Session 11 - LMC, Dwarf Galaxies.
Display session, Monday, January 15
North Banquet Hall, Convention Center

## [11.08] HST Observations of Andromeda I, a dSph Galaxy in the Halo of M31

T. E. Armandroff (KPNO/NOAO), G. S. D. Costa (MSSSO), N. Caldwell (FLWO), P. Seitzer (U. Mich.)

Images have been obtained with the Hubble Space Telescope WFPC2 camera of the central regions 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 systems, the horizontal branch of And I is predominantly red, though blue horizontal branch (HB) stars and RR Lyrae variable stars are also found. A comparison with the giant branches of standard Galactic globular clusters indicates a mean abundance for And I of [Fe/H] \approx -1.5 dex. For such an abundance, the horizontal branch morphology expected on the basis of the (HB morphology, [Fe/H]) relation followed by the majority of Galactic globular clusters, has relatively many more blue HB stars, and many fewer red HB stars, than is shown by the And I data. In this sense we can then say that And I shows the ``second parameter effect" in the same way as do many of the Galactic dSph systems. Within our Galaxy the second parameter effect is generally interpreted as a manifestation of age differences among the systems in the Galactic halo. This age difference interpretation of the second parameter effect is a cornerstone of the chaotic halo formation model, advocated initially by Searle and Zinn, in which the outer Galactic halo is built out of the destruction of dwarf galaxy satellites over an extended interval of perhaps \sim3--5 Gyr. Our results for And I are consistent with the hypothesis that the outer halo of M31 formed in a similar chaotic manner to the outer halo of our Galaxy.