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Faint End of the Galaxy Luminosity Function : A Chronometer for Structure Formation ?

There is accumulating evidence that the faint end of the galaxy lumi-nosity function might be very different in different locations. The lumi-nosity function might be sharply rising in rich clusters and flat or declining in regions of low density. If galaxies form according to the model of hierarchical clustering then there should be many small halos compared to the number of big halos. If this theory is valid then there must be a mechanism that eliminates at least the visible component of galaxies in low density regions. A plausible mechanism is photoionization of the intergalactic medium at a time before the epoch of galaxy formation in low density regions but after the epoch of formation for systems that ultimately end up in rich clusters. The dynamical timescales are found to accommodate this hypothesis in a flat universe with Ω m < 0.4. If this idea has validity, then upon surveying a variety of environments it is expected that a dichotomy will emerge. There should be a transition between high density / high frequency of dwarfs to lower density / low frequency of dwarfs. This transition should ultimately be understood by the matching of three timing considerations: (i) the collapse timescale of the transition density, (ii) the timescale of reionization, and (iii) the linkage given by the cosmic expansion timescale as controlled by the dark matter and dark energy content of the universe.