Photoionization and the formation of dwarf galaxies

Abstract

It has been argued that a UV photoionizing background radiation field suppresses the formation of dwarf galaxies, and may even inhibit the formation of larger galaxies. In order to test this, we present gas-dynamical simulations of the formation of small objects in a CDM universe with and without a photoionizing background. The objects are selected from a collisionless simulation at a redshift of 2.4, and rerun at higher resolution including the effects of gas dynamics and using a hierarchical grid of particles. Five objects, each with a circular speed of 46 km s−1, are simulated. The presence of the photoionizing background has a moderate effect on the amount of gas that collapses in these objects, reducing the amount of cold collapsed gas by 40 to 90 per cent. Analysis of the smaller objects found in the higher resolution simulation indicates that the photoionizing background significantly affects the formation of objects with a virialized halo mass less than 4 × 109 M⊙ and circular speeds less than 37 km s−1. Although formation of the larger objects is only moderately affected, the ionization balance is greatly changed by the presence of the background radiation field. Typical lines of sight through the objects have 4 orders of magnitude less neutral hydrogen column density when the photoionizing background is included.