Colossal gas halo surrounding Andromeda galaxy discovered

A massive halo of gas has been found surrounding the Andromeda Galaxy, which is next door to our Milky Way, scientists from the Universities of Notre Dame and Wisconsin, both in the US, reported this week. The gas cloud was observed using NASA’s Hubble Space Telescope.

The halo stretches approximately one million light-years from Andromeda to halfway to the Milky Way. The discovery will give astronomers insight into the evolution and structure of giant spiral galaxies like Andromeda and the Milky Way.

The researchers published their findings in the The Astrophysical Journal (citation below).

Andromeda halo

The Andromeda galaxy is about 6 times larger and 1,000 times more massive than previously measured. (Image: NASA/STScI)

Halos control star-formation rates

Team leader, Notre Dame astrophysicist Nicolas Lehner, said:

“Halos are the gaseous atmospheres of galaxies. The properties of these gaseous halos control the rate at which stars form in galaxies.”


The gigantic halo is believed to contain at least as much mass in its diffuse gas as half of all the stars in the Andromeda Galaxy, also known as Messier 31 or M31, the most massive galaxy in the Local Group of galaxies, which includes our Milky Way and about 45 others.

Andromeda with its estimated 1 trillion stars, has about twice as many stars as the Milky Way. It lies about 2.5 million light-years away and is estimated to be about 25% more luminous than our galaxy.

Halo gas is invisible

The study demonstrates that this halo is a significant feature of the Andromeda Galaxy, and has an apparent size about 100 times the diameter of the moon. However, the gas in the halo is ‘invisible’.

To locate and study the halo, the scientists looked at bright objects in the background whose light was affected by the intervening gas within the halo.

Quasars, super-bright celestial bodies very far away in space, are ideal for such a study because they are so luminous. They shine very brightly because of the presence of gas falling onto supermassive black holes in their cores.

Co-author, J. Christopher Howk, associate professor of physics at at Notre Dame, said:

“As the light from the quasars travels toward Hubble, the halo’s gas will absorb some of that light and make the quasar appear a little darker in just a very small wavelength range.”

“By measuring the dip in brightness, we can tell how much halo gas from M31 there is between us and that quasar.”

Largest nearby halo ever seen

Scientists have observed halos around other galaxies using the Hubble Space Telescope. However, this is the largest one ever observed so near to Earth outside our galaxy.

Because of its proximity, the research team have used 18 quasars projected at different distances from Andromeda to determine its presence and how big it is.

Prof. Lehner said:

“This is a new milestone because typically only one quasar is used to probe the halos of galaxies beyond the Local Group.”

“Here we have assembled a large sample of quasars that directly demonstrate the true extent of the halo of a single massive galaxy.”

Prof. Lehner and colleagues used Hubble’s unique capability to study ultraviolet light at high spectroscopic resolution where spectral characteristics can be observed and accurately be modeled, revealing fundamental data about the nature and extent of the halo gas of galaxies.

The scientists gathered and analysed five years’ worth of data from the Hubble archive to perform this study. They hope to amass a large sample of quasars Hubble has observed to study in more detail the intimate relationship between the halo and its galaxy.

The research team says over Andromeda’s lifetime it has expelled nearly half of all the heavy elements made by its stars beyond the galaxy’s 200,000 light-year-diameter stellar disk.

They predict that if the Milky Way also possesses a similar halo to Andromeda’s, the two galaxies’ halos may merge long before the two massive galaxies collide and eventually form a gargantuan elliptical galaxy starting in about 4 billion years’ time.

Citation: “Evidence for a Massive, Extended Circumgalactic Medium Around the Andromeda Galaxy,” Nicolas Lehner1, J. Christopher Howk1, and Bart P. Wakker. The Astrophysical Journal. Published 4 May, 2015. DOI: 10.1088/0004-637X/804/2/79.