ALMA shows that gas reservoirs of star-forming disks over the past 3 billion years are not predominantly molecular

TL;DR

This study combines ALMA and Arecibo data to show that atomic hydrogen dominates the cold gas in massive star-forming galaxies over the past 3 billion years, challenging the idea that molecular gas is always predominant.

Contribution

It provides the first direct measurement of the molecular-to-atomic hydrogen ratio in massive galaxies at intermediate redshift, revealing atomic hydrogen's dominance over the past three billion years.

Findings

Atomic hydrogen dominates the cold gas mass in these galaxies.

Gas velocity dispersions are low despite high gas fractions.

The gas balance is similar to local main-sequence disks.

Abstract

Cold hydrogen gas is the raw fuel for star formation in galaxies, and its partition into atomic and molecular phases is a key quantity for galaxy evolution. In this Letter, we combine Atacama Large Millimeter/submillimeter Array and Arecibo single-dish observations to estimate the molecular-to-atomic hydrogen mass ratio for massive star-forming galaxies at $z\sim$ 0.2 extracted from the HIGHz survey, i.e., some of the most massive gas-rich systems currently known. We show that the balance between atomic and molecular hydrogen in these galaxies is similar to that of local main-sequence disks, implying that atomic hydrogen has been dominating the cold gas mass budget of star-forming galaxies for at least the past three billion years. In addition, despite harboring gas reservoirs that are more typical of objects at the cosmic noon, HIGHz galaxies host regular rotating disks with low gas velocity dispersions suggesting that high total gas fractions do not necessarily drive high turbulence in the interstellar medium.