The link between the Star Formation History and [alpha/Fe]

TL;DR

This study empirically links [alpha/Fe] abundance ratios with galaxy star formation histories, revealing that higher [alpha/Fe] correlates with shorter formation timescales and older stellar populations across galaxy masses.

Contribution

It provides an empirical correlation between [alpha/Fe] and SFH using spectral indices and non-parametric spectral fitting, challenging previous simple models.

Findings

High [alpha/Fe] (>+0.2) galaxies have the shortest formation times (<2Gyr).

Old stellar populations (>10Gyr) are common even in low-mass ellipticals.

[alpha/Fe] strongly correlates with the time to assemble the stellar component.

Abstract

(Abridged) The abundance ratios between key elements such as iron and alpha-process elements carry a wealth of information on the star formation history (SFH) of galaxies. So far, simple chemical evolution models have linked [alpha/Fe] with the SFH timescale, correlating large abundance ratios with short-lived SFH. We provide an empirical correlation between [alpha/Fe] (measured from spectral indices) and the SFH (determined via a non-parametric spectral-fitting method). We offer an empirical version of the iconic outline of Thomas et al. (2005), relating star formation timescale with galaxy mass, although our results suggest, in contrast, a significant population of old (>10Gyr) stars even for the lowest mass ellipticals. In addition, the abundance ratio is found to be strongly correlated with the time to build up the stellar component, showing that the highest [alpha/Fe] (>+0.2) are attained by galaxies with the shortest half-mass formation time (<2Gyr), or equivalently, with the smallest (<40%) fraction of populations younger than 10Gyr. These observational results support the standard hypothesis that star formation incorporates the Fe-enriched interstellar medium into stars, lowering the high abundance ratio of the old populations.