Atomic Gas Dominates the Baryonic Mass of Star-forming Galaxies at $z\approx 1.3$

Abstract

We present a comparison between the average atomic gas mass, $⟨M_{Atom}⟩$ (including HI and He), the average molecular gas mass, $⟨M_{Mol}⟩$, and the average stellar mass, $⟨M_{\ast }⟩$, of a sample of star-forming galaxies at $z\approx 0.75-1.45$, to probe the baryonic composition of galaxies in and during the epoch of peak star-formation activity in the universe. The $⟨M_{Atom}⟩$ values of star-forming galaxies in two stellar-mass matched samples at $z=0.74-1.25$ and $z=1.25-1.45$, were derived by stacking their HI 21cm signals in the GMRT-CAT$z1$ survey. We find that the baryonic composition of star-forming galaxies at $z\gtrsim 1$ is dramatically different from that at $z\approx 0$. For star-forming galaxies with $⟨M_{\ast }⟩\approx 10^{10}{M}_{\odot }$, the contribution of stars to the total baryonic mass, $M_{Baryon}$, is $\approx 61\%$ at $z\approx 0$, but only $\approx 16\%$ at $z\approx 1.3$, while molecular gas constitutes $\approx 6\%$ of the baryonic mass at $z\approx 0$, and $\approx 14\%$ at $z\approx 1.3$. Remarkably, we find that atomic gas makes up $\approx 70\%$ of $M_{Baryon}$ in star-forming galaxies at $z\approx 1.3$. We find that the ratio $⟨M_{Atom}⟩/⟨M_{\ast }⟩$ is higher both at $z\approx 1.0$ and at $z\approx 1.3$ than in the local Universe, with $⟨M_{Atom}⟩/⟨M_{\ast }⟩\approx 1.4$ at $z\approx 1.0$, and $\approx 4.4$ at $z\approx 1.3$, compared to its value of $\approx 0.5$ today. Further, we find that the ratio $⟨M_{Atom}⟩/⟨M_{Mol}⟩$ in star-forming galaxies with $⟨M_{\ast }⟩\approx 10^{10}{M}_{\odot }$ is $\approx 2.3$ at $z\approx 1.0$ and $\approx 5.0$ at $z\approx 1.3$. Overall, we find that atomic gas is the dominant component of the baryonic mass of star-forming galaxies at $z\approx 1.3$, during the epoch of peak star-formation activity in the universe.

9 Pages, 4 Figures; Accepted for publication in ApJ Letters. Updated with minor changes to match version in press

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• The HI mass function of star-forming galaxies at $\mathbf{z}\sim 0.35$
• The [CII] 158 $\mu$m emission line as a gas mass tracer in high redshift quiescent galaxies