The Nature of the Antlia Galaxy: A New Dwarf Irregular in the Outskirts of the Local Group.

The Antlia dwarf galaxy, recently found to be a possible local Group member, is analyzed using VI photometry. The galaxy is resolved into a large number of stars and although the galaxy is intrinsically faint and of low surface brightness, its stellar populations reveal characteristics more typical of faint star-forming dIrs rather than dEs. Significant star formation is currently going on in the central part of Antlia although little or no star formation is taking place in the outer regions. This indicates a two-component (core-halo) morphology which appears to be common, not only in large spirals (disk-halo), but in dwarf galaxies as well. The SFR averaged over the lifetime of the galaxy is estimated to be Ψ‾ 1 Gyr/A∼3-9×1010 Msun yr-1 pc-2 while the more recent star formation, averaged over the last 1 Gyr is much higher (Ψ1 Gyr/A∼3- 9×1010 Msun yr-1 pc-2 for the central region). The total mass locked into stars and stellar remnants is estimated to be M*∼2-4×106 Msun, estimated from the TRGB is 1.32±0.06 Mpc, which places Antlia just beyond the Local Group, and makes it a close companion of the dwarf galaxy NGC 3109 (Δr≥30 kpc), although it is not clear whether they are gravitationally bound.