Decoding Galaxy Evolution with Gas-phase and Stellar Elemental Abundances

Elemental abundances of gas and stars are sensitive diagnostics of the main processes that drive galaxy evolution: gas inflow, star formation, enrichment, and gas outflow. The relation between galaxy stellar mass and gas-phase oxygen abundance, known as the mass-metallicity relation (MZR), is one of the strongest constraints on galaxy evolution models. However, the popular strong line methods of measuring oxygen abundance have large systematic uncertainties. We employ a more robust direct method to measure the metallicities of ~200,000 star-forming galaxies from the Sloan Digital Sky Survey stacked in bins of stellar mass and star formation rate (SFR) to significantly enhance the signal-to-noise ratio of the weak auroral lines required for the direct method. The direct method MZR has a steeper slope, a lower turnover mass, and a factor of 2-3 greater dependence on the SFR than strong line MZRs.;Gas-phase abundances reflect a galaxy's current abundances, but stellar abundances encode its complete enrichment history. We use a sample of 35 microlensed bulge dwarf and subgiant stars, whose brightness increased by a factor of 100-1000 due to lensing from an intervening star, to investigate the formation of the Galactic bulge. We apply principal component abundance analysis (PCAA)---a principal component decomposition of relative abundances [X/Fe]---to this sample to characterize its distribution in the 12-dimensional space defined by their elemental abundances. The first principal component PC1, which describes the abundance patterns of most stars in the sample, shows a strong contribution from alpha-elements, reflecting the relative contributions of core-collapse and Type Ia supernovae. The second principal component PC2 is characterized by a Na-Ni correlation, the likely product of metallicity-dependent core-collapse supernova (CCSN) yields. Applying PCAA to a sample of local disk dwarfs yields a nearly identical PC1, suggesting broadly similar alpha-enrichment histories. However, the disk PC2 is dominated by a Y-Ba correlation, likely indicating a contribution of s-process enrichment from long-lived asymptotic giant branch stars that is absent from the bulge PC2 because of rapid bulge formation.;Detailed interpretations of stellar abundance patterns require chemical evolution modeling, but these models are sensitive to the treatment of star formation efficiency (SFE), outflow, stellar yields, and mixing of stellar populations. The two main features in [alpha/Fe]--[Fe/H] are the [Fe/H] of the knee (where the high [alpha/Fe] plateau turns downwards towards solar ratios), and the equilibrium abundance to which the simulations quickly asymptote. A higher SFE increases the [Fe/H] of the knee but does not change the equilibrium abundance. Conversely, a higher outflow mass-loading parameter does not impact the [Fe/H] of the knee, but it decreases the equilibrium [Fe/H]. Adopting different CCSN yields makes a modest impact on the evolution of alpha-elements and Fe but produces a dramatically different evolution for elements with a strong metallicity dependence to their yields, like Na and Al. The most natural explanation for reproducing the bimodality in [alpha/Fe]--[Fe/H] involves mixing stellar populations born at different galactocentric radii with unique enrichment histories.