The Formation of Disk Galaxies

We present a scenario for the formation of disks which explains not only the properties of normal galaxies, but the properties of the population of low surface brightness galaxies (LSBs) as well. We use a gravitationally self-consistent model for disk collapse to calculate the observable properties of disk galaxies as a function of mass and angular momentum of the initial protogalaxy. The model naturally produces both smooth, asymptotically flat rotation curves and exponential surface brightness profiles over many disk scale lengths. In this scenario, low mass and/or high angular momentum halos naturally form low baryonic surface density disks, which will tend to be low surface brightness. Theoretical and numerical calculations suggest galaxy halos should form with a wide range of mass and angular momenta, and thus, the disks which form within these halos should have a wide range of surface brightnesses and scale lengths. We use theoretical predictions for the distribution of halo masses and angular momenta to explicitly calculate the expected number density of disk galaxies as a function of central surface brightness and disk scale length. The resulting distribution is compared to the observed properties of galactic disks, and is shown to explain the range of observed disk properties, including the cutoff in the maximum disk scale length as a function of surface brightness. We also show that disk instabilities explain the observed lack of high surface density e-mail address: [email protected] Hubble Fellow Current Address: Columbia Astrophysics Lab, Mail Code 5247, 550 West 120th Street, New York, NY 10027