The luminosity function of dwarf galaxies in four spiral-rich clusters

We measure luminosity functions in the cores of four spiral-rich, poorclusters of galaxies at median redshift $z = 0.016$. In the red magnitude range-14 < M_R < -10, our data imply that the luminosity functions phi(L) \proptoL^{alpha} are steep, -1.8 < alpha < -1.6, in the central 200-300 kpc of Abell262 and of the NGC 507 Group. Abell 194 also shows signs of a steep luminosityfunction, alpha < -1.6, in this magnitude range. In Pegasus, the dwarf galaxydensity is too low to let us constrain alpha. The NGC 507 Group and Abell 194have been interpreted as clusters that are forming today, based on morphologyand velocity structure. The high spiral galaxy fraction in Abell 262 relativeto clusters like Virgo and Coma also suggests that it is young. We thereforesuggest that steep luminosity functions in the range -14 < M_R < -10 may be auniversal feature of young clusters and possibly of the field. If this is true,then the observed paucity of gas-rich galaxies in such environments suggeststhat we are finding galaxies similar to the low-surface-brightness,dark-matter-dominated dwarf spheroidal galaxies seen locally and in Virgo. Thisinterpration is also consistent with the distribution of colors and sizes ofthe faint galaxies in Abell 262. If we are indeed detecting dwarf spheroidalgalaxies and if they are as numerous relative to bright galaxies in the fieldas they are in the young clusters observed here, then the contribution of theirhalos to the cosmological mass density is Omega_{dSph halo} \approx 0.01. Thisis much smaller than values of Omega derived from dynamical measurements.Comment: 43 pages, 14 figs, postscript, MNRAS in press fig 1 (45 Mb) available from autho