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A. M. N. Ferguson (IoA, Cambridge), R. F. G. Wyse (JHU, Baltimore), J. S. Gallagher (University of Wisconsin, Madison), R. A. Johnson (IoA, Cambridge)
Andromeda IV is an enigmatic object first discovered during van den Bergh's search for dwarf spheroidal companions to M31. Being bluer, smaller and more compact than the other dwarf spheroidals, it has been suggested that Andromeda IV is either a large star cloud in the outer disk of M31 or a background, possibly Local Group, galaxy. If a star cloud, And IV would have a size of ~200 pc and lie at a deprojected distance of 25 kpc in M31's disk, and would thus be an interesting probe of extreme outer disk star formation. We present a deep WFPC2 study of Andromeda IV along with supporting ground-based observations (deep Halpha imagery & long-slit spectroscopy) that are used to discern the true nature of this puzzling object.
We have also used these data to construct the deepest colour-magnitude-diagram to date of the stellar populations in the outskirts of M31. At the location probed by our WFPC2 field, the halo and disk populations are expected to contribute in approximately equal numbers to the observed stellar density. We discuss interesting new contraints on the nature of the outer disk and halo giant populations in M31, derived from the combination of our Andromeda IV data with archival WFPC2 fields.