Atomic hydrogen in $z=0$ galaxies in the COLIBRE simulations
We present an overview of the properties of atomic hydrogen (HI) in galaxies at redshift $z=0$ in the COLIBRE cosmological galaxy formation simulations. COLIBRE directly simulates the multi-phase interstellar medium (ISM) and includes new methods to follow radiative cooling to $\approx 10\,\rm{K}$, non-equilibrium chemistry coupled to dust, and dust formation, growth \& destruction. We focus on the $(200\,\rm{cMpc})^3$ volume with resolution elements of $2\times 10^{6}\,\rm{M}_{\odot}$. No HI properties informed the calibration of model parameters and yet we find very good agreement with the observed $M_{\star}$-$M_{\rm{HI}}$ relation, the HI mass and velocity width functions, the HI mass-size relation, and (baryonic) Tully-Fisher relations, albeit with some mismatches at the extremes of the galaxy population. HI surface density profiles are realistic, except at large radii around massive ($M_{\star}\gtrsim 10^{10}\,\rm{M}_{\odot}$) galaxies where we find a population of possibly spurious circumgalactic HI clouds, whose abundances and masses decrease, however, with increasing mass resolution). Using smaller simulations to vary some of the uncertain model parameters, we find that the main scaling relations are at most mildly affected, although this may not extend to detailed HI morphokinematics. Convergence across mass resolution levels a factor of $8$ higher and lower is good: the median HI mass and HI `occupation fraction' as a function of halo mass, and the HI mass function, are remarkably stable, although the highest resolution model overproduces HI sources by a factor of $\approx2$ at $M_{\rm{HI}}\gtrsim 3\times 10^{9}\,\rm{M}_{\odot}$. Upturns in the low-mass and low-velocity slopes of the HI mass and velocity width functions with potentially subtle observational signatures are predicted.