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 $\Delta m_r \equiv ({m}_{r,\, {\rm sat}} -\,
{m}_{r,\, {\rm main}}) \sim 6$-$8$, 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 $\Delta m_r = 12$, 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.
