HI content at cosmic noon – a millimeter-wavelength perspective

In order to understand galaxy growth evolution, it is critical to constrain the evolution of its building block: gas. Mostly comprised by Hydrogen in its neutral (H i) and molecular (H₂) phases, the latter is the one mostly directly associated to star formation, while the neutral phase is considered the long-term gas reservoir. In this work, we make use of an empirical relation between dust emission at millimetre wavelengths and total gas mass in the interstellar medium (MHI plus MH₂) in order to retrieve the H i content in galaxies. We assemble an heterogeneous sample of 335 galaxies at 0.01< z< 6.4 detected in both mm-continuum and carbon monoxide (CO), with special focus on a blindly selected sample to retrieve H i cosmological content when the Universe was ~ 2-6\, Gyr old (1< z< 3). We find no significant evolution with redshift of the MHI/MH₂ ratio, which is about $1-3$ (depending on the relation used to estimate MHI). This also shows that MH₂-based gas depletion times are underestimated overall by a factor of $2-4$. Compared to local Universe H i mass functions, we find that the number density of galaxies with MHI 1010.5\,M_ significantly decreased since 8–12 Gyr ago. The specific sample used for this analysis is associated to 20–50 per cent of the total cosmic H i content as estimated via Damped Lyman-α Absorbers. In IR luminous galaxies, H i mass content decreases between z~ 2.5 and z~ 1.5, while H₂ seems to increase. We also show source detection expectations for SKA surveys.