2002ApJ...569..573T

There is accumulating evidence that the faint end of the galaxy luminosity function might be very different in different locations. The luminosity function might be rising in rich clusters and flat 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 that most dwarf galaxies form in low-density regions but after the epoch of formation for similar 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 small halos exist but simply cannot be located because they have never become the sites of significant star formation, they still might have dynamical manifestations. These manifestations are hard to identify in normal groups of galaxies because small halos do not make a significant contribution to the global mass budget. It could be entertained, however, that there are clusters of halos where there are only small systems, clusters that are at the low-mass end of the hierarchical tree. There may be places where only a few small galaxies managed to form, enough for us to identify and use as test probes of the potential. It turns out that such environments might be common. Four probable groups of dwarfs are identified within 5 Mpc, and the assumption that they are gravitationally bound suggests M/L_B∼300-1200M_☉/L_☉, a factor of 6±2 times higher than typical values for groups with luminous galaxies.