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. 
Louis E. Strigari, Risa H. Wechsler
astro-ph.CO astro-ph.GA