The SAMI Galaxy Survey: Trends in [\alpha/Fe] as a Function of Morphology and Environment

TL;DR

This study measures alpha-element enhancement in over 2000 galaxies, revealing how environment and morphology influence star formation histories and chemical enrichment, especially in elliptical galaxies.

Contribution

It provides new index-based measurements of [$eta$/Fe] across a large galaxy sample, highlighting environmental and morphological effects on chemical evolution.

Findings

Elliptical galaxies in dense environments are more alpha-enhanced.

Alpha-enhancement correlates with velocity dispersion and environment.

Star formation in dense environments truncates earlier by about 1 Gyr.

Abstract

We present a new set of index-based measurements of [$\alpha$/Fe] for a sample of 2093 galaxies in the SAMI Galaxy Survey. Following earlier work, we fit a global relation between [$\alpha$/Fe] and the galaxy velocity dispersion $\sigma$ for red sequence galaxies, [$\alpha$/Fe]=(0.378$\pm$0.009)log($\sigma$/100)+(0.155$\pm$0.003). We observe a correlation between the residuals and the local environmental surface density, whereas no such relation exists for blue cloud galaxies. In the full sample, we find that elliptical galaxies in high-density environments are $\alpha$-enhanced by up to 0.057$\pm$0.014 dex at velocity dispersions $\sigma$<100 km/s, compared with those in low-density environments. This $\alpha$-enhancement is morphology-dependent, with the offset decreasing along the Hubble sequence towards spirals, which have an offset of 0.019$\pm$0.014 dex. At low velocity dispersion and controlling for morphology, we estimate that star formation in high-density environments is truncated $\sim1$ Gyr earlier than in low-density environments. For elliptical galaxies only, we find support for a parabolic relationship between [$\alpha$/Fe] and $\sigma$, with an environmental $\alpha$-enhancement of at least 0.03 dex. This suggests strong contributions from both environment and mass-based quenching mechanisms. However, there is no evidence for this behaviour in later morphological types.