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Abstract

The molecular gas content of high-redshift galaxies is a highly sought-after property. However, H₂ is not directly observable in most environments, so its mass is probed through other emission lines (e.g. CO, [C i], [C ii]), or through a gas-to-dust ratio. Each of these methods depends on several assumptions, and are best used in parallel. In this work, we extend an additional molecular gas tracer to high-redshift studies by observing hydrogen deuteride (HD) emission in the strongly lensed z = 5.656 galaxy SPT0346−52 with ALMA. While no HD(1–0) emission is detected, we are able to place an upper limit on the gas mass of M_H₂<6.4×10¹¹M_⊙⁠. This is used to find a limit on the L′_CO conversion factor of      α_CO<5.8M_⊙(Kkms⁻¹pc²)−1⁠. In addition, we construct the most complete spectral energy distribution of this source to date, and fit it with a single-temperature modified blackbody using the nested sampling code multinest, yielding a best-fitting dust mass M_dust = 10^{8.92 ± 0.02} M_⊙, dust temperature 78.6 ± 0.5 K, dust emissivity spectral index β = 1.81 ± 0.03, and star formation rate SFR = 3800 ± 100 M_⊙ yr⁻¹. Using the continuum flux densities to estimate the total gas mass of the source, we find     M_H₂<2.4×10¹¹M_⊙ , assuming subsolar metallicity. This implies a CO conversion factor of α_CO < 2.2, which is between the standard values for MW-like galaxies and starbursts. These properties confirm that SPT0346−52 is a heavily starbursting, gas-rich galaxy.