Photoionization Feedback in Low‐Mass Galaxies at High Redshift

The cosmic ultraviolet (UV) ionizing background impacts the formation ofdwarf galaxies in the low-redshift universe (z=3) by suppressing gas infallinto galactic halos with circular velocities up to v(circ)=75 km/s. Using aone-dimensional, spherically symmetric hydrodynamics code (Thoul & Weinberg1995), we examine the effect of an ionizing background on low-mass galaxiesforming at high redshifts (z>10). We find that the importance ofphotoionization feedback is greatly reduced, because (1) at high redshift,dwarf-galaxy sized objects can self-shield against the ionizing background, (2)collisional cooling processes at high redshift are more efficient, (3) theamplitude of the ionizing background at high redshift is lower, and (4) theionizing radiation turns on when the perturbation that will become the dwarfgalaxy has already grown to a substantial overdensity. We find that because ofthese reasons, gas can collect inside halos with circular velocities as low asv(circ)=10 km/s. This result has important implications for the reionizationhistory of the universe.Comment: submitted to ApJ, 10 emulateapj pages with 4 figures include