While studying a prominent globular cluster using the Hubble Space Telescope, astronomers realized part of what they had thought was the cluster is actually a previously unknown galaxy. We discover new galaxies at a phenomenal rate, but this one is unusually close, exceptionally old, and curiously distant from any other galaxy.
Thirty light-years is an almost unimaginably long way by Earthly standards. It would take the fastest machine ever built thousands of years to reach a star that distant. Yet even galaxies a million times further away are a part of our cosmic neighborhood. Not quite in our local group of galaxies, but one of the closest galaxies beyond this little huddle.
From where we stand, the newly discovered galaxy, which has been named Bedin 1, lies almost directly behind the very bright globular cluster NGC 6752, notable for containing stars that skip a normal stage in stellar development. Hubble was collecting data on the white dwarf stars within the cluster to get a more accurate measure of its age.
A team led by LR Bedin of the Osservatorio Astronomico di Padova noticed some of the stars photographed by Hubble were substantially further away than the 13,000 light-years at which the cluster sits. They report in Monthly Notices of the Royal Astronomical Society (preprint available to read here) that on closer examination they concluded these stars formed part of a dwarf spheroidal galaxy.
As the name suggests dwarf spheroidal galaxies are small by galactic standards – just 3,000 light-years across in Bedin 1's case, compared to the Milky Way's 100,000.
The most unusual thing about Bedin 1 is how isolated it is from larger galaxies. Most of the dwarf spheroidal galaxies we know are in orbit around the Milky Way, and dwarf galaxies, in general, are usually located quite close to galactic giants. However, it is at least 2 million light-years from Bedin 1 to NGC 6744, its nearest large galaxy. This would be an astonishing distance if it is indeed a satellite galaxy, and one that marks it as possibly the most isolated dwarf galaxy we have yet found.
Bedin 1's age has been estimated at 13 billion years old, not that much younger than the universe itself. Star formation within Bedin 1 apparently ceased 10 billion years ago as it ran out of gas. The absence of hot, bright stars makes Bedin 1 particularly faint, even given its small size.
Most small galaxies have been altered by their interactions with larger objects, but Bedin 1's isolation means it provides an exceptional opportunity to see what happens to an object this size when it is left to evolve in peace, uninfluenced by mighty neighbors.