Hubble maps a giant halo round the Andromeda galaxy

Scientists were amazed to discover this tenuous, nearly invisible halo of diffuse plasma extends 1.3 million light-years in the galaxy–roughly halfway to our Milky Way–and as far as 2 million light-years in some instructions. This means that Andromeda’s halo is currently bumping into the halo of our galaxy.

At a distance of 2.5 million Alight-years, the majestic spiral Andromeda galaxy it 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.[ NASA, ESA,J.
DePasquale and E. Wheatley
(STScl) and Z. Levay]

They also found that the halo has a layered structure, with just two main nested and distinct shells of gasoline. This is the most comprehensive analysis of a halo surrounding a galaxy. “Understanding the huge halos of gas surrounding galaxies is immensely important,” explained coinvestigator Samantha Berek of Yale University at New Haven, Connecticut. “This reservoir of gasoline contains fuel for future star formation within the galaxy, in addition to outflows from events such as supernovae. It’s filled with hints regarding the past and future growth of the galaxy, and we are finally able to examine it in great detail in our nearest galactic neighbor.” “We find the inner shell that extends to about a half million lightyears is a lot more complex and dynamic,” explained study leader Nicolas Lehner of the University of Notre Dame in Indiana. “The outer shell is smoother and hotter. This distinction is a probable result from the impact of supernova activity in the galaxy’s disk more immediately affecting the internal halo.”
A signature of this activity is the group’s discovery of a large amount of heavy elements in the gaseous halo of Andromeda. Heavier elements are cooked up in the interiors of stars then ejected into space — occasionally as a celebrity dies. The halo is then infected with this substance from stellar explosions. The Andromeda galaxy, also called M31, is a royal spiral of possibly as many as 1 trillion celebrities and equal in size to our Milky Way. At a period of 2.5 million light-years, it’s so near us that the galaxy appears as a cigarshaped smudge of light high in the autumn sky. When its gaseous halo could be viewed with the naked eye, then it would be about three times the width of the Big Dipper. This would easily be the largest feature on the night skies. The quasars are scattered behind the halo, allowing scientists to research multiple regions. Looking through the halo at the quasars’ light, the team observed how this light is absorbed by the Andromeda halo and the way that absorption changes in different regions. The immense Andromeda halo is made from very rarified and ionized gas which doesn’t emit radiation that’s readily detectable. Thus, tracing the absorption of light coming out of a background source is a better way to probe this substance.
The researchers used the unique capability of Hubble’s Cosmic Origins Spectrograph (COS) to examine the ultraviolet light from the quasars. Ultraviolet light is absorbed by Earth’s atmosphere, which makes it impossible to observe with ground-based telescopes. The group utilized COS to discover ionized gas from carbon monoxide, silicon and oxygen. An atom becomes ionized when radiation strips one or more electrons from it.
Andromeda’s halo has been probed before by Lehner’s team. In 2015, they found that the halo is big and massive. But there was little sign of its sophistication; today, it is mapped out in more detail, resulting in its mass and size being far more correctly determined.

This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars 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. INASA, ESA, and E. Wheatley(STScl)]

“Formerly, there was hardly any information– only six quasars–within 1 million light-years of the galaxy. This new program provides more info on this interior part of Andromeda’s halo,” explained co-investigator “Probing gas within this radius is important, as it represents something of a gravitational sphere of influence to Andromeda.”
Since we reside inside the Milky Way, scientists can’t readily interpret the signature of our galaxy’s halo. However, they believe the halos of Andromeda and the Milky Way has to be very similar since these two galaxies are rather similar. The two galaxies are on a collision course, and will merge to form a giant elliptical galaxy starting about 4 billion years from now.
Scientists have studied gaseous halos of distant galaxies, but those galaxies are much smaller on the sky, meaning the amount of bright enough background quasars to probe their halo is generally only one per galaxy. Spatial information is therefore essentially lost.

This diagram shows the light from a background quasar passing through the vast, gaseous halo around the neighboring Andromedagalaxy(M31), as spectroscopically measured by the Hubble Space Tele-scope. The colored regions show absorption from two components that make up the halo. For ionized sillcon,a significant absorption is shown in both plots. The more highly ionized carbon is absorbed by only one component. Astronomers can tell the two components apart because their line-of-sight motions, known as radial velocity, cause a Doppler shift that changes the wavelength of light being absorbed.
NASA, ESA, and E. Wheatley(STSc)

“This is really a unique experimentation because only with Andromeda do we now have information on its own halo along not only one or two sightlines, but over 40,” clarified Lehner.
Actually, Andromeda is the only galaxy in the universe for which this experimentation could be accomplished today, and only with Hubble. Just using an ultraviolet-sensitive future space telescope will be able to routinely undertake this type of experiment beyond the roughly 30 galaxies comprising the Local Group. “So Project AMIGA has also given us a glimpse of this future,” reasoned Lehner.