Dwarf galaxies in voids: suppressing star formation with photoheating

Abstract

We study the structure formation in cosmological void regions using high-resolution hydrodynamical simulations. Despite being significantly underdense, voids are populated abundantly with small dark matter haloes which should appear as dwarf galaxies if their star formation is not suppressed significantly. We here investigate to which extent the cosmological ultraviolet (UV) background reduces the baryon content of dwarf galaxies, and thereby limits their cooling and star formation rates. Assuming a Haardt & Madau UV background with reionization at redshift z= 6, our samples of simulated galaxies show that haloes with masses below a characteristic mass of Mc(z= 0) = 6.5 × 109 h−1 M⊙ are baryon-poor, but in general not completely empty, because baryons that are in the condensed cold phase or are already locked up in stars resist evaporation. In haloes with mass M≲Mc, we find that photoheating suppresses further cooling of gas. The redshift- and UV-background-dependent characteristic mass Mc(z) can be understood from the equilibrium temperature between heating and cooling at a characteristic overdensity of δ≃ 1000. If a halo is massive enough to compress gas to this density despite the presence of UV-background radiation, gas is free to ‘enter’ the condensed phase and cooling continues in the halo, otherwise it stalls. By analysing the mass accretion histories of dwarf galaxies in voids, we show that they can build up a significant amount of condensed mass at early times before the epoch of reionization. Later on, the amount of mass in this phase remains roughly constant, but the masses of the dark matter haloes continue to increase. Consequently, photoheating leads to a reduced baryon fraction in void dwarf galaxies, endows them with a rather old stellar population, but still allows late star formation to some extent. We estimate the resulting stellar mass function for void galaxies. While the number of galaxies at the faint end is significantly reduced due to photoheating, additional physical feedback processes may be required to explain the apparent paucity of dwarfs in observations of voids.