Joint Optical- <scp>Hi</scp> mock catalogues and prospects for upcoming <scp>Hi</scp> surveys

Abstract Atomic hydrogen (Hi) regulates star formation as cold gas fuels star formation. It represents a key phase in the baryon cycle. Redshifted 21 cm line emission serves as a key tracer for investigating Hi in the interstellar medium (ISM) and circumgalactic medium (CGM) in distant galaxies, and enables the study of galaxy evolution. Direct detections of Hi are currently limited to z ≤ 0.4 due to the inherently weak 21 cm emission line. Ongoing and upcoming large radio surveys aim to detect 21 cm emission from galaxies up to z ≳ 1. We present a novel approach for creating optical-Hi joint mock catalogues for upcoming SKA precursor surveys: MIGHTEE-Hi, LADUMA and WALLABY. Incorporation of optical properties along with Hi makes these mock catalogues a powerful tool for making predictions for surveys and provides a benchmark for exploring the Hi science (e.g., conditional Hi MF and optical-to-Hi scaling relations) expected from these surveys. As a case study, we show the use of the joint catalogues for predicting the expected outcome of stacking detection for average Hi mass in galaxies that are below the threshold for direct detection. We show that combining stacking observations with only the number of direct detections puts a strong constraint on the Hi mass function. We also show that this can be done separately for parts of the color-magnitude diagram for galaxies, enabling a fine grained study of galaxy evolution. This may be used to set priors for the full determination of the Hi mass function.