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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.