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

Abstract

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](1-0) (and higher-$J$ CO lines) from the ALMA Spectroscopic Survey of the HUDF (ASPECS), we determine $3\ddot{I}ƒ$ 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 $3\ddot{I}ƒ$ upper limits of 9 $\hat{I}¼$Jy and $1.2$ $\hat{I}¼$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 $\hat{I}{\pm }_{\mathrm{CO}}\ge 10$ and $\hat{I}{´}_{\mathrm{GDR}}\ge 1200$, consistent with the sub-solar metallicity estimated for these galaxies ($12+\log (O/H)\approx 7.8\pm 0.2$). 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].

26 pages, 12 figures, 3 tables. Accepted for publication in ApJ