The Delayed Formation of Dwarf Galaxies
Abstract
One of the largest uncertainties in understanding the effect of a background UV field on galaxy formation is the intensity and evolution of the radiation field with redshift. This work attempts to shed light on this issue by computing the quasi-hydrostatic equilibrium states of gas in spherically symmetric dark matter halos (roughly corresponding to dwarf galaxies) as a function of the amplitude of the background UV field. We integrate the full equations of radiative transfer, heating, cooling and non-equilibrium chemistry for nine species: H, H+, H-,H2, H2+, He, He+, He++, and e-. As the amplitude of the UV background is decreased the gas in the core of the dwarf goes through three stages characterized by the predominance of ionized (H+), neutral (H) and molecular (H2) hydrogen. Characterizing the gas state of a dwarf galaxy with the radiation field allows us to estimate its behavior for a variety of models of the background UV flux. Our results indicate that a typical radiation field can easily delay the collapse of gas in halos corresponding to 1-σ CDM perturbations with circular velocities less than 30 km/s.
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