HI content at cosmic noon – a millimeter-wavelength perspective

ABSTRACT 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 (M ₇₈ plus M ₇䃒) 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 M ₇₈/M ₇䃒 ratio, which is about 1-3 (depending on the relation used to estimate M ₇₈). This also shows that M ₇䃒-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 M ₇₈ 10^10. 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- 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.