The Chemical Evolution of the Universe
A Thesis proposal by Prof. Mike Dopita (RSAA),
Chiaki Kobayashi (RSAA) and Lisa Kewley (U. Hawaii)
To understand how galaxies in the early universe evolved into those that we see locally requires an understanding of the chemical and star formation history of galaxies over cosmic time. Large spectroscopic redshift surveys have sparked an enormous effort aimed at elucidating the star formation history of galaxies, but we still lack a solid understanding of the chemical history of galaxies. This has now become a major driver of astronomical research.
Cosmological simulations such as those by Kobayashi can now predict the evolution of chemical abundances in galaxies. These lack a solid observational ground because:
1. a solid local benchmark sample is lacking,
2. there are insufficient observations of galaxies at early epochs, and
3. the absolute calibration of the chemical abundance scale is currently discrepant by more than a factor of two.
The aim of the proposed research project is to gain an unprecedented understanding of the chemical history of galaxies by exploiting the capabilities of the new Wide Field Spectrograph (WiFeS) supported by the theory developed by the project supervisors.
Figure 1. The throughput and an engineering cutaway of the new WiFeS spectrograph at Siding Srring Observatory. WiFeS will operate on the 2.3m telescope, and will provide spectra of R=3000 and R=7000 in the visible region of the spectrum over a field of view of 25x38 arc sec. Commissioning is in November 2008.
The project consists of 3 major themes, all or some of which can be integrated into the final thesis:
Calibration of the Nebular Abundance Scale. Currently, significant unresolved discrepancies, (typically more than factor of two! ) exist between different abundance calibrations of the abundance scale of H II regions, on which all measurements of abundances in distant galaxies depends. To resolve these problems, we propose to obtain extremely high quality (S/N ~ 106; see figure 2) spectra of H II regions both in our galaxy and in local galaxies, and to derive abundances using both traditional and new, unconventional techniques. In addition, we will use our new HST WFC3 narrow-band images along with WiFeS spectroscopy of M83 to compare abundances of shock-excited interstellar gas in old supernova remnants and those of nearby H II regions.
Figure 2. A high S/N spectrum of NGC6302, emphasizing the very many faint recombination lines of heavy elements that can be discerned against the nebular continuum at the resolution offered by the WiFeS. The S/N ratio of the faint lines of this spectrum is limited by the noise in the nebular continuum. The dynamical range required to capture both the faint and the brightest lines is enormous, in excess of 105.A Local Benchmark Sample: Winds, missing Baryons & the chemical abundance “floor”. We will use WiFeS to measure the chemical abundances of the lowest-mass dwarf galaxies and isolated extragalactic HII regions are identified in our Survey of Ionization in Neutral Gas Galaxies (SINGG; Meurer et al. 2006). SINGG is an H-alpha survey of galaxies detected in atomic hydrogen with the Parkes HIPASS survey. SINGG includes many low surface brightness, low luminosity galaxies as well as HII regions that are so isolated that it is not clear if they belong to any galaxy. The spectroscopy will enable us to answer the following questions:
- What is the mass : chemical abundance relationship for low mass galaxies? This provides sensitive constraints on the mass fraction of the interstellar medium lost to inter-galactic space
by galaxy winds. We want to know whether matter lost from dwarf galaxies can account for
the “missing baryon” problem - the discrepancy between the baryon mass currently contained in
visible galaxies, and the baryon mass inferred from the Standard Model of cosmology.
- What is the total mass of Oxygen in each of the sample of dwarf galaxies? This is determined from the chemical abundance and the total HI mass of these galaxies. Since we know how many massive stars have formed, we can obtain tight constraints on the fraction of heavy elements that have been lost to the inter-galactic medium by galactic winds.
- Is there is a chemical abundance floor in the local Universe? Such a floor, predicted to be
about 1/100 solar, would result from pollution of inter-galactic gas by heavy elements ejected in
starburst-powered galactic winds, or by black hole jet-powered outflows from massive galaxies.
Figure 3. A Dwarf galaxy (lower left) and an isolated star formation region (upper right) from the SINGG survey. This image is a composite of H-alpha, R and UV from the Galex satellite. These will be typicl tagets for the WiFeS spectrograph.
Chemical Abundances over Cosmic Time We aim to compare local abundances with abundances measured in absorption lines of gamma-ray bursts and with independent emission-line abundances derived for their host galaxies. Long-Duration Gamma Ray Bursts (GRBs) are highly-beamed sources associated with super-massive supernova explosions. GRBs can be more luminous than entire galaxies, and can be detected through the observable Universe. GRBs produce an extremely bright optical continuum on which we detect absorption lines from the local interstellar medium, directly probing the chemical evolutionary state of the host galaxy. The optical afterglow fades extremely rapidly; after a day it is detectable only with the largest telescopes. High signal-to-noise spectra obtained on a rapid-response small telescope yield more reliable metallicities than from spectra obtained on 8m-class telescopes an hour or two later (e.g., Figure 4).
WiFeS is the perfect instrument to obtain prompt time-resolved spectroscopy of the GRB afterglow. In absorption spectroscopy, WiFeS can observe GRBs anywhere in the redshift range 1.8 < z < 5.5, and in emission lines, in the range 0 < z < 0.8. The Swift GRB satellite detects 150-200 GRBs per year. We xpect to get excellent spectra on about 10 of these per year.
Figure 4. A spectrum of GRB 06510B at a red-shift of z=4.92. The identification of the main absorbing species is shown. We derive an abundance of only 0.03 of solar in this object. WiFeS will obtain spectra of this quality on about 10 gamma-ray burst sources per year.