The Origin of the Density Distribution of Disk Galaxies: A New Problem for the Standard Model of Disk Formation

We present new models for the formation of disk galaxies that improve upon previous models by following the detailed accretion and cooling of the baryonic mass, and by using realistic distributions of specific angular momentum. Under the assumption of detailed angular momentum conservation the disks that form have density distributions that are more centrally concentrated than an exponential. We examine the influence of star formation, bulge formation, and feedback on the outcome of the surface brightness distributions of the stars. Whereas the models are succesful in reproducing high surface brightness disks, they fail to produce exponential disks of low surface brightness. In addition, the models predict truncation radii in the gas that occur at too high surface density. This signals a new problem for the standard model of disk formation: if the baryonic component of the protogalaxies out of which disk galaxies form have the same angular momentum distribution as the dark matter, disks are too compact.