Atomic Gas Dominates the Baryonic Mass of Star-forming Galaxies at z ≈ 1.3

Abstract We present a comparison between the average atomic gas mass, 〈M Atom〉 (including hydrogen and helium), the average molecular gas mass, 〈M Mol〉, and the average stellar mass, 〈M ⋆〉, of a sample of star-forming galaxies at z ≈ 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 21 cm signals in the GMRT-CATz1 survey. We find that the baryonic composition of star-forming galaxies at z ≳ 1 is dramatically different from that at z ≈ 0. For star-forming galaxies with 〈M ⋆〉 ≈ 1010 M ⊙, the contribution of stars to the total baryonic mass, M Baryon, is ≈61% at z ≈ 0, but only ≈16% at z ≈ 1.3, while molecular gas constitutes ≈6% of the baryonic mass at z ≈ 0, and ≈14% at z ≈ 1.3. Remarkably, we find that atomic gas makes up ≈70% of M Baryon in star-forming galaxies at z ≈ 1.3. We find that the ratio 〈M Atom〉/〈M ⋆〉 is higher at both z ≈ 1.3 and at z ≈ 1.0 than in the local universe, with 〈M Atom〉/〈M ⋆〉 ≈ 1.4 at z ≈ 1.0 and ≈ 4.4 at z ≈ 1.3, compared to its value of ≈0.5 today. Further, we find that the ratio 〈M Atom〉/〈M Mol〉 in star-forming galaxies with 〈M ⋆〉 ≈ 1010 M ⊙ is ≈2.3 at z ≈ 1.0 and ≈5.0 at z ≈ 1.3. Overall, we find that atomic gas is the dominant component of the baryonic mass of star-forming galaxies at z ≈ 1.3, during the epoch of peak star formation activity in the universe.