The tiered radio extragalactic continuum (T-RECS) simulation II: H i emission and continuum-H i cross-correlation

In this paper, we extend the Tiered Radio Extragalactic Continuum Simulation (T-RECS) to include H i emission. The H i T-RECS model is based on the most recent H i mass function estimates, combined with prescriptions to convert H i mass to total integrated H i flux. It further models source size, morphology, and kinematics, including rotational velocity and H i line width. The continuum T-RECS model is updated to improve the agreement with deeper number counts available at 150 MHz. The model for star-forming galaxies (SFGs) is also modified according to the most recent indications of a star formation rate (SFR)-radio luminosity relation, which depends primarily on stellar mass rather than redshift. We further introduce prescriptions to associate an H i mass with the T-RECS radio continuum SFG and active galactic nuclei (AGN) populations. This gives us a way to meaningfully associate counterparts between H i and continuum catalogues, thus building H ix continuum simulated observations. Clustering properties of the sources in both H i and the continuum are reproduced by associating the galaxies with dark matter haloes in a cosmological simulation. We deliver a set of mock catalogues as well as the code to produce them, which can be used for simulating observations and predicting results from radio surveys with existing and forthcoming radio facilities, such as the Square Kilometre Array (SKA).