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Star-forming galaxies are believed to replenish their atomic gas reservoir,which is consumed in star-formation, through accretion of gas from theircircumgalactic mediums (CGMs). However, there are few observational constraintstoday on the gas accretion rate in external galaxies. Here, we use our recentmeasurement of the scaling relation between the atomic hydrogen (HI) mass$M_{HI}$ and the stellar mass $M_*$ in star-forming galaxies at $z \approx0.35$, with the relations between the star-formation rate (SFR) and $M_*$, andthe molecular gas mass $M_{Mol}$ and $M_*$, and the assumption thatstar-forming galaxies evolve along the main sequence, to determine theevolution of the neutral gas reservoir and the average net gas accretion rateonto the disks of star-forming galaxies over the past 4 Gyr. For galaxies with$M_* \gtrsim 10^9 M_{\odot}$ today, we find that both $M_*$ and $M_{HI}$ in thedisk have increased, while $M_{Mol}$ has decreased, since $z \sim 0.35$. Theaverage gas accretion rate onto the disk over the past 4 Gyr is similar to theaverage SFR over this period, implying that main-sequence galaxies havemaintained a stable HI reservoir, despite the consumption of gas instar-formation. We obtain an average net gas accretion rate (over the past 4Gyr) of $\approx 6 M_{\odot} yr^{-1}$ for galaxies with the stellar mass of theMilky Way. At low redshifts, $z \lesssim 0.4$, the reason for the decline inthe cosmic SFR density thus appears to be the inefficiency in the conversion ofatomic gas to molecular gas, rather than insufficient gas accretion from theCGM.

The Gas Accretion Rate of Star-forming Galaxies over the last 4 Gyr