Unveiling the Physical Conditions in Star-Forming Galaxies at the Peak of Galaxy Assembly

Citation

Strom, Allison Leigh (2018) Unveiling the Physical Conditions in Star-Forming Galaxies at the Peak of Galaxy Assembly. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9N58JJN. https://resolver.caltech.edu/CaltechTHESIS:09082017-170618214

Abstract

Galaxies at the peak of cosmic star formation (z~2-3) exhibit significantly higher star formation rates and gas fractions at fixed stellar mass than nearby galaxies. These z~2-3 galaxies are also distinct in terms of their nebular spectra, reflecting important differences not only in the physical conditions of their interstellar medium (e.g., electron density and gas-phase metallicity), but also in the details of their massive stellar populations. Jointly observing galaxies' HII regions, at rest-UV and rest-optical wavelengths, and massive stars, at rest-UV wavelengths, is central to constructing a framework for understanding the differences between z~2-3 and z~0 star-forming galaxies and also vital for self-consistently explaining the trends observed in the high-z population.

This thesis presents the main results from the near-infrared (NIR) component of the Keck Baryonic Structure Survey (KBSS), a targeted spectroscopic survey of z~2-3 galaxies that uniquely combines observations in the rest-UV (1000-2000Å) and rest-optical (3500-7500Å) bandpasses. The NIR spectroscopic campaign conducted using Keck/MOSFIRE, described in Chapter 2, includes observations over 1200 high-z galaxies and represents one of the largest samples of high-quality rest-optical spectra of z~2-3 galaxies ever assembled. These measurements offer new insights regarding the physical conditions in galaxies forming during one of the most active periods in the universe's history.

Chapter 3 describes the rest-optical spectra of ~380 KBSS galaxies at z~2-2.7 and shows that the primary difference between HII regions in z~2.3 galaxies and those at z~0 is an enhancement in the degree of nebular excitation. KBSS galaxies are also 10 times more massive than z~0 galaxies with similar ionizing spectra and have higher gas-phase N and O abundances at fixed excitation. These results indicate the presence of harder ionizing radiation fields at fixed gas-phase enrichment relative to typical z~0 galaxies, consistent with Fe-poor stellar population models that include massive binaries.

Chapter 4 builds on this analysis to develop a new technique for determining the physical conditions in individual high-z galaxies -- independent of diagnostics tuned to local calibration samples. This method produces self-consistent measurements of the chemical enrichment and excitation conditions in ~150 galaxies at z~2-2.7.

Together, these results provide compelling evidence that the distinct chemical abundance patterns observed in z~2-3 star-forming galaxies result from systematic differences in their star formation histories relative to galaxies with similar stellar masses today. The thesis concludes by considering the importance of accounting for differences in galaxies' past star formation when interpreting spectroscopic observations and briefly discusses opportunities for extending the framework for analyzing high-z galaxies presented here to future studies of star-forming galaxies throughout cosmic time.

Thesis Files