The secular evolution of galactic disks with a star formation threshold: The origins of low surface brightness spirals, magellanic irregulars, and blue compact dwarfs

The long-term evolution of disk galaxies of extremely late morphological type is examined in relation to the possible dynamical effect of a star formation threshold in the interstellar gas. Although having common features such as high gas content, blue optical colors, and dominant dark matter, extremely late galaxies exhibit a remarkable diversity in their structure, ranging from low surface brightness disks and central bars to the centrally concentrated star-forming regions that characterize blue compact dwarf galaxies. I make the hypothesis that the observed structural variety has been caused by secular evolution processes taking place in the galactic disks, probably driven by some kind of viscosity. A simple model of the secular evolution of galactic disks is devised, and how the mass redistribution process depends upon the galaxy mass and density is examined. It is found that the existence of a threshold surface density of the gas disk, below which the star formation is heavily damped, has a profound effect on the viscosity-driven evolution of late-type galaxies, i.e., galaxies with a small mass and/or a low density. More massive and/or denser galaxies, corresponding to luminous high surface brightness galaxies (with a Hubble type of similar to Scd and earlier), hardly suffer from this effect and are governed by a different regime of mass transport. The most important result found in the present extensive model calculation is that the viscous evolution model of galactic disks, coupled with the existence of a star formation threshold in the interstellar gas density, predicts the formation of gas-rich, low-mass galaxies that have a prominent central component at the present cosmological epoch. The most plausible candidates for such systems include blue compact dwarf galaxies and extremely late-type disk galaxies possessing a central bar.