Hubble Investigates The Enormous Halo Of The Andromeda Galaxy

The size of Andromeda's halo if it was visible to the naked eye. NASA, ESA, J. DePasquale and E. Wheatley (STScI) and Z. Levay

Galaxies are surrounded by a spherical halo where diffuse hot gas, dark matter, and a few stars reside. The Hubble Space Telescope has now studied the halo of Andromeda, the closest galaxy to the Milky Way of a similar size. If you think the Andromeda galaxy is big, its halo will blow you away.

The halo stretches over 1 million light-years from the galaxy, which in itself is 220,000 light-years across. The study, published in The Astrophysical Journal, highlights how the halo is not a simple structure but actually complex due to the internal processes of the galaxy.

“We find the inner shell that extends to about a half million light-years is far more complex and dynamic,” study leader Nicolas Lehner, from the University of Notre Dame, said in a statement. “The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy’s disk more directly affecting the inner halo.”

The team detected the presence of heavy elements in the gaseous halo. This is a clear sign of where the material came from. Heavy elements are produced in stars and distributed across space when stars reach the end of thier lives, especially when they go supernova.

Illustration of the extent of the halo and the quasars used to probe it. NASAESA, and E. Wheatley (STScI)

These incredible observations were possible thanks to Project AMIGA (Absorption Map of Ionized Gas in Andromeda), which observed 46 quasars (extremely bright galaxies) in the line of sight of where the halo was expected to be. These were used as “backlights” to illuminate the diffused halo so that it could be studied by Hubble.

“Previously, there was very little information – only six quasars – within 1 million light-years of the galaxy. This new program provides much more information on this inner region of Andromeda’s halo,” explained co-investigator J. Christopher Howk, also of Notre Dame. “Probing gas within this radius is important, as it represents something of a gravitational sphere of influence for Andromeda.”

We do not have instruments to conduct these kind of studies of other major galaxies, so AMIGA serves as a proxy for what might go on around other spiral galaxies in the cosmos.

“Understanding the huge halos of gas surrounding galaxies is immensely important,” explained co-investigator Samantha Berek of Yale University. “This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It’s full of clues regarding the past and future evolution of the galaxy, and we’re finally able to study it in great detail in our closest galactic neighbor.”

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