THE FORMATION OF DISK GALAXIES

We present a scenario for the formation of disks that explains not onl y the properties of normal galaxies but also the properties of the pop ulation or low surface brightness galaxies (LSBs) as well. We use a gr avitationally self-consistent model for disk collapse to calculate the observable properties of disk galaxies as a function of mass and angu lar momentum of the initial protogalaxy. The model naturally produces both smooth, asymptotically flat rotation curves and exponential surfa ce brightness profiles over many disk scale lengths. In this scenario, low-mass and/or high angular momentum halos naturally form low baryon ic surface density disks, which will tend to be low surface brightness . Theoretical and numerical calculations suggest galaxy halos should f orm with a wide range of mass and angular momenta, and thus, the disks that form within these halos should have a wide range of surface brig htnesses and scale lengths. We use theoretical predictions for the dis tribution of halo masses and angular momenta to calculate explicitly t he expected number density of disk galaxies as a function of central s urface 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 i n the maximum disk scale length as a function of surface brightness. W e also show that disk instabilities explain the observed lack of high surface density disks. The calculated distribution of disk properties also suggests that there are large numbers of galaxies that remain und etected owing to biases against galaxies with either low surface brigh tness or small scale length. We quantify this by calculating the diffe rence between the intrinsic luminosity function and the luminosity fun ction that would be measured in a galaxy survey with a given limiting surface brightness. We show that current measurements of the galaxy lu minosity function may be missing more than half of all L galaxies, an d an even larger fraction of faint galaxies, given the correlation bet ween mass and surface brightness. The likely underestimate of the lumi nosity density is also expected to be large. We discuss how this affec ts observations of the ''faint blue galaxy'' population. We also inves tigate the dynamics of galaxies as a function of surface brightness. W e show that, in the absence of any systematic change in the ratio of d isk mass to disk luminosity, galaxies of all surface brightnesses shou ld lie on the same Tully-Fisher relation. Our models also show systema tic changes in the shape of the rotation curve as a function of angula r momentum, which leads to LSBs having slowly rising rotation curves. Furthermore, because high angular momentum LSB disks have their baryon ic mass spread over a larger area than normal galaxies of comparable m ass, LSB disks contribute very little to the observed dynamics of the galaxy. Thus, LSBs provide a very effective tracer of the shape and ma ss profile of the dark matter halo, out to proportionally larger radii than is possible to observe with normal galaxy rotation curves.