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 ⊙ , from the MUSE Hubble Ultra Deep Field (HUDF) Survey. Selected by their Ly α λ 1216 emission and H F160W -band magnitude, the galaxies show an average 〈 EW Ly α 0 〉 ≈ 20 Å, below the typical selection threshold for Lyα emitters ( EW Ly α 0 > 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 Ly α by 〈Δ v (Ly α )〉 = 346 km s −1 , with a 100 to 600 km s −1 range. Stacking 12 CO J = 4 → 3 and [C i ] 3 P 1 → 3 P 0 (and higher- J CO lines) from the ALMA Spectroscopic Survey of the HUDF, we determine 3 σ upper limits on the line luminosities of 4.0 × 10 8 K km s −1 pc 2 and 5.6 × 10 8 K km s −1 pc 2 , respectively (for a 300 km s −1 line width). Stacking the 1.2 mm and 3 mm dust-continuum flux densities, we find a 3 σ upper limits of 9 μ Jy and 1.2 μ 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 α CO ≥ 10 and δ GDR ≥ 1200, consistent with the subsolar metallicity estimated for these galaxies ( 12 + log ( O / H ) ≈ 7.8 ± 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 ].