2023ApJ...956L..15B

Star-forming galaxies are believed to replenish their atomic gas reservoir, which is consumed in star formation, through accretion of gas from their circumgalactic mediums (CGMs). However, there are few observational constraints today on the gas accretion rate in external galaxies. Here, we use our recent measurement of the scaling relation between the atomic hydrogen (H I) mass M_H I_ and the stellar mass M_* in star-forming galaxies at z ≈ 0.35, with the relations between the star formation rate (SFR) and M_*, and the molecular gas mass M_Mol and M_*, and the assumption that star-forming galaxies evolve along the main sequence, to determine the evolution of the neutral gas reservoir and the average net gas accretion rate onto the disks of star-forming galaxies over the past 4 Gyr. For galaxies with M_* ≳ 10⁹M_⊙ today, we find that both M_* and M_H I_ in the disk have increased, while M_Mol has decreased, since z ≈ 0.35. The average gas accretion rate onto the disk over the past 4 Gyr is similar to the average SFR over this period, implying that main-sequence galaxies have maintained a stable H I reservoir, despite the consumption of gas in star formation. We obtain an average net gas accretion rate (over the past 4 Gyr) of ≈6 M_⊙ yr^–1 for galaxies with the stellar mass of the Milky Way. At low redshifts, z ≲ 0.4, the reason for the decline in the cosmic SFR density thus appears to be the inefficiency in the conversion of atomic gas to molecular gas, rather than insufficient gas accretion from the CGM.