Three 鈥渘aked鈥� galaxies, not surrounded by cloaks of dark matter, have been found by an international team of astronomers.
The discovery overturns the long-held assumption that all galaxies are enclosed by dark matter halos. 鈥淭he result was certainly not what we expected,鈥� says team member Michael Merrifield, at the University of Nottingham, UK.
Virtually all the matter in the Universe is dark, i.e. it does not emit radiation that can be observed by astronomers. But it can be detected by measuring the velocity of objects in the galaxy鈥檚 outlying regions. The further away from the centre of the galaxy the object lies, the more susceptible it is to surrounding dark matter, and the faster it moves.
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Astronomers have been measuring this phenomenon in spiral galaxies since the 1970s, using the bright radio signal of large gas clouds as their tracer. However, elliptical galaxies, thought to form by the collision of spiral galaxies, do not harbour large gas clouds and so had been overlooked.
The new research was made possible by a custom-built spectrograph that was able to detect a different tracer in elliptical galaxies 鈥� planetary nebulae. The spectrograph was installed at the William Herschel Telescope on La Palma.
So slow
The astronomers watched planetary nebulae in the outer regions of three elliptical galaxies and found them moving so slowly that the presence of dark matter is virtually ruled out.
The result raises the intriguing possibility that most elliptical galaxies are devoid of dark matter halos. But the team will wait until the entire survey of 25 galaxies is finished before drawing any conclusions, says Merrifield. He hopes that will be in a couple of years.
According to some theories of galaxy formation, dark matter halos are essential to form galaxies quickly. 鈥淚ts certainly hard to see how you can have a galaxy without dark matter,鈥� says Merrifield鈥檚 colleague Aaron Romanowsky. 鈥淭his result could imply that galaxy formation and evolution are more complicated than we thought.鈥�
Merrifield agrees: 鈥淲e are driven towards thinking that something in the galaxy鈥檚 subsequent evolution has stripped away the dark matter. It鈥檚 now down to the theorists to come up with ideas to explain this finding.鈥�
The research will be presented at the UK National Astronomy Meeting in Dublin, Ireland on Wednesday.