Measuring the average molecular gas content of star-forming galaxies at $z=3-4$

We study the molecular gas content of 24 star-forming galaxies at $z=3-4$, with a median stellar mass of ${10}^{9.1}$ M${}_{\odot }$, from the MUSE Hubble Ultra Deep Field (HUDF) Survey. Selected by their Lyman-alpha-emission and H-band magnitude, the galaxies show an average EW $\approx 20$ angstrom, below the typical selection threshold for Lyman Alpha Emitters (EW $>25$ angstrom), and a rest-frame UV spectrum similar to Lyman Break Galaxies. We use rest-frame optical spectroscopy from KMOS and MOSFIRE, and the UV features observed with MUSE, to determine the systemic redshifts, which are offset from Lyman alpha by 346 km s${}^{-1}$, with a 100 to 600 km s${}^{-1}$ range. Stacking CO(4-3) and CI (and higher-$J$ CO lines) from the ALMA Spectroscopic Survey of the HUDF (ASPECS), we determine $3\sigma$ upper limits on the line luminosities of $4.0\times {10}^8$ K km s${}^{-1}$pc${}^2$ and $5.6\times {10}^8$ K km s${}^{-1}$pc${}^2$, respectively (for a 300 km s${}^{-1}$ linewidth). Stacking the 1.2 mm and 3 mm dust continuum flux densities, we find a $3\sigma$ upper limits of 9 $\mu$Jy and $1.2$ $\mu$Jy, respectively. The inferred gas fractions, under the assumption of a ‘Galactic’ CO-to-H${}_2$ conversion factor and gas-to-dust ratio, are in tension with previously determined scaling relations. This implies a substantially higher ${\alpha }_{\mathrm{C}\mathrm{O}}\ge 10$ and $\delta_{\rm GDR} \ge 1200$, consistent with the sub-solar metallicity estimated for these galaxies ($12+\log (O\mathrm{/}H)\approx 7.8\pm 0.2$). The low metallicity of $z \ge 3$ star-forming galaxies may thus make it very challenging to unveil their cold gas through CO or dust emission, warranting further exploration of alternative tracers, such as [CII].