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_⊙, 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 $\langle {\mathrm{EW}}_{\mathrm{Ly}\alpha }^{0}\rangle \approx 20$ 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 α_CO ≥ 10 and δ_GDR ≥ 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].