A new dwarf member of the Local Group of galaxies has been identified by CCD images obtained at the CTIO 1.5-m telescope. The discovery was made by Alan Whiting, George Hau (both of Cambridge), and Mike Irwin (RGO), based on a candidate identified during visual inspection of the UK Schmidt southern sky survey plates. The newly discovered galaxy, in the constellation of Antlia, lies in a region of space previously thought to be devoid of nearby dwarf galaxies. The Antlia dwarf is of special interest as it is isolated in the Local Group, rather than being a satellite of the Milky Way or M31. It can thus be used as a probe of the dynamical evolution of the Local Group.
Visual inspection of 894 UK Schmidt Telescope photographic plates covering the entire southern sky had been used to discover many uncatalogued large, diffuse low surface brightness objects. Digitization of candidates on the RGO PDS microdensitometer confirmed their potentially interesting nature, and followup deep CCD imaging on the CTIO 1.5-m telescope showed that two such objects could be clearly resolved into stars. Red giants visible in the Antlia dwarf place it in the Local Group, while blue and red supergiants visible in the second candidate dwarf (in the constellation Argo) suggest that it lies beyond the Local Group.
On the sky survey plate Antlia looks like a fuzzy low surface brightness patch some 3' in diameter. This appearance is characteristic of potential new Local Group dwarf members and a host of interlopers such as old planetary nebulae, Galactic reflection nebulae, and distant low surface brightness galaxies. Whiting, Hau, and Irwin thus required followup CCD imaging at CTIO to classify their dwarf candidates.
The Antlia dwarf appears to be similar to other dwarf spheroidal galaxies.
The brightest stars are easily resolved and the galaxy appears smooth and
devoid of any obvious concentrations of stars or clusters. A deep H image
reveals no obvious star forming regions and there are no young hot blue
stars apparent in any of the deep CCD images taken. The color and luminosity
distribution of Antlia stars is typical of the Milky Way satellite dwarf
spheroidals; comparison with these and other nearby galaxies suggests
that Antlia is roughly 1 Mpc distant. Antlia is most similar in
appearance to the Tucana dwarf, the only other isolated Local Group dwarf
spheroidal galaxy known. At a distance of 1 Mpc the apparent size of
Antlia on the deep CCD images implies that it is only 1-2 kpc in diameter
and probably only contains a million or so stars, placing it firmly at
the faint end of the galaxy luminosity function.
Being far away from both the Milky Way and M31, Antlia will help to constrain the age and total mass of the Local Group via timing arguments. Members of the Local Group were born close together in space but with sizeable relative velocities. Knowing their present distances and velocities plus the masses of the larger members makes it possible to work backwards to derive an age of the Universe independent of Ho.