2021ApJ...916...12B

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 Lyα λ1216 emission and H_F160W-band magnitude, the galaxies show an average <EW_Lyα⁰>~20 Å, below the typical selection threshold for Lyα emitters (EW_Lyα⁰> 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 x 10⁸ K km s^–1pc² and 5.6 x 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 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].