Milky Way breakthrough: Mysterious space ‘halo’ offers ‘clue to galaxy’s past’

Using the Hubble Space Telescope, astronomers have mapped the enormous halo of gas surrounding the Andromeda Galaxy. The most detailed of its kind, the map shows how the halo – electrically charged gas – surrounding this spiral galaxy extends by about 1.3 million light-years towards the Milky Way. This is around half the distance of the galaxy, and as much as two million light-years in some directions.

Peculiarly, the halo is invisible to the naked eye.

However, researchers say that if it was visible, it would be around three times the width of the Plough or the Big Dipper.

This would make it the biggest feature in the night sky.

Team member of the study, Samantha Berek at Yale University, told the BBC’s Science Focus magazine how the discovery might further understanding of our own universe.

She said: “Understanding the huge halos of gas surrounding galaxies is immensely important.

“This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae.

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“It’s full of clues regarding the past and future evolution of the galaxy.

“We’re finally able to study it in great detail in our closest galactic neighbour.”

The team found that Andromeda’s halo is composed of two distinct layers.


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The “inner” shell has a more complex structure than the outer shell.

This is likely a result of supernovae in the galaxy’s disc.

Violent explosions – the deaths of giant stars – eject heavy elements into space, which were also detected in high amounts within the halo.

Ultraviolet light from 43 distant quasars was studied in order to map additional detail about the halo.

The researchers used Hubble’s Cosmic Origins Spectrograph instrument to analyse how this background light was absorbed by the halo’s gas in different regions.

This revealed variations in the gas’s structure.

Andromeda is thought to be similar in size and shape to the Milky Way.

The findings, therefore, could provide significant and unprecedented insights into our own galactic halo, which is far more difficult to map from our own position inside the Milky Way.

You can read next month’s issue of BBC Science Focus by subscribing here.

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