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Say hello to the halo

A partial view of the large spiral galaxy M81.

A partial view of the large spiral galaxy M81. Japan's Subaru telescope has studied its outskirts, looking for "fossil" remains of smaller galaxies that were devoured by M81.

  • Galaxy M81, 11.7 million light-years away
  • It’s outskirts are different to the Milky Way’s
  • Halo forms through merger of galaxies

Astronomers have used Japan’s giant Subaru telescope to study the outskirts of large spiral galaxy, in an effort to understand more about galaxy growth.

Astronomers think that large galaxies such as our Milky Way, become bigger over time by gobbling up smaller galaxies.

In the case of our Galaxy, there’s plenty of evidence for this process – several actual small galaxies have been spotted crashing into the Milky Way, attracted by its huge gravity. And swarms of stars all moving together within the Milky Way are thought to be the remnants of past episodes of galactic cannibalism.

But the “gobbling up” process would not be absolute. Many stars from the smaller galaxies would get left behind, loitering on the outskirts of the Milky Way in a region astronomers call the “halo“.

By studying galaxy halos, astronomers can learn more about these “fossil” remains of past galaxies, and thereby learn more about the process of large galaxy growth.

There are still lots of things we don’t know about the Milky Way’s halo, or galaxy halos in general for that matter. For a start, it’s hard to study our Galaxy’s halo from the “inside”, and most other galaxies are too far away to detailed observations to be made.

Japan's giant Subaru Telescope in Hawaii.

Japan's giant Subaru Telescope in Hawaii.

Outer limits

Enter Japan’s huge 8.2-metre Subaru telescope, situated in Hawaii. Astronomers have used it to make observations of the outskirts of a spiral galaxy called M81, the largest of a group of 30-plus galaxies over 11 million light-years from Earth.

They managed to identify a faint outer region to the galaxy, beyond its bright main section. They also gathered information on enough individual stars in this region to analyse its chemical properties.

They found that what they saw does not quite fit in with the conventional notion of a galaxy halo. It has basically the same type of spread of stars as the Milky Way’s halo, but overall it could be several times brighter and contain nearly twice as much matter in the form of heavy elements.

Questions raised include: does the definition of a halo need to be widened? Do M81’s outskirts have a different structure to the Milky Way’s? Is this because M81 gobbled more or different galaxies in its past, compared to the Milky Way?

Either way, it seems to be becoming clearer that the outer limits of galaxies are more complex than previously expected.

Written by Jonathan Nally, Images courtesy NAOJ.

Bursting ‘bubbles’ give our Galaxy gas

The regions of our Galaxy the researchers studied

The regions of our Galaxy the researchers studied. More gas clouds were found in the region on the right than in the region on the left.

  • 650 Milky Way gas clouds studied
  • Each contains 700 times the mass of the Sun
  • Clouds might recycle gas in and out of the Galaxy

Like bubbles bursting on the surface of a glass of champagne, ‘bubbles’ in our Galaxy burst and leave ‘flecks’ of material in the form of clouds of hydrogen gas, researchers using CSIRO’s Parkes telescope have found.

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 disc of our Galaxy is surrounded by a thinner halo of stars, gas and ‘dark matter’.

The Parkes radio telescope

The Parkes radio telescope

The halo clouds skim the surface of our Galaxy, sitting 400 to 10,000 light-years outside the Galactic disc. 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 disc and halo,” Dr McClure-Griffiths said.

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

Gas is “spritzing” up our Galaxy

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.

An image made with the Parkes radio telescope of some of the 'halo clouds' above the main body of our Galaxy.

An image made with the Parkes radio telescope of some of the 'halo clouds' above the main body of our Galaxy.

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 disc into the lower halo.

“We’ve found this churned-up gas is ‘spritzing’ the surface of the Galactic disc 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 overnight at a news conference at a meeting of the American Astronomical Society in Miami, Florida.

Adapted from information issued by CSIRO / A Ford (U. Michigan), N. McClure-Griffiths (CSIRO Astronomy and Space Science) / NASA / JPL-Caltech / David McClenaghan.