Astronomers in Cambridge have discovered a new member of the Local Group of Galaxies in a region of space previously thought to be devoid of nearby dwarf galaxies. The newly discovered galaxy is in the constellation of Antlia and is important because it is one of a very select club of isolated Local Group members. Most Local Group galaxies are satellites of the Milky Way and Andromeda systems leaving only a few outliers to use as probes of the dynamical evolution of the Local Group and for characterising the unperturbed evolution of nearby dwarf galaxies. Astronomers had previously overlooked the galaxy because of its low surface brightness relative to the night sky.
The discovery was made by two research students, Alan Whiting and George Hau , working at the Institute of Astronomy, University of Cambridge, with Dr. Mike Irwin of the Royal Greenwich Observatory, and was made public at the National Astronomy Meeting in Southampton on 10th April 1997. A visual inspection of 894 UK Schmidt Telescope photographic survey plates covering the entire Southern sky had been used to discover many previously uncatalogued large, diffuse low surface brightness objects. Digitisation of these candidates on the Royal Greenwich Observatory PDS microdensitometer confirmed their potentially interesting nature and followup deep CCD imaging on the 1.5m telescope at CTIO showed that two of these objects clearly resolved into stars. The red giants visible in the Antlia dwarf place it in the Local Group; while the blue and red supergiants visible in the Argo dwarf suggest that it lies beyond the Local Group.
V,R,I-band CCD images were combined to produce real colour images of Antlia and Argo . The brightest stars of Antlia (neutral colour) are readily resolved placing it within the boundaries of the Local Group. Argo on the other hand has both hot blue star forming regions in addition to evolved red supergiants. Argo has also been detected in neutral hydrogen and as such appears to be a typical dwarf irregular galaxy located just beyond the Local Group at a distance of about 4 Mpc.
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