# High gas fraction in a CO-selected main-sequence galaxy at $z > 3$

## TL;DR

This study reports the detection of a highly gas-rich main-sequence galaxy at z > 3, revealing a high molecular gas fraction and supporting the idea that gas content increases with redshift, using CO line observations.

## Contribution

First measurement of molecular gas fraction in a z > 3 main-sequence galaxy using CO(5-4) observations, confirming high gas fractions at early cosmic times.

## Key findings

- High molecular gas fraction (~0.9) at z > 3.
- Gas excitation similar to z ~ 1.5 MS galaxies.
- Deep CO searches effectively identify high-z gas-rich galaxies.

## Abstract

We report NOrthern Extended Millimetre Array (NOEMA) observations of warm molecular gas traced by CO($5-4$) in a $z \sim 3.2$ gas-rich main-sequence galaxy (MS), initially serendipitously detected in CO($3-2$) emission in `blind' deep NOEMA observations. Our target shows a gas excitation consistent with that seen in $z \sim 1.5$ MS galaxies ($L'_{\rm CO( 5 - 4)}/L'_{\rm CO (3 - 2)} = 0.41 \pm 0.14$), albeit toward the low end, as well as a similar star formation efficiency based on the CO($3-2$) line luminosity and the $L_{\rm IR}$. However, it shows a high molecular gas fraction ($f_{\rm gas} = 0.9\pm 0.2$) as compared to $z\sim 1.5$ MS galaxies ($f_{\rm gas} \sim 0.4$), consistent with a cosmologically increasing gas fraction beyond $z\gtrsim3$ and our current understanding of scaling relations between $z$, $f_{\rm gas}$, the stellar mass $M_*$, and the specific star formation rate sSFR. Our results are consistent with recent findings by the COLDz and ASPECS molecular line scan surveys and suggest that deep searches for CO emission are a powerful means to identify gas-rich, star-forming galaxies at high redshift.