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On the Nature of Andromeda IV

Lying at a projected distance of 40′ or 9 kpc from the center of M31, Andromeda IV is an enigmatic object first discovered during van den Bergh's search for dwarf spheroidal companions to M31. Since it is bluer, more compact, and has a higher surface brightness than other known dwarf spheroidals, it has been suggested that And IV is either a relatively old "star cloud" in the outer disk of M31 or a background dwarf galaxy. We present deep Hubble Space Telescope WFPC2 observations of And IV and the surrounding field, which, along with ground-based long-slit spectroscopy and Hα imagery, are used to decipher the true nature of this puzzling object. We find compelling evidence that And IV is a background galaxy seen through the disk of M31. The moderate surface brightness ( ∼ 24), very blue color (V-I ≲ 0.6), low current star formation rate (∼0.001 M⊙ yr-1), and low metallicity (∼10% Z⊙) reported here are consistent with And IV being a small dwarf irregular galaxy, perhaps similar to Local Group dwarfs such as IC 1613 and Sextans A. Although the distance to And IV is not tightly constrained with the current data set, various arguments suggest it lies in the range 5 ≲ D ≲ 8 Mpc, placing it well outside the confines of the Local Group. It may be associated with a loose group of galaxies, containing major members UGC 64, IC 1727, and NGC 784. We report an updated position and radial velocity for And IV.