Astronomers find new member of Local Group

Dim, diminutive, and isolated, a newly discovered galaxy called Antlia doesn't look like much -- which is precisely why astronomers are thrilled to have found it.

 Named for the constellation in which it resides, Antlia is the newest member of the Local Group, a collection of 30 or so galaxies dominated by two massive ones -- the Milky Way and Andromeda. Most of the small galaxies in the Local Group are gravitationally bound to one of these large galaxies. Antlia, in contrast, lives in isolation at the edge of the Local Group, unfettered by external gravitational influences. This nearby galaxy, which lies about 3 million light-years from the Milky Way, provides astronomers with a chance to detail how small galaxies evolve when they aren't enslaved by larger ones.

"This is an outlier, an object in the middle of nowhere that shows what happens in the absence of gravitational disruption," notes codiscoverer Michael Irwin of the Royal Greenwich Observatory in Cambridge, England.

 Only one other small galaxy, a faint body called Tucana, goes it alone in the Local Group, he adds. Because of their size, shape, and featureless appearance, both galaxies are classified as dwarf spheroidals. Such galaxies have proved intriguing because they are thought to contain a much lower proportion of visible matter to dark matter -- hypothetical, unseen material -- than other galaxies do. In addition, several cosmological theories predict that the universe contains many more small galaxies than large ones, notes Rosemary F.G. Wyse of Johns Hopkins University in Baltimore.

 Before researchers discovered Tucana a few years ago, some astronomers suggested that dwarf spheroidals were simply remnants of larger galaxies that had been ripped apart by the massive galaxies they orbited. The discovery of Antlia cements the notion that dwarf spheroidals can arise in the absence of a gravitational tussle.

 Irwin and his colleagues, Alan Whiting and George Hau of the University of Cambridge, reported their findings this week in Southampton, England, at the Royal Astronomical Society's annual National Astronomy Meeting. They identified Antlia, as well as another dwarf galaxy, Argo, which lies a little too far away to qualify as a member of the Local Group, by searching 894 survey photographs of the southern sky. Computer analysis then homed in on the most likely candidates among the several dim, diffuse objects detected, and observations last month with the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in La Serena, Chile, revealed the two to be nearby galaxies.

 Antlia's status as an outlier offers another benefit, Irwin notes. As a way of estimating the age of the universe without measuring the cosmological rate of expansion, researchers can take the current positions, velocities, and masses of members of the Local Group and turn the clock backward, calculating the time of the Big Bang as the moment when all the galaxies would have overlapped. Because Antlia hasn't interacted with other Local Group members, its prior motion can be reconstructed more accurately, Irwin says. --