STAR FORMATION AND THE INTERSTELLAR MEDIUM IN LOW SURFACE BRIGHTNESS GALAXIES. III. WHY THEY ARE BLUE, THIN AND POOR IN MOLECULAR GAS

We present N -body simulations of Low Surface Brightness (LSB) galaxies and their Interstellar Medium to in- vestigate the cause for their low star formation rates (SFR). Due to their massive halos, stellar disks of LSB galaxies are very stable and thin. Lack of dust makes the projected edge-on surface brightness of LSB galaxies comparable to the projected edge-on surface brightness of dust-rich High Surface Brightness (HSB) galaxies of similar size. We show that the low surface densities found in LSB galax- ies are by themselves not enough to explain the slow evolution of LSB galaxies. A low metal content of the gas is essential. As a consequence the gas cools inefficiently, resulting in an almost negligible cold gas fraction. We show that LSB galaxies must have molecular gas fractions of less than 5 percent. Our best model has a SFR which is on average low but fluctuates strongly. This causes the large spread in colors of LSB galaxies. From a distribution of birthrate parameters we conclude that the presently-known and modeled gas-rich blue LSB galaxies constitute the majority of the total population of gas-rich LSB disk galaxies. We deduce the existence of an addi- tional red, quiescent and gas-rich population which constitutes less than 20 percent of the total population. This does not rule out the existence of a large number of gas-poor LSB galaxies. These must however have had an evolutionary history dramati- cally different from that of the gas-rich galaxies.