Dense Gas History of the Universe: from ASPECS to the ngVLA

TL;DR

This paper reviews the evolution of cosmic molecular gas density over time using CO observations, highlighting its role in star formation history and discussing future capabilities of the ngVLA for detailed studies.

Contribution

It synthesizes current knowledge of molecular gas evolution up to high redshifts and discusses the potential of the next-generation VLA to advance this field.

Findings

Molecular gas density peaks around z~2 and declines at higher redshifts.

Deep CO observations trace gas back to z~5, near the Big Bang.

Future ngVLA observations will enable detailed imaging of early galaxy gas dynamics.

Abstract

We review the evolution of the cosmic average molecular gas density to large look-back times, using observations of rotational transitions of CO. Molecular gas is the fuel for star formation in galaxies. Deep searches for CO emission from distant galaxies have delineated the density of molecular gas back to $z \sim 5$, or within 1~Gyr of the Big Bang. The results show a rise and fall in the gas density that parallels, and likely drives, the rise and fall of the cosmic star formation rate density. We present the potential for the next generation Very Large Array to image the distribution and dynamics of the molecular gas in early galaxies, and to make a precise measurement of the dense gas history of the Universe.