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

TL;DR

This study estimates the gas accretion rate of star-forming galaxies over the last 4 billion years, showing that galaxies maintain their atomic gas reservoirs mainly through accretion, with implications for understanding star formation decline.

Contribution

It provides observational constraints on gas accretion rates in external galaxies and links these rates to the evolution of gas reservoirs and star formation activity.

Findings

Average net gas accretion rate is about 6 solar masses per year for Milky Way-like galaxies.

Galaxies have increased stellar and atomic gas mass while molecular gas decreased since z~0.35.

Gas accretion sustains star formation despite the decline in molecular gas conversion efficiency.

Abstract

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 (HI) mass $M_{HI}$ and the stellar mass $M_*$ in star-forming galaxies at $z \approx 0.35$, 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 $M_* \gtrsim 10^9 M_{\odot}$ today, we find that both $M_*$ and $M_{HI}$ in the disk have increased, while $M_{Mol}$ has decreased, since $z \sim 0.35$. 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 HI reservoir, despite the consumption of gas in star-formation. We obtain an average net gas accretion rate (over the past 4 Gyr) of $\approx 6 M_{\odot} yr^{-1}$ for galaxies with the stellar mass of the Milky Way. At low redshifts, $z \lesssim 0.4$, 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.