The Luminosity Function in Groups of Galaxies

Abstract

With targeted imaging of groups in the local volume, the regions of collapse around bright galaxies can be clearly identified by the distribution of dwarfs and luminosity functions can be established to very faint levels. In the case of the M 81 Group there is completion to M_R ~ −9. In all well studied cases, the faint end slopes are in the range − 1.35 < α < −1.2, much flatter than the slope for the bottom end of the halo mass spectrum anticipated by ΛCDM hierarchical clustering theory. Small but significant variations are found with environment. Interestingly, the populations of dwarf galaxies are roughly constant per unit halo mass. With the numbers of dwarfs as an anchor point, evolved environments (dominated by early morphological types) have relatively fewer intermediate luminosity systems and at least one relatively more important galaxy at the core. The variations with environment are consistent with a scenario of galaxy merging. However it is questionable if the universal dearth of visible dwarf systems is a consequence of an astrophysical process like reionization.