Suppressing the formation of dwarf galaxies via photoionization

In hierarchical clustering theories, some sort of feedback mechanism is required to prevent most of the baryonic material collapsing into subgalactic objects at high redshifts. It is argued that a photoionizing background of the magnitude suggested by applying the Gunn–Peterson constraint to high–redshift quasars would strongly suppress the cooling of a hydrogen–helium plasma and so inhibit the formation of dwarf galaxies. The effectiveness of this mechanism depends on the spectrum of the photoionizing radiation, and so galaxy formation could depend on the proximity of protogalactic perturbations to unusual sources of hard photons such as luminous quasars. This could introduce large-scale spatial variations in the galaxy distribution.