The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda I

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Abstract
Images have been obtained with the Hubble Space Telescope WFPC2 camera of Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of M31. The resulting color-magnitude diagrams reveal for the first time the morphology of the horizontal branch in this system. We find that, in a similar fashion to many of the galactic dSph companions, the horizontal branch (HB) of And~I is predominantly red. Combined with the metal abundance of this dSph, this red HB morphology indicates that And I can be classified as a ``second parameter'' system in the outer halo of M31. This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy.Comment: 26 pages using aas2pp4.sty, including 2 tables and 7 figures, to be published in AJ. Figure 1 is in gif form. To include in main ps file, use xv to create a ps file called Da_Costa.fig1.ps and uncomment appropriate lines in .tex fil
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