You may have seen the Andromeda Galaxy (M31) without even realizing it. The giant spiral galaxy appears as a gray, spindle-shaped blob in the night sky, visible to the naked eye in the right position. It is the largest galaxy near us and has been studied extensively by astronomers.
Now astronomers have used the Hubble Space Telescope to create a huge halo of hot gas from Andromeda.
Scientists call the halo of gases around galaxies a peripheral medium (CGM). CGM is diffuse, and almost invisible. But as scientists acquire the technology to study it more closely, they are beginning to understand the important role it plays in galactic evolution. They believe that CGM is an important source of star-forming material, and that it controls the galaxy’s gas supply.
“It’s full of links to the galaxy’s past and future evolution, and we’ve finally been able to study it in detail in our immediate galactic neighborhood.”
Samantha Barrack, co-investigator at Yale University in New Haven, Connecticut.
In a new study, a team of researchers used the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) to map Andromeda’s CGM. The study is entitled “Project Amiga: The Circumalectic Medium of Andromeda.” The lead author is Nicholas Lehner from the University of Notre Dame in Indiana. The study is published in the Astrophysical Journal.
Studies show that the halo of Andromeda is the largest object in the night sky, we cannot see it. It extends 1.3 million light years from the center of Andromeda, about half of our galaxy. In some directions, it still extends to 20 million light years. And the halo of Andromeda is really haunting the entrance to the galaxy.
The CGM is even more detailed than the researchers thought. It has two layered parts: the inner shell of the gas resides inside the outer shell. The inner shell is more dynamic, and the outer shell is warmer and smoother. The team of researchers believes that the inner shell is more dynamic and unstable due to the exit from the supernova.
Nicholas Lehner, study leader at the University of Notre Dame in Indiana, explains, “We find an inner shell whose let-shell area is very complex and dynamic up to a million light years.” “The outer shell is smooth and warm. This difference is a possible consequence of the effect of supernova activity on the galaxy disk directly affecting the inner halo, ”Lehner said in a press release.
It is not just the dynamic state of the inner halo that points to the supernova. It is also a composition of gas. The team discovered many heavy elements in the gas, which are formed in the hearts of giant stars, and spread into space by spreading supernovae.
The gas in CGM emits a little energy, but it is very difficult to see. Researchers studied it by seeing ultraviolet light from distant quarters as it passed through the halo. That ultraviolet light is absorbed by the Earth’s atmosphere, so it cannot be observed from the ground. But Hubble can see it from its position in low-Earth orbit (LO.).
The team has found 43 quarters “behind” Andromeda from our point of view. Because they are spread across the width and breadth of the galaxy, researchers have been able to study haloes in many places. They observed how ultraviolet light from distant quarters is absorbed differently in different regions of the CGM. The team used Hubble’s COS to detect ionized gases from carbon, silicon and oxygen.
This is not the first time that leading researcher Lehner has studied Andromeda by observing light from distant quarters. In 2015 he and his colleagues published a pilot study of Andromeda based on light from just six quarters. That study showed how big and wide Andromeda’s CGM is, but it doesn’t reveal all the complications. The work was called “Evidence for Evidence, an Extended Circulomectic Medium Around the Andromeda Galaxy” and was also published in the Astrophysical Journal.
“Previously, in the 1 million light-years of the galaxy, there was only six quarters-very little information. This new program provides more information on this inner region of Andromeda’s halo, ”said Notre Dame co-investigator J. Christopher Hawke explained. “It is important to examine the gas within this radius, as it represents something of a gravitational field for Andromeda.”
The team also measured the speed of the gas in the interior and exterior halls. In this way they determined that the inner shell was more dynamic than the outer shell. The inner shell shows multiple velocity components, while the outer shell shows only one velocity component. Measurements of velocity also allowed them to determine that the outer halo is bound to Andromeda by gravity.
“This is the basis for capturing the complexity of the galaxy halo outside our own galaxy.”
Nicholas Lehner, study leader at the University of Notre Dame in Indiana.
“It’s important to understand the vast accumulation of gas around the galaxy,” explained Samantha Barrack, a co-investigator at Yale University in New Haven, Connecticut. “This reservoir of gas is the fuel for the formation of future stars inside the galaxy, as well as the flow of events such as supernovae. It is full of links to the galaxy’s past and future evolution, and we have finally been able to study it in detail in our immediate galactic neighborhood. “
Andromeda is really our only chance to study CGM in such detail. Our position inside the galaxy makes it impossible to study the galaxy’s own CGM. And none of our large galaxies are close enough to our current technology to study this way. Distant galaxies appear so small that there are not enough background quarks for spectroscopy. Each quarter behind the galaxy provides a line of sight for scientists.
“This is truly a unique experiment because with Andromeda alone we have not just one or two visual cues on its halo, but more than 40 pieces of information,” Lehner explained. “This is the basis for capturing the complexity of the galaxy halo outside our own galaxy.”
Although we cannot study the CGM of the Milky Way directly, researchers say that they can estimate some of its properties based on this study. In his study, he writes, “Probably. MW also has a nice and warm ionized CGM,” and the CGM of the Milky Way and Andromeda “probably should already overlap and interact with each other.”
As it stands now, Andromeda is the only galaxy that can be investigated in this way. But in the future, that will change. The LUVIR (Large UV / Optical / IR Surveyor), like the future UV space telescopes, with its enormous 15mm mirror, should allow scientists to study the CGM of the galaxy outside our local group. In that sense, this study gives us a glimpse of some possible future results.
“So Project AMG has also given us a glimpse of the future,” Lehner said.
