The dark matter galaxy that could be orbiting our Milky Way

A satellite galaxy made almost entirely of dark matter could be orbiting the Milky Way, a new image has revealed.

It is one of 24 neighbouring galaxies spotted by the Sloan Digital Sky Survey, which has recorded the night sky in greater detail than ever before.

The results have doubled the number of known dwarf galaxies orbiting the Milky Way.

The latest finding is particularly significant as it appears to have a very low light-to-mass ratio - a billion times less bright than the Milky Way itself.

Yale Professor of Astronomy Marla Geha, who led the team that made the discovery, believes that the galaxy, named 'Segue 1' is mainly composed of dark matter.

The scientists studying the galaxy believe this is due to the fact that despite having a mass of a million suns, it is not nearly as bright as it should be.
'I’m excited about this object,' she said. 'Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass.

'These dwarf galaxies tell us a great deal about galaxy formation, for example, different theories about how galaxies form predict different numbers of dwarf galaxies versus large galaxies. So just comparing numbers is significant.'

The discovery of dark matter is often significant to astronomers, who believe it makes up around 85 per cent of the mass in the universe. Despite the fact that it does not absorb or emit light, it is possible to examine it's gravitational effect on surrounding matter. The scientists hope that discoveries lke Segue 1 will provide clues about how galaxies evolve.

'The galaxies I now consider bright used to be the least luminous ones we knew about,' Geha continued. 'It’s a totally new regime. This is a story that’s just unfolding.'

The team's findings will shortly be published in The Astrophysical Journal.