Bursting 'bubbles' the origin of galactic gas clouds, astronomers find

Their study explains the origin of these clouds for the first time.

Swinburne University PhD student Alyson Ford (now at the University of Michigan) and her supervisors; Dr Naomi McClure-Griffiths (CSIRO Astronomy and Space Science) and Felix Lockman (US National Radio Astronomy Observatory), have made the first detailed observations of 'halo' gas clouds in our Galaxy.

Just as Earth has an atmosphere, the main starry disk of our Galaxy is surrounded by a thinner halo of stars, gas and 'dark matter'.

The halo clouds skim the surface of our Galaxy, sitting 400 to 10 000 light-years outside the Galactic disk. They are big: an average-sized cloud contains hydrogen gas 700 times the mass of the Sun and is about 200 light-years across.

"We're studying the clouds to understand what role they play in recycling material between the disk and halo," Dr McClure-Griffiths said.

"The clouds can fall back down into the main body of the Galaxy, returning gas to it."

The researchers studied about 650 clouds and found striking differences between them in different areas of the Galaxy. One part of the Galaxy had three times as many clouds as another next to it, and the clouds were twice as thick.

The region with lots of thick clouds is where lots of stars form, while the region with fewer clouds also forms fewer stars.

But the halo clouds aren't found exactly where stars are forming right now. Instead, they seem to be linked to earlier star formation.

Massive stars grow old quickly. After a few million years they shed material into space as a 'wind' and then explode.

This violence creates bubbles in the gas in space, like the holes in a Swiss cheese.

"Stellar winds and explosions sweep up gas from the Galactic disk into the lower halo.

"We've found this churned-up gas is 'spritzing' the surface of the Galactic disk in the form of halo clouds."

A star-forming region is active for less than a million years, but a super-bubble in the Galaxy takes 20 or 30 million years to form.

"Just as yeast takes a while to make wine bubbly, stars take a while to make the Galaxy bubbly," Dr McClure-Griffiths said.

The halo clouds are distinct from a larger population of 'high-velocity clouds' that also sail outside the galaxy. The halo clouds move in tandem with the rotating Galaxy, while the high-velocity clouds scud along much faster.

This study is the first to accurately locate the halo clouds in relation to the main body of the Galaxy. Its findings were presented at a news conference at a meeting of the American Astronomical Society in Miami, Florida.