The discovery of three 鈥渘aked鈥� galaxies 鈥� not cloaked in dark matter 鈥� shocked astronomers in 2003 and prompted an urgent hunt for more. But a detailed new X-ray observation of an isolated galaxy has not reproduced the finding, leading researchers to suggest galaxies may form or evolve in multiple ways.
Conventional theories propose that galaxies form within clumps 鈥� or halos 鈥� of dark matter. Although it cannot be seen directly by astronomers, dark matter can be detected by its gravitational effect on objects in or around these galaxies.
In 2003, astronomers led by Aaron Romanowsky, then at the University of Nottingham, UK, used an optical telescope to study how gas clouds in blob-shaped 鈥渆lliptical鈥� galaxies move. The team found three galaxies that appeared to have little or no dark matter around them.
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Two of the galaxies were grouped with other galaxies, making it conceivable that their dark matter halos had been stripped away by interactions with their neighbours. But one naked galaxy was out on its own 鈥� apparently too isolated to have ever interacted with other galaxies.
Lonely as a cloud
That lone galaxy prompted Ewan O鈥橲ullivan, at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, to track down other isolated galaxies 鈥� which are rare compared to those in groups 鈥� to discover whether they had dark matter halos.
He used NASA鈥檚 Chandra X-ray Telescope to study one in detail 鈥� a galaxy called NGC 4555 which shows no large galaxies within about 400 million light-years. The galaxy is bathed in a hot, X-ray-emitting gas cloud, implying the galaxy is swaddled in a dark matter halo with 300 times the cloud鈥檚 mass.
鈥淭his is the first time anyone鈥檚 done decent X-ray studies of isolated ellipticals,鈥� O鈥橲ullivan told New 杏吧原创. His results will be published in the 1 November issue of the Monthly Notices of the Royal Astronomical Society.
Integrating observations
But even though his result backs standard theories of galaxy formation, O鈥橲ullivan says the discoveries by Romanowsky 鈥� who he is working with 鈥� provoke some interesting ideas.
One is that dark matter halos do not form in the way predicted by the standard 鈥渃old dark matter鈥� model. But O鈥橲ullivan says: 鈥淚 think it鈥檚 more likely they are either stripped of their dark matter after they form or they form with much less dark matter than predicted.鈥�
He hopes to observe the X-ray emission of about 30 other lone elliptical galaxies. Already, preliminary results suggest one of Romanowsky鈥檚 two naked-galaxies-in-groups shows more X-ray emitting gas than expected, which could imply a dark matter halo. Chandra is also scheduled to view Romanowsky鈥檚 puzzling isolated galaxy to further the search for evidence of dark matter.
鈥淭here鈥檚 quite a bit of debate about how elliptical galaxies form,鈥� says O鈥橲ullivan. If Romanowsky鈥檚 result holds up, he says, 鈥渋t鈥檚 possible they鈥檙e forming through multiple methods鈥�.
Journal reference: Monthly Notices of the Royal Astronomical Society (vol 354, p 935)