NEUTRALUNIVERSEMACHINE: Predictions of H I gas in different theoretical models

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China
² University of Chinese Academy of Sciences, Beijing 100049, China
³ Tianjin Normal University, Binshuixidao 393, 300387 Tianjin, China
⁴ INAF-Astronomical Observatory of Trieste, Via G.B. Tiepolo, 11 I-34143 Trieste, Italy
⁵ IFPU – Institute for Fundamental Physics of the Universe, via Beirut 2, I-34151 Trieste, Italy
⁶ DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, DK-2100 Copenhagen, Denmark

^⋆ Corresponding author: guohong@shao.ac.cn

Received: 22 October 2024
Accepted: 23 May 2025

Abstract

We investigate the distribution and evolution of H I gas in different theoretical models, including hydrodynamical simulations of IllustrisTNG and SIMBA, the semi-analytical model of GAEA, and the empirical model of NEUTRALUNIVERSEMACHINE (NUM). By comparing model predictions for the H I mass function (HIMF), H I-halo and H I-stellar mass relations, conditional H I mass function (CHIMF) and the halo occupation distribution (HOD) of H I-selected galaxies, we find that all models show reasonable agreement with the observed HIMF at z ∼ 0. However, the differences become much larger at higher redshifts of z = 1 and z = 2. The HIMF of NUM shows remarkable agreement with the observation at z = 1, whereas other models predict much smaller amplitudes of HIMF at the high-mass end. Comparisons of CHIMF distributions indicate that the HIMF is dominated by haloes of 10 < log(M_vir/M_⊙) < 11 and 11 < log(M_vir/M_⊙) < 12 at the low- and high-mass ends, respectively. From the H I HODs of central galaxies, we find that TNG100 overpredicts the number of central galaxies with high M_H I in massive haloes, and that GAEA shows a very strong depletion of H I gas in quenched centrals of massive haloes. The main cause of these differences is the AGN feedback mechanisms implemented in different models.