Gas and star formation from HD and dust emission in a strongly lensed galaxy

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 MH2 < 6.4 × 1011M_. This is used to find a limit on the L’CO conversion factor of αCO < 5.8M_(K km s−1 pc2)−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 Mdust = 108.92 ± 0.02M_, dust temperature 78.6 ± 0.5K, dust emissivity spectral index β = 1.81 ± 0.03, and star formation rate SFR = 3800 ± 100M_ yr−1. Using the continuum flux densities to estimate the total gas mass of the source, we find MH2 < 2.4 × 1011M_ , assuming subsolar metallicity. This implies a CO conversion factor of αCO < 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.