Research

Galactic Chemical Evolution

Population synthesis modeling of the spectra of elliptical galaxies provides insights into the ages and chemical abundances of their stellar populations. At z ~ 0, ellipticals of higher stellar mass or higher velocity dispersion exhibit systematically older ages, higher metallicities, and larger enhancements of α-elements relative to iron. Recent studies of early-type galaxies at z ~ 0.7 show near-solar metallicity ([Fe/H] ~ 0) and strong α-enhancement ([Mg/Fe] ~ 0.2−0.3), but the trends with velocity dispersion are weaker than those at z ~ 0. The elevated [Mg/Fe] of high-redshift ellipticals and the most massive low-redshift ellipticals are interpreted as a sign of rapid star formation, with many stars forming before Type Ia supernovae (SNIa) have had time to make large contributions to the abundance of iron-peak elements. 

Under the supervision of David Weinberg, I am using parameterized galactic chemical evolution (GCE) models to predict the light-weighted age, [Mg/H], and [Mg/Fe] of observed elliptical populations.  We focus on Mg because it is a well-measured element that is expected to come entirely from core-collapse supernovae (CCSN). Similarly, Fe is a well-measured element with contributions from both CCSN and SNIa. We aim to address three broad questions:

1.  Given these yields motivated by independent observations, can GCE models with reasonable parameter values reproduce the ages, metallicities, and α-enhancements of observed ellipticals at z ~ 0 and z ~ 0.7?

2.  If so, what do the observed properties imply about the star formation histories, star formation efficiencies, and outflows of these galaxies?

3.  If not, what modifications might be required to the stellar yields or other model assumptions, and what do they imply about the distinctive features of elliptical galaxies?

While the high metallicities and α-enhancements of ellipticals are usually attributed to "rapid star formation," this phrase is imprecise.  In our models, there are four distinct timescales in the star formation history: the time at which star formation begins, the duration of an initial linear rise, the e-folding timescale of a subsequent decline, and the time at which star formation ceases entirely.  The star formation efficiency,  defined as the ratio of the star formation rate to the gas mass, defines another important time scale.  One additional goal of our modeling is to tease apart the role of these distinct timescales in governing ages and chemical enrichment.