Collaborative Research: Spectral diagnostics of heavy elements at high redshift

interstellar space. Understanding the enrichment history of galaxies is therefore crucial to understanding star formation and feedback processes central to galaxy evolution. Absorption lines of damped Lyman-alpha (DLA) and sub-DLA absorbers in the spectra of background quasars or gamma-ray burst (GRB) afterglows provide the most sensitive tools to measure the heavy element content of distant galaxies. High-resolution spectroscopy with 8-m class telescopes points to some interesting puzzles. Quasar DLAs show smaller than expected metallicities, and show relatively little evolution over ~70% of the age of the universe. By contrast, a significant fraction of sub-DLAs are highly enriched: some sub-DLAs were supersolar in metallicity as far back as >10 Gyr ago! Furthermore, GRB DLAs may be more metal-rich than quasar DLAs. Proposed 20-30 m class telescopes will soon allow us to reach quasars/GRBs at much higher redshifts and obtain spectra of numerous background star-forming galaxies, enabling vast statistical studies. This proposal addresses gaps in the existing atomic database, now the greatest weakness in deriving abundances from spectra. Both oscillator strengths and recombination coefficients are highly uncertain or unknown for most elements beyond Mg. These elements are crucial for the study of high-redshift galaxies as their ground-accessible, unsaturated lines give accurate abundances. We will improve those atomic data needed for high-redshift galaxies and apply spectral simulations to observational datasets. We will do the following: 1) calculate more accurate oscillator strengths for transitions of all relevant ions of Al to Zn using state of the art quantal codes, 2) calculate dielectronic recombination coefficients for all relevant ions of key elements, e.g., Al, Fe, Zn, 3) incorporate these data into our widely used photoionization code Cloudy, 4) apply the new Cloudy models to existing/new spectral data, including our Magellan/VLT Echelle spectra of > 60 DLAs/sub-DLAs, and data for ~200 other absorbers toward quasars/GRBs obtained from VLT/Keck archives or published literature, 5) the last and most crucial step is to provide feedback to the atomic theory from the observational and modeling results.