Star formation efficiency across large-scale galactic environments

Abstract

ABSTRACT Environmental effects on the formation and evolution of galaxies have been one of the leading questions in galaxy studies during the past few decades. In this work, we investigate the relationship between the star formation activity of galaxies and their environmental matter density using the cosmological hydrodynamic simulation Simba. The galactic star formation activity indicators that we explore include the star formation efficiency (SFE), specific star formation rate (sSFR), and molecular hydrogen mass fraction ($f^{*}_{\mathrm{H_2}}$), and the environment is considered as the large-scale environmental matter density, calculated based on the stellar mass of nearby galaxies on a 1 h−1 Mpc grid using the cloud in cell method. Our sample includes galaxies with $9\lt \log \frac{M_*}{{\rm M}_{\odot }}$ at 0 < z < 4, divided into three stellar mass bins to disentangle the effects of stellar mass and environment on the star formation activity of galaxies. For low- to intermediate-mass galaxies at low redshifts (z < 1.5), we find that the star formation efficiency of those in high-density regions are ∼0.3 dex lower than those in low-density regions. However, there is no significant environmental dependence of the star formation efficiency for massive galaxies over all our redshift range, and low- to intermediate-mass galaxies at high redshifts (z > 1.5). We present a scaling relation for the depletion time of cold molecular hydrogen (tdepl = 1/SFE) as a function of galaxy parameters including environmental density. Our findings provide a framework for quantifying the environmental effects on the star formation activities of galaxies as a function of stellar mass and redshift. The most significant environmental dependence is seen at later cosmic times (z < 1.5) and towards lower stellar masses ($9\lt \log \frac{M_*}{{\rm M}_{\odot }}\lt 10$). Future large galaxy surveys can use this framework to look for the environmental dependence of the star formation activity and examine our predictions.