Hubble maps giant halo around Andromeda Galaxy

In a landmark study, scientists using NASA's Hubble Space Telescope have mapped the immense envelope of gas, called a halo, surrounding the Andromeda galaxy, our nearest large galactic neighbor.

Scientists were surprised to find that this tenuous, nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy—about halfway to our Milky Way—and as far as 2 million light-years in some directions. This means that Andromeda's halo is already bumping into the halo of our own galaxy.

They also found that the halo has a layered structure, with two main nested and distinct shells of gas. This is the most comprehensive study of a halo surrounding a galaxy.

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Credit: NASA, ESA, and E. Wheatley (STScI)

This illustration shows the location of the 43 quasars scientists used to probe Andromeda's gaseous halo. These quasars--the very distant, brilliant cores of active galaxies powered by black holes--are scattered far behind the halo, allowing scientists to probe multiple regions.

Looking through the immense halo at the quasars' light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo's material.

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Credit: NASA, ESA, J. DePasquale and E. Wheatley (STScI), and Z. Levay (background image)

This illustration depicts the gaseous halo of the Andromeda galaxy if it could be seen with the naked eye. At a distance of 2.5 million light-years, the majestic spiral Andromeda galaxy is so close to us that it appears as a cigar-shaped smudge of light high in the autumn sky.

If its gaseous halo could be seen with the naked eye, it would be about three times the width of the Big Dipper—easily the biggest feature on the nighttime sky.

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Credit: NASA, ESA, Digitized Sky Survey 2 and T.M. Brown (STScI)

This video zooms in on the location of Hubble observations in the Andromeda Galaxy's giant stellar stream. An area which appears virtually empty in the wide-field view is revealed as being packed with many stars

The halo is a relatively sparse sphere of stars and dark matter which surrounds a galaxy's disc.

Credit: NASA, ESA, Digitized Sky Survey 2 and T.M. Brown (STScI)

This video zooms in on the location of Hubble observations in the Andromeda Galaxy's giant stellar stream. An area which appears virtually empty in the wide-field view is revealed as being packed with many stars

The halo is a relatively sparse sphere of stars and dark matter which surrounds a galaxy's disc.