integration under these conditions on a 500 km s-1 wide line.
Clearly, the inner Narrow Line Region will have to be significantly clumped (either spatially or in velocity) to be easily detected at full spectral resolution with NIFS.
Table 10: Average emission line surface brightnesses (from Veilleux, Goodrich, & Hill 1997).
|
Object |
Aperture (″) |
J Continuum (mag arcsec-2) |
K Continuum (mag arcsec-2) |
Pb 1.282 μm (W cm‑2 arcsec‑2) |
[Fe II] 1.257 μm (W cm‑2 arcsec‑2) |
H2
1-0 S(1) 2.122 μm (W cm‑2 arcsec‑2) |
H I Brg 2.166 μm (W cm‑2 arcsec‑2) |
|
|
|
|
|
|
|
|
|
|
Mrk 176 |
3×3 |
14.3 |
14.6 |
4.2×10-23 |
1.8×10-22 |
2.4×10-23 |
6.2×10-24 |
|
Mrk 477 |
3×3 |
15.5 |
14.5 |
1.2×10-21 |
8.6×10-22 |
1.0×10-22 |
1.1×10-22 |
|
Mrk 573 |
3×3(J) 1.5×1.5(K) |
14.3 |
13.0 |
2.9×10-22 |
2.7×10-22 |
5.6×10-23 |
1.2×10-22 |
|
Mrk 1066 |
3×3(J) 1.5×1.5(K) |
13.8 |
12.4 |
1.3×10-21 |
1.3×10-21 |
6.0×10-22 |
6.0×10-22 |
|
NGC 1068 |
1.5×1.5 |
… |
8.2 |
… |
… |
1.3×10-21 |
5.6×10-21 |
|
NGC 5728 |
3×3 |
15.0 |
13.9 |
2.4×10-22 |
3.3×10-22 |
7.8×10-23 |
2.5×10-23 |
|
NGC 7212 |
1.5×1.5 |
… |
14.3 |
… |
… |
8.9×10-23 |
1.2×10-22 |
In
summary, 22 radio-extended Seyfert galaxies are expected to be measurable with
the ALTAIR natural guide star system. Only the brighter ones of these are expected
to be detectable with NIFS in less than one night per object unless the inner
Narrow Line Region is significantly clumped.
1.4.6 Extrasolar Planets
NIFS may be used to search for planets around nearby stars. This is a long shot! All known extrasolar planet detected by Doppler techniques are too close to the star to be resolved. These systems may contain other gas giant planets at larger radii and it may be possible to resolve these planets, but current expectations are that they would be too faint to detect with NIFS. It is more realistic to consider measuring spectra of very low mass stellar and substellar binary companions. Nevertheless, we use the list of stars known to possess planets at http://exoplanets.org as a source list (Appendix B, §11.4) for assessing guide star availability for this class of observation (Table 11). The large fraction of objects accessible with the ALTAIR natural guide star system is obviously due to the presence of a bright host star at the field center.
Table 11: Guide Star Availability for Stars With Extrasolar Planets.
|
|
ALTAIR NGS System |
ALTAIR LGS System |
||||
|
Y |
? |
|||||