Effects of metal enrichment and metal cooling in galaxy growth and cosmic star formation history

TL;DR

This study uses cosmological hydrodynamic simulations to show that metal cooling significantly boosts galaxy growth and star formation, especially at lower redshifts, by enhancing star formation efficiency and gas accretion.

Contribution

It demonstrates the dual mechanisms by which metal cooling influences galaxy evolution, highlighting effects on star formation and gas accretion over cosmic time.

Findings

Star formation rate increases by 50% at z=1 with metal cooling.

Gas mass fraction in galaxies decreases when metal cooling is included.

Number of massive galaxies rises at z=1 due to metal cooling.

Abstract

We present the results of a numerical study on the effects of metal enrichment and metal cooling on galaxy formation and cosmic star formation (SF) history using cosmological hydrodynamic simulations. We find following differences in the simulation with metal cooling when compared to the run without it: (1) the cosmic star formation rate (SFR) is enhanced by about 50 & 20% at z=1 & 3, respectively; (2) the gas mass fraction in galaxies is lower; (3) the total baryonic mass function (gas + star) at z=3 does not differ significantly, but shows an increase in the number of relatively massive galaxies at z=1; (4) the baryonic mass fraction of intergalactic medium (IGM) is reduced at z<3 due to more efficient cooling and gas accretion onto galaxies. Our results suggest that the metal cooling enhances the galaxy growth by two different mechanisms: (1) increase of SF efficiency in the local interstellar medium (ISM), and (2) increase of IGM accretion onto galaxies. The former process is effective throughout most of the cosmic history, while the latter is effective only at z<3 when the IGM is sufficiently enriched by metals owing to feedback.