H i content at cosmic noon – a millimetre-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 (H |$_2$|⁠) 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 |$_{\rm HI}$| plus M |$_{\rm H_2}$|⁠) in order to retrieve the H  i content in galaxies. We assemble an heterogeneous sample of 335 galaxies at |$0.01\lt z\lt 6.4$| 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 |$\sim 2-6\,$|  Gyr old (⁠|$1\lt z\lt 3$|⁠). We find no significant evolution with redshift of the M |$_{\rm HI}$| /M |$_{\rm H_2}$| ratio, which is about |$1-3$| (depending on the relation used to estimate M |$_{\rm HI}$|⁠). This also shows that M |$_{\rm H_2}$| -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 |$_{\rm HI}\gtrsim 10^{10.5}\,$| M |$_\odot$| 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- |$\alpha$| Absorbers. In IR luminous galaxies, H  i mass content decreases between |$z\sim 2.5$| and |$z\sim 1.5$|⁠ , while H |$_2$| seems to increase. We also show source detection expectations for SKA surveys. [ABSTRACT FROM AUTHOR]