The observed and predicted spatial distribution of Milky Way satellite galaxies

ABSTRACT We review evidence that the census of Milky Way satellites similar to those knownmay be incomplete at low latitude due to obscuration and in the outer halo due to adecreasing sensitivity to dwarf satellites with distance. We evaluate the possible im-pact that incompleteness has on comparisons with substructure models by estimatingcorrections to the known number of dwarfs using empirical and theoretical models.Under the assumption that the true distribution of Milky Way satellites is uniformwith latitude, we estimate a 33% incompleteness in the total number of dwarfs due toobscuration at low latitude. Similarly, if the radial distribution of Milky Way satellitesmatches that of M31, or that of the oldest sub-halos or the most massive sub-halosin a simulation, then we estimate a total number of Milky Way dwarfs ranging from1 – 3 times the known population. Although the true level of incompleteness is quiteuncertain, the fact that our extrapolations yield average total numbers of MW dwarfsthat are realistically 1.5 – 2 times the known population, shows that incompletenessneeds to be taken seriously when comparing to models of dwarf galaxy formation.Interestingly, the radial distribution of the oldest sub-halos in a ΛCDM simulation ofa Milky Way-like galaxy possess a close match to the observed distribution of M31’ssatellites, which suggests that reionization may be an important factor controlling theobservability of sub-halos. We also assess the prospects for a new SDSS search forMilky Way satellites to constrain the possible incompleteness in the outer halo.Key words: galaxies: haloes — galaxies: Local Group — galaxies: dwarf — methods:N-body simulations