The tiniest galaxies in the Universe have been discovered lurking in a nearby galaxy cluster. As well being the first new class of galaxy for 70 years, the diminutive galaxies could help solve one of cosmology鈥檚 trickiest problems.
Astronomers led by Steve Phillipps, at the University of Bristol, UK, and Michael Drinkwater, University of Queensland, Australia, first found the galactic minnows in a survey of the nearby Fornax cluster of galaxies, using the 3.9 metre Anglo-Australian telescope.
鈥淲e took spectra of everything so we wouldn鈥檛 miss anything,鈥� says Phillipps, and the strategy paid off. The team found themselves with seven objects that had previously been thought to be stars in the Milky Way, but were quite obviously members of the Fornax cluster. The puzzle was that, for galaxies, they were minuscule.
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The team has now carried out a detailed analysis, using data from ESO鈥檚 Very Large Telescope and the Hubble Space Telescope. They conclude that the galaxies represent a new type of galaxy and have named them Ultra Compact Dwarfs (UCDs).
An entire UCD would fit into the volume of space separating the Earth from the brightest stars in the constellation of Orion, while the mass of a UCD is roughly between one and 10 per cent that of a standard dwarf galaxy. UCDs contain a few tens of millions of stars, but all squeezed together in conditions reminiscent of a dwarf galaxy鈥檚 nucleus.
Stripped down
That set the astronomers thinking because, in order to explain the number of large galaxies observed in clusters, cosmologists always predict many more dwarf galaxies than are actually seen.
The UCDs could solve the problem. 鈥淲e think the UCDs were once larger objects,鈥� Phillipps explains. Then, something stripped them down into their current configuration.
鈥淲e believe that the gravitational fields of larger galaxies in the cluster robbed some dwarf galaxies of their outer stars, leaving just the central nucleus, which is what we see as a UCD,鈥� he says.
If UCDs were once dwarf galaxies, then astronomers have found at least a partial explanation for their deficit in the Universe today. Phillipps is cautiously optimistic: 鈥淲e haven鈥檛 found all the 鈥榣ost鈥� dwarfs by any means, but any we do find skews the distribution in the right direction. And, so far, we have probably only picked up the brightest UCDs.鈥�
Journal reference: Nature (vol 423, p 519)