# The Molecular Gas Reservoirs of $z\sim 2$ Galaxies: A comparison of CO(1-0) and dust-based molecular gas masses

## TL;DR

This study validates the use of single-band long-wavelength dust emission as a reliable method to estimate molecular gas masses in high-redshift star-forming galaxies, showing agreement with CO(1-0) measurements within a factor of two.

## Contribution

It demonstrates that dust continuum emission at long wavelengths can accurately estimate molecular gas masses at z~2, supporting its use as a practical alternative to CO observations.

## Key findings

- Dust-based gas masses agree with CO-based masses within a factor of two.
- The uncertainty mainly arises from assumptions about dust properties.
- Single-band dust observations are reliable for z~2 galaxies.

## Abstract

We test the use of long-wavelength dust continuum emission as a molecular gas tracer at high redshift, via a unique sample of 12, z~2 galaxies with observations of both the dust continuum and CO(1-0) line emission (obtained with the Atacama Large Millimeter Array and Karl G. Jansky Very Large Array, respectively). Our work is motivated by recent, high redshift studies that measure molecular gas masses (\ensuremath{\rm{M}_{\rm{mol}}}) via a calibration of the rest-frame $850\mu$m luminosity ($L_\mathrm{850\mu m,rest}$) against the CO(1-0)-derived \ensuremath{\rm{M}_{\rm{mol}}}\ of star-forming galaxies. We hereby test whether this method is valid for the types of high-redshift, star-forming galaxies to which it has been applied. We recover a clear correlation between the rest-frame $850\mu$m luminosity, inferred from the single-band, long-wavelength flux, and the CO(1-0) line luminosity, consistent with the samples used to perform the $850\mu$m calibration. The molecular gas masses, derived from $L_\mathrm{850\mu m,rest}$, agree to within a factor of two with those derived from CO(1-0). We show that this factor of two uncertainty can arise from the values of the dust emissivity index and temperature that need to be assumed in order to extrapolate from the observed frequency to the rest-frame at 850$\mathrm{\mu m}$. The extrapolation to 850$\mathrm{\mu m}$ therefore has a smaller effect on the accuracy of \Mmol\ derived via single-band dust-continuum observations than the assumed CO(1-0)-to-\ensuremath{\rm{M}_{\rm{mol}}}\ conversion factor. We therefore conclude that single-band observations of long-wavelength dust emission can be used to reliably constrain the molecular gas masses of massive, star-forming galaxies at $z\gtrsim2$.