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 109.1 M⊙, from the MUSE Hubble Ultra Deep Field (HUDF) Survey. Selected by their Lyα λ1216 emission and HF160W-band magnitude, the galaxies show an average $\langle {\mathrm{EW}}_{\mathrm{Ly}\alpha }^{0}\rangle \approx 20$ Å, below the typical selection threshold for Lyα emitters (${\mathrm{EW}}_{\mathrm{Ly}\alpha }^{0}\gt 25$ Å), 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 Lyα by 〈Δv(Lyα)〉 = 346 km s−1, with a 100 to 600 km s−1 range. Stacking 12CO J = 4 → 3 and [C i]3P1 → 3P0 (and higher-J CO lines) from the ALMA Spectroscopic Survey of the HUDF, we determine 3σ upper limits on the line luminosities of 4.0 × 108 K km s−1pc2 and 5.6 × 108 K km s−1pc2, respectively (for a 300 km s−1 line width). Stacking the 1.2 mm and 3 mm dust-continuum flux densities, we find a 3σ upper limits of 9 μJy and 1.2 μJy, respectively. The inferred gas fractions, under the assumption of a "Galactic" CO-to-H2 conversion factor and gas-to-dust ratio, are in tension with previously determined scaling relations. This implies a substantially higher αCO ≥ 10 and δGDR ≥ 1200, consistent with the subsolar metallicity estimated for these galaxies ($12+\mathrm{log}({\rm{O}}/{\rm{H}})\approx 7.8\pm 0.2$). The low metallicity of z ≥ 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 [C ii].