Nature versus Nurture: Revisiting the Environmental Impact on Star Formation Activities of Galaxies

Abstract

We present a systematic study of the environmental impact on star formation activities of galaxies using a mass-complete sample of ∼170k galaxies at z < 4 from the latest COSMOS2020 catalog. At z < 1, we find that the mean star formation rate (SFR) of all galaxies decreases with increasing density of the environment. However, when we only consider star-forming galaxies, the mean SFR becomes independent of the environment at z < 1. At z > 2, we observe a clear positive correlation between the SFR and the density of the environment for all the galaxies. On the other hand, the stellar mass of the galaxies increases significantly with the environment at all redshifts except for star-forming galaxies at z < 1. The fraction of quiescent galaxies increases with increasing density of the environment at z < 2, and the morphology–density relation is confirmed to be present up to z ∼ 1. We also find that environmental quenching is negligible at z > 1, whereas mass quenching is the dominant quenching mechanism for massive galaxies at all redshifts. Based on these results, we argue that stellar mass-regulated physical processes might be the major driving force for star formation activities of galaxies. At low redshift (z < 1) massive galaxies are quenched primarily due to their high mass, resulting in a normal SFR–density relation. At high redshift (z > 2) most of the galaxies are star-forming ones tightly following the star-forming main sequence, and the difference in their stellar mass in different environments naturally leads to a reversal of the SFR–density relation.