Evolution of Cosmic Molecular Gas Mass Density From z ~ 0 to z = 1 -1.5

TL;DR

This study estimates the evolution of cosmic molecular gas mass density from the local universe to redshifts 1-1.5, showing a tenfold increase at higher redshifts, which correlates with increased star formation rates.

Contribution

It combines stellar mass functions and molecular gas fractions from recent observations to constrain cosmic molecular gas density evolution between z~0 and z=1-1.5.

Findings

Cosmic molecular gas density at z=1-1.5 is about ten times higher than in the local universe.

The cosmic star formation rate density at z~1-2 is about ten times larger than locally.

The results support the link between molecular gas availability and star formation activity at different epochs.

Abstract

We try to constrain the cosmic molecular gas mass density at $z =1-1.5$ and that in the local universe by combining stellar mass functions of star-forming galaxies and their average molecular gas mass fractions against the stellar mass. The average molecular gas mass fractions are taken from recent CO observations of star-forming galaxies at the redshifts. The cosmic molecular gas mass density is obtained to be $\rho_{\rm H_2} = (6.8-8.8)~\times~10^7~M_\odot~{\rm Mpc}^{-3}$ at $z=1-1.5$ and $6.7 \times 10^6~M_\odot~{\rm Mpc}^{-3}$ at $z \sim 0$ by integrating down to $0.03~M^\ast$. Although the values have various uncertainties, the cosmic molecular gas mass density at $z =1-1.5$ is about ten times larger than that in the local universe. The cosmic star formation rate density at $z \sim 1-2$ is also about ten times larger than that in the local universe. Our result suggests that the large cosmic molecular gas mass density at $z=1-1.5$ accounts for the large cosmic star formation rate density at $z \sim 1 -2$.