Abstract
ABSTRACT We investigate the environmental dependence of the stellar populations of galaxies in Sloan Digital Sky Survey Data Release 7 (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 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.
Highlights
The environment in which galaxies form has long been thought to play an important role in shaping their evolution, as observations of galaxy clusters in the local Universe had revealed that early-type galaxies are preferentially found in dense environments, while late-type galaxies dominate the galaxy population in low density environments (Dressler 1980)
We obtain the following results: (i) Similar to earlier works, such as Pasquali et al (2010), we find that satellites are both more metal-rich (< 0.1 dex) and older (< 2 Gyr) than centrals of the same stellar mass, which has often been interpreted as an indication that the environment plays an important role in shaping both the chemical evolution and star formation histories of galaxies
After separating star-forming, green valley and passive galaxies, we find that the true environmental dependence is much weaker, with star-forming, green valley and passive satellites being only marginally more metal-rich (< 0.03 dex) and older (< 0.5 Gyr) than star-forming, green valley and passive centrals of the same stellar mass
Summary
The environment in which galaxies form has long been thought to play an important role in shaping their evolution, as observations of galaxy clusters in the local Universe had revealed that early-type galaxies are preferentially found in dense environments, while late-type galaxies dominate the galaxy population in low density environments (Dressler 1980). Peng et al (2015) and Trussler et al (2020) studied the stellar metallicities of galaxies in the SDSS (z ∼ 0) and found that green valley and passive galaxies are significantly more metal-rich than star-forming galaxies of the same stellar mass, indicating that galaxies typically undergo significant chemical enrichment during the quenching phase. Using gas regulator models (Peng & Maiolino 2014b), they find that the large stellar metallicity difference between star-forming and passive galaxies implies that for galaxies at all masses, quenching must have involved an extended phase of starvation.
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