History of the gas fuelling star formation in eagle galaxies

TL;DR

This study uses the EAGLE simulation to explore how gas accretion influences star formation and chemical inhomogeneities in galaxies across cosmic time, aligning predictions with observations.

Contribution

It provides a detailed analysis of gas accretion sources and their impact on star formation and metallicity in galaxies, including the role of dark galaxy mergers.

Findings

Low metallicity regions correlate with enhanced star formation in low-mass and outer galaxy regions.

External gas contribution increases at higher redshifts, influencing star formation.

Dark galaxy mergers significantly contribute to stellar mass in low-mass galaxies even at present day.

Abstract

Theory predicts that cosmological gas accretion plays a fundamental role fuelling star formation in galaxies. However, a detailed description of the accretion process to be used when interpreting observations is still lacking. Using the state-of-the-art cosmological hydrodynamical simulation eagle, we work out the chemical inhomogeneities arising in the disk of galaxies due to the randomness of the accretion process. In low-mass systems and outskirts of massive galaxies, low metallicity regions are associated with enhanced star-formation, a trend that reverses in the centers of massive galaxies. These predictions agree with the relation between surface density of star formation rate and metallicity observed in the local spiral galaxies from the MaNGA survey. Then, we analyse the origin of the gas that produces stars at two key epochs, z simeq 0 and z simeq 2. The main contribution comes from gas already in the galaxy about 1 Gyr before stars are formed, with a share from external gas that is larger at high redshift. The accreted gas may come from major and minor mergers, but also as gravitationally unbound gas and from mergers with dark galaxies (i.e., haloes where more than 95 % of the baryon mass is in gas). We give the relative contribution of these sources of gas as a function of stellar mass (8 < log Mstar < 11). Even at z = 0, some low-mass galaxies form a significant fraction of their total stellar mass during the last Gyr from mergers with dark galaxies.