Neutral hydrogen (HI) gas content of galaxies at $z \approx 0.32$

TL;DR

This study uses spectral stacking of GMRT observations to measure the average neutral hydrogen content of galaxies at z ≈ 0.32, finding little evolution in cosmic HI density over the last 4 billion years.

Contribution

First application of HI spectral stacking to measure average HI content of galaxies at z ≈ 0.32, providing new insights into cosmic HI density evolution.

Findings

95% of neutral gas in blue, star-forming galaxies

Lower mass galaxies are more gas-rich

HI density at z ≈ 0.32 matches previous low-redshift results

Abstract

We use observations made with the Giant Metrewave Radio Telescope (GMRT) to probe the neutral hydrogen (HI) gas content of field galaxies in the VIMOS VLT Deep Survey (VVDS) 14h field at $z \approx 0.32$. Because the HI emission from individual galaxies is too faint to detect at this redshift, we use an HI spectral stacking technique using the known optical positions and redshifts of the 165 galaxies in our sample to co-add their HI spectra and thus obtain the average HI mass of the galaxies. Stacked HI measurements of 165 galaxies show that 95 per cent of the neutral gas is found in blue, star-forming galaxies. Among these galaxies, those having lower stellar mass are more gas-rich than more massive ones. We apply a volume correction to our HI measurement to evaluate the HI gas density at $z \approx 0.32$ as $\Omega_{HI}=(0.50\pm0.18) \times 10^{-3}$ in units of the cosmic critical density. This value is in good agreement with previous results at z < 0.4, suggesting no evolution in the neutral hydrogen gas density over the last $\sim 4$ Gyr. However the $z \approx 0.32$ gas density is lower than that at $z \sim 5$ by at least a factor of two.