The Expected Mass Function for Low‐Mass Galaxies in a Cold Dark Matter Cosmology: Is There a Problem?

It is well known that the mass function forₕalos_ in CDM cosmology is a relatively steep power law for low masses, possibly too steep to be consistent with observations. But how steep is thegalaxy_ mass function? We have analyzed the stellar and gas mass functions of the first massive luminous objects formed in a CDM universe, as calculated in the numerical simulation described in Gnedin (2000ab). We found that while the dark matter mass function is steep, the stellar and gas mass functions are flatter for low mass objects. The stellar mass function is consistently flat at the low mass end. Moreover, while the gas mass function follows the dark matter mass function until reionization at z~7, between z=7 and z=4, the gas mass function also flattens considerably at the low mass end. At z=4, the gas and stellar mass functions are fit by a Schechter function with ~ -1. 2 +/- 0. 1, significantly shallower than the dark matter halo mass function and consistent with some recent observations. The baryonic mass functions are shallower because (a) the dark matter halo mass function is consistent with the Press-Schechter formulation at low masses n (M) M^-2 and (b) heating/cooling and ionization processes appear to cause baryons to collect in halos with the relationship Mb Md⁴ at low masses. Combining (a) and (b) gives n (Mb) Mb^-5/4, comparable to the simulation results. Thus, the well known observational fact that low mass galaxies are underabundant as compared to expectations from numerical dark matter simulations or Press-Schechter modeling of CDM universes emerges naturally from these results, implying that perhaps no ``new physics'' beyond the standard model is needed.