On the Nature of Andromeda Iv

Lying at a projected distance of 40 or 9 kpc from the centre of M31, Andromeda IV is an enigmatic object first discovered during van den Bergh’s search for dwarf spheroidal companions to M31. Being bluer, more compact and 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 HST 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 (μ̄V ∼24), very blue colour (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 dataset, 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. Subject headings: galaxies: dwarf – galaxies: individual (Andromeda IV) – galaxies: irregular – galaxies: ISM – galaxies: stellar content