Dwarf Galaxies Also Have Stellar Halos

We present evidence for the existence of an old stellar halo in the dwarfirregular galaxy WLM, an isolated member of the Local Group. The halo consistsof population-II stars, with low metallicities and age $\geq 10^{10}$yr. Thefinding of a halo in a dwarf irregular galaxy argues for a generic mode ofgalaxy formation that requires a halo in the presence of a disk, regardless ofgalaxy size. This implies that formation mechanisms are similar along thespiral Hubble sequence, and favors the scenarios where the formation takesplace during the original collapse and accretion of the protogalactic gasclouds. Halo formation also appears not necessarily to be related to thepresence of a bulge or a nucleus, since WLM lacks both of these components.Comment: ApJ (Letters), in press. LaTeX, using macro aasms.sty, 13 pages. 4 figures available on reques