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Faint Dwarfs in Nearby Groups

The number and distribution of dwarf satellite galaxies remain a critical test of cold dark matter-dominated structure formation on small scales. Until recently, observational information about galaxy formation on these scales has been limited mainly to the Local Group. We have searched for faint analogues of Local Group dwarfs around nearby bright galaxies, using a spatial clustering analysis of the photometric catalog of the Sloan Digital Sky Survey (SDSS) Data Release 8. Several other recent searches of SDSS have detected clustered satellite populations down to $Δm_r \equiv ({m}_{r,\, {\rm sat}} -\, {m}_{r,\, {\rm main}}) \sim 6$-$8$, using photometric redshifts to reduce background contamination. SDSS photometric redshifts are relatively imprecise, however, for faint and nearby galaxies. Instead we use angular size to select potential nearby dwarfs, and consider only the nearest isolated bright galaxies as primaries. As a result, we are able to detect an excess clustering signal from companions down to $Δm_r = 12$, four magnitudes fainter than most recent studies. We detect an over-density of objects at separations $< 400$ kpc, corresponding to about $4.6 \pm 0.5$ satellites per central galaxy, consistent with the satellite abundance expected from the Local Group given our selection function. Although the sample of satellites detected is incomplete by construction, since it excludes the least and most compact dwarfs, this detection provides a lower bound on the average satellite luminosity function, down to luminosities corresponding to the faintest "classical" dwarfs of the Local Group.