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. Low angular momentum haloes yield disk galaxies with a significant bulge component and with a stellar disk that is close to exponential, in good agreement with observations. High angular momentum haloes, on the other hand, produce stellar disks that are much more concentrated than an exponential, in clear conflict with observations. At large radii, the models reveal distinct truncation radii in both the stars and the cold gas. The stellar truncation radii owe to our implementation of star formation threshold densities, and are in excellent agreement with observations. The truncation radii in the density distribution of the cold gas reflect the maximum specific angular momentum of the gas that has cooled. We find that these truncation radii occur at HI surface densities of roughly 1 M⊙ pc , in conflict with observations. We examine various modifications to our models, including feedback, viscosity, and dark matter haloes with constant density cores, but show that the models consistently fail to produce bulge-less disks with exponential surface brightness profiles. 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.