H i content at cosmic noon – a millimetre-wavelength perspective

Abstract

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 (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.