Deep Hubble Space Telescope STIS color-magnitude diagrams of the dwarf irregular galaxy WLM: Detection of the horizontal branch

We have obtained V- and I-band photometry for 1886 stars down to I = 27 and y = 28 in the held of the dwarf irregular galaxy WLM, using deep Space Tel escope Imaging Spectrograph (STIS) CL- and LP-band images taken in the para llel mode with the Hubble Space Telescope. The photometry is used to study the horizontal branch identified in WLM. The horizontal branch, extending b lueward from the red giant clump, is an unambiguous signature of an old pop ulation. We demonstrate that it is possible to reach the horizontal branch of an old population at a distance of similar or equal to 1 Mpc using STIS, with relatively short exposure times. From the VI color-magnitude diagram, we obtain an accurate distance modulus (m - M)(0) = 24.95 +/- 0.13 for WLM by using the Y magnitude of the horizontal branch, and by adopting E(V-I)= 0.03. The implications are that (1) WLM formed stars at high redshift and (2) the old population of WLM can be representative of a protogalactic frag ment, related to those accreted to form the Milky Way halo.