The weak imprint of environment on the stellar populations of galaxies

TL;DR

This study finds that the environmental impact on galaxy stellar populations is minimal when accounting for galaxy type, with weak effects observed mainly in passive and green valley satellites, emphasizing the importance of galaxy classification in such analyses.

Contribution

The paper demonstrates that environmental effects on galaxy stellar populations are weaker than previously thought when separating galaxies by star formation activity, highlighting the importance of galaxy classification.

Findings

Weak environmental dependence of stellar metallicity and age after galaxy classification.

No environmental effects observed for star-forming galaxies.

Weak metallicity increase in passive and green valley satellites with environment.

Abstract

We investigate the environmental dependence of the stellar populations of galaxies in SDSS DR7. Echoing earlier works, we find that satellites are both more metal-rich (<0.1 dex) and older (<2 Gyr) than centrals of the same stellar mass. However, after separating star-forming, green valley and passive galaxies, we find that the true environmental dependence of both stellar metallicity (<0.03 dex) and age (<0.5 Gyr) is in fact much weaker. We show that the strong environmental effects found when galaxies are not differentiated result from a combination of selection effects brought about by the environmental dependence of the quenched fraction of galaxies, and thus we strongly advocate for the separation of star-forming, green valley and passive galaxies when the environmental dependence of galaxy properties are investigated. We also study further environmental trends separately for both central and satellite galaxies. We find that star-forming galaxies show no environmental effects, neither for centrals nor for satellites. In contrast, the stellar metallicities of passive and green valley satellites increase weakly (< 0.05 dex and < 0.08 dex, respectively) with increasing halo mass, increasing local overdensity and decreasing projected distance from their central; this effect is interpreted in terms of moderate environmental starvation (`strangulation') contributing to the quenching of satellite galaxies. Finally, we find a unique feature in the stellar mass--stellar metallicity relation for passive centrals, where galaxies in more massive haloes have larger stellar mass (~0.1 dex) at constant stellar metallicity; this effect is interpreted in terms of dry merging of passive central galaxies and/or progenitor bias.