The observed and predicted spatial distribution of Milky Way satellite galaxies

We review evidence that the census of Milky Way satellites similar to those known may be incomplete at low latitude due to obscuration and in the outer halo due to a decreasing sensitivity to dwarf satellites with distance. We evaluate the possible impact that incompleteness has on comparisons with substructure models by estimating corrections to the known number of dwarfs using empirical and theoretical models. Under the assumption that the true distribution of Milky Way satellites is uniform with latitude, we estimate a 33 per cent incompleteness in the total number of dwarfs due to obscuration at low latitude. Similarly, if the radial distribution of Milky Way satellites matches that of M31, or that of the oldest sub-haloes or the most massive sub-haloes in a simulation, then we estimate a total number of Milky Way dwarfs ranging from 1–3 times the known population. Although the true level of incompleteness is quite uncertain, the fact that our extrapolations yield average total numbers of MW dwarfs that are realistically 1.5–2 times the known population shows that incompleteness needs to be taken seriously when comparing models of dwarf galaxy formation. Interestingly, the radial distribution of the oldest sub-haloes in a ΛCDM simulation of a Milky Way-like galaxy is a close match to the observed distribution of M31's satellites, which suggests that reionization may be an important factor controlling the observability of sub-haloes. We also assess the prospects for a new SDSS search for Milky Way satellites to constrain the possible incompleteness in the outer halo.