HI Mass Function High-Mass Excess

The HI mass function describes the number density of galaxies as a function of their neutral hydrogen mass. At the low-mass end, the HI mass function has a steep slope that is sensitive to the abundance of gas-rich dwarf galaxies — a regime where ΛCDM predictions and observations have historically been in tension with the missing satellite problem. At the high-mass end, the HI mass function shows a sharp exponential cutoff above ~10¹⁰ solar masses, beyond which HI-rich galaxies become extremely rare. However, systematic surveys including ALFALFA have found a slight excess of very HI-rich galaxies at the highest masses compared to ΛCDM semi-analytic model predictions that incorporate AGN feedback to suppress gas cooling in massive halos. This high-mass excess, while modest, persists across different survey treatments and suggests that either the AGN feedback prescription suppresses gas retention too efficiently in massive halos, or that there is a source of HI at high masses not captured in standard models.

Successive Collision Theory explains the high-mass HI excess through the pre-existing gas reservoirs from the colliding pockets. In SCT, the most massive galaxies in the present-day universe assembled from debris regions that were themselves among the most massive structures in the two pre-existing pockets — regions with higher-than-average gas-to-stellar mass ratios because the angular momentum barriers in those regions had prevented efficient star formation despite high total mass. These pre-existing massive gas reservoirs were incorporated into the post-collision debris field as the raw material for the most massive present-day galaxies, contributing HI that is additional to what standard post-collision star formation histories predict. The result is that the most HI-massive galaxies in the present-day universe contain a contribution from pre-existing gas that survived the collision thermalization epoch and has not yet been consumed by star formation or expelled by AGN feedback.

The angular momentum inheritance mechanism provides a second contribution to the high-mass HI excess by protecting gas in massive halos from efficient AGN feedback stripping. In SCT, the gas in massive halos has inherited organized angular momentum that partially supports it against infall toward the central black hole — the same centrifugal barrier that flattens rotation curves and maintains disk stability at all mass scales. This angular momentum support reduces the efficiency with which AGN feedback can expel gas from the most massive halos, because the organized rotation of the gas provides centrifugal resistance to the outward pressure of AGN-driven winds. The net effect is that massive galaxies in SCT retain more HI at late times than ΛCDM models with isotropic AGN feedback predict, producing the excess at the high-mass end of the HI mass function while leaving the low-mass end largely unaffected — consistent with the observed shape of the discrepancy.