2013ApJ...773...17S

A number of mechanisms have been proposed to connect star-forming dwarf irregular galaxies with the formation of non-star-forming dwarf spheroidal galaxies (dSphs), but distinguishing between these mechanisms has been difficult. We use the Via Lactea (VL) dark matter only cosmological simulations to test two well-motivated simple hypotheses–transformation of irregulars into dSphs by tidal stirring and ram pressure stripping following a close passage to the host galaxy, and transformation via mergers between dwarfs–and predict the radial distribution and inferred formation times of the resulting dSphs. We compare this to the observed distribution in the Local Group and show that (1) the observed dSph distribution far from the Galaxy or M31 can be matched by the VL halos that have passed near the host galaxy at least once, though significant halo-to-halo scatter exists, (2) models that require two or more pericenter passages for dSph-formation cannot account for the dSphs beyond 500 kpc such as Cetus and Tucana, and (3) mergers predict a flat radial distribution of dSphs and cannot account for the high dSph fraction near the Galaxy, but are not ruled out at large distances. The models also suggest that for dSphs found today beyond 500 kpc, mergers tend to occur significantly earlier than dwarf-host encounters, thus leading to a potentially observable difference in stellar populations. We argue that tidal interactions are sufficient to reproduce the observed distribution of dSphs if and only if a single pericenter passage is sufficient to form a dSph.