2020MNRAS.498.4109J

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_< 6.4×10¹¹ M_☉. 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_< 2.4×10¹¹ M_☉ , 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.