The Cosmic Abundance of Classical Milky Way Satellites
  We study the abundance of satellites akin to the brightest, classical dwarf
spheroidals around galaxies similar in magnitude and isolation to the Milky Way
and M31 in the Sloan Digital Sky Survey. From a combination of photometric and
spectroscopic redshifts, we bound the mean and the intrinsic scatter in the
number of satellites down to ten magnitudes fainter than the Milky Way.
Restricting to magnitudes brighter than Sagittarius, we show that the Milky Way
is not a significant statistical outlier in its population of classical dwarf
spheroidals. At fainter magnitudes, we find an upper limit of 13 on the mean
number of satellites brighter than the Fornax dwarf spheroidal. Methods to
improve these limits that utilize full photometric redshift distributions hold
promise, but are currently limited by incompleteness at the very lowest
redshifts. Theoretical models are left to explain why the majority of dark
matter subhalos that orbit Milky Way-like galaxies are inefficient at making
galaxies at the luminosity scale of the brightest dwarf spheroidals, or why
these subhalos predicted by Lambda-CDM do not exist.
