What Shapes the Luminosity Function of Galaxies?

We investigate the physical mechanisms that shape the luminosity function.Beginning with the mass function of dark matter halos, we show how gas cooling,photoionization, feedback, merging and thermal conduction affect the shape ofthe luminosity function. We consider three processes whereby supernovae canaffect the forming galaxy: (1) reheating of disk gas to the halo temperature;(2) expansion of the diffuse halo gas; (3) expulsion of cold disk gas from thehalo. While feedback of form (1) is able to flatten the faint end of theluminosity function, this alone does not produce the sharp cut-off observed atlarge luminosities. Feedback of form (2) is also unable to solve this problem.The relative paucity of very bright galaxies can only be explained if coolingin massive halos is strongly suppressed. Conduction is a promising mechanism,but an uncomfortably high efficiency is required to suppress cooling to thedesired level. If, instead, superwinds are responsible for the lack of brightgalaxies, then the total energy budget required to obtain a good match to thegalaxy luminosity function greatly exceeds the energy available from supernovaexplosions. The mechanism is only viable if the formation of centralsupermassive black holes play a crucial role in limiting the amount of starsthat form in the host galaxy. (abridged)Comment: 10 pages, 5 figures. Submitted to Ap