The Effects of a Photoionizing Ultraviolet Background on the Formation of Disk Galaxies

We use high resolution N-body/gasdynamical simulations to investigate theeffects of a photoionizing UV background on the assembly of disk galaxies inhierarchically clustering universes. We focus on the mass and rotationalproperties of gas that can cool to form centrifugally supported disks in darkmatter halos of different mass. Photoheating can significantly reduce theamount of gas that can cool in galactic halos. Depending on the strength of theUV background field, the amount of cooled gas can be reduced by up to $50\%$ insystems with circular speeds in the range $80$-$200$ \kms. The magnitude of theeffect, however, is not enough to solve the ``overcooling'' problem thatplagues hierarchical models of galaxy formation if the UV background is chosento be consistent with estimates based on recent observations of QSO absorptionsystems. Photoionization has little effect on the collapse of gas at highredshift and affects preferentially gas that is accreted at late times. Sincedisks form inside-out, accreting higher angular momentum gas at later times,disks formed in the presence of a UV background have spins that are evensmaller than those formed in simulations that do not include the effects ofphotoionization. This exacerbates the angular momentum problem that afflictshierarchical models of disk formation. We conclude that photoionization cannotprovide the heating mechanism required to reconcile hierarchically clusteringmodels with observations. Energy feedback and enrichment processes from theformation and evolution of stars must therefore be indispensable ingredientsfor any successful model of the formation of disk galaxies.