Dwarf Galaxies Also Have Stellar Halos

Abstract

We present evidence for the existence of an old stellar halo in the dwarf irregular galaxy WLM, an isolated member of the Local Group. The halo consists of population-II stars, with low metallicities and age $\geq 10^{10}$yr. The finding of a halo in a dwarf irregular galaxy argues for a generic mode of galaxy formation that requires a halo in the presence of a disk, regardless of galaxy size. This implies that formation mechanisms are similar along the spiral Hubble sequence, and favors the scenarios where the formation takes place during the original collapse and accretion of the protogalactic gas clouds. Halo formation also appears not necessarily to be related to the presence of a bulge or a nucleus, since WLM lacks both of these components.