News in Science

'Candles' light up Milky Way's dark side

Far side stars A new picture of the far side of our galaxy has come to light following the discovery of stars hidden in a giant hydrogen cloud flaring out from the galactic disk.

The findings, reported in the journal Nature, will help scientists better understand the structure of the Milky Way.

"If a significant number of similar stars could be identified in the outer reaches of the galaxy, they could be used as tracers ... to probe the distribution of dark matter," says study author Dr Patricia Whitelock of the South African Astronomical Observatory.

The far side of the Milky Way is difficult to study, because it's blocked from Earth's view by billions of stars, gas clouds and dust.

Previous observations looking on either side of the centre, but away from the central region itself, suggest that the gaseous disk of the Milky Way shows a marked flaring from about 45,000 light years out from the galactic centre.

"The flaring of the gas disc is observed in some other directions away from the Galactic centre [but] nothing whatsoever was known about the flaring behind the Galactic centre," says Whitelock.

It's been hypothesised that dark matter causes these clouds on the far side of the galaxy's disk to flare, resulting in the thickening of the outer regions of the disk.

Standard candles

Whitelock and colleagues used infrared photometry to peer through the clouds, identifying five Cepheid variable stars.

Cepheid variables are young stars which regularly change in luminosity. The timing and amount of the change, is directly linked to the mass of the star.

Scientists can use these stars as standard candles to determine astronomical distances.

Whitelock and colleagues were able to place these stars between 39,000 and 66,000 light years out from the centre of the galaxy. Our Sun and solar system are 27,000 light years out.

More importantly, the authors determined these five stars are between 3000 and 6000 light years above or below the galactic disk, which has an average thickness of about 1000 light years.

The location of these stars matches the pattern of flaring of atomic hydrogen clouds out from the galactic disk.

The presence of these relatively young stars so far from the galactic plane is puzzling, unless they're in the flared outer region of the disk.

"In view of what is seen on either side of the [galactic] centre we assume that gas flaring exists where we find the Cepheids," says Whitelock.

Dark matter

By constraining the location of the flaring, scientists will improve their understanding of the distribution of dark matter around our galaxy, says astronomer Professor Gary Da Costa of the Australian National University .

"The usual assumption is that dark matter is distributed spheroidally around the galaxy like a sphere or football," says Da Costa.

"But there are some suggestions that you need to have dark matter in the disk, to model the flaring we see. This new data gives you accurate distances to the flare, and may help us to understand whether that disk of dark matter is needed or not."