WHEN STAR-FORMATION STOPS - GALAXY EDGES AND LOW SURFACE BRIGHTNESS DISKS

The star formation rate per unit gas mass should be low wherever the average gas pressure is low because most compressions in these regions, whether from old supernova shocks, gravitational instabilities, cloud collisions, supersonic turbulence, or spiral waves, cannot produce gas that is dense enough to cool to low thermal temperatures. All of the normal routes to star formation shut off when the average pressure is much less than the minimum critical pressure for the coexistence of two thermal phases. We calculate this minimum critical pressure P-min and the corresponding densities for gas in thermal equilibrium for a variety of radiation fields, metallicities, and cosmic-ray fluxes. Low total pressure usually correlates with low gas surface density in a galaxy, so if the thermal pressure, which is only part of this total, decreases as the total pressure decreases, then the low surface density regions that are at large radii in normal galaxies or in low surface brightness galaxies should have sufficiently low thermal pressures to prevent star formation in all but the highest pressure fluctuations. Star formation stops because without the possibility of low equilibrium temperatures, no amount of cooling, and no likely amount compression subsequent to cloud formation (as in turbulence), can produce clump masses that exceed the thermal Jeans mass. We predict that H I in the outer parts of galaxies and in regions with low ratios of pressure to radiation field should have most of their mass at temperatures of several thousand kelvins, and very little of their mass in a dense form appropriate for star formation.