Hubble maps giant glory around Andromeda Galaxy

In a landmark study, scientists using NASA’s Hubble Space Telescope have mapped out the incredible cover of gas, called a halo, the Andromeda Galaxy, our nearest large galactic neighbor. Scientists were surprised to find that this soft, almost invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy – about halfway to our Milky Way – and up to 2 million light-years in some directions. This means that the halo of Andromeda already collides with the halo of our own galaxy.

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

“Understanding the enormous halos of gas-surrounding galaxies is very important,” said co-researcher Samantha Berek of Yale University in New Haven, Connecticut. “This reservoir of gas contains fuel for future star formation in the galaxy, as well as streams of events such as supernovae. neighbor. “

“We find that the inner shell, which extends to nearly half a million light-years, is much more complex and dynamic,” said study leader Nicolas Lehner of the University of Notre Dame in Indiana. “The outer shell is smoother and hotter. This difference is a likely result of the impact of supernova activity in the galaxy’s disk that has a more direct impact on the inner halo.”

A signature of this activity is the team’s discovery of a large number of heavy elements in the gaseous halo of Andromeda. Heavier elements are sought out in the interior of stars and then thrown into space – sometimes violently when a star dies. The halo is then contaminated with this material from star explosions.

The Andromeda galaxy, also called M31, is a majestic spiral of perhaps 1 trillion stars and comparable in size to our Milky Way. At a distance of 2.5 million light-years, it is so close to us that the galaxy appears like a cigar-shaped cloud of light high in the autumn sky. If its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper. This would easily be the biggest feature in the night sky.

Through a program called Project AMIGA (Andromeda Ionized Gas Absorption Map), the study examined the light of 43 quasars – the very distant, brilliant nuclei of active galaxies driven by black holes – that lie far beyond Andromeda. The quasars are scattered behind the halo, allowing scientists to study multiple regions. Looking through the halo to the light of the quasars, the team observed how this light is absorbed by the Andromeda halo and how that recording changes in different regions. The incredible Andromeda halo is made of very rare and ionized gas that does not emit radiation that is easy to detect. Therefore, detecting the recording of light coming from a background source is a better way to examine this material.

The researchers used the unique capability of Hubble’s Cosmic Origins Spectrograph (COS) to study the ultraviolet light of quasars. Ultraviolet light is absorbed by the Earth’s atmosphere, making it impossible to observe with telescopes on the ground. The team used COS to detect ionized gases of carbon, silicon and oxygen. An atom is ionized when radiation strikes one or more electrons.

Andromeda’s halo has been previously investigated by Lehner’s team. In 2015, they discovered that the Andromeda halo is large and massive. But there was little hint of their complexity; now it has been mapped in more detail, leading to its size and mass being determined much more precisely.

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

Because we live in the Milky Way, scientists cannot easily interpret the signature of the glory of our own galaxy. However, they believe that the halos of Andromeda and the Milky Way should be very similar, because these two galaxies are quite similar. The two galaxies are on a collision course, and will merge to form a giant elliptical galaxy starting over 4 billion years from now.

Scientists have studied gaseous halos of distant galaxies, but those galaxies are much smaller in the sky, which means that the number of bright enough background quasars to study their halo is normally only one per galaxy. Spatial information is therefore essentially lost. With its proximity to Earth, Andromeda’s gaseous halo looms large in the sky, allowing for much more extensive sampling.

“This is truly a unique experiment, because we only have information with Andromeda about their halo along not just one or two lines of sight, but more than 40,” Lehner explained. “This is groundbreaking for capturing the complexity of a galaxy halo outside our own Milky Way.”

In fact, Andromeda is the only galaxy in the universe for which this experiment can now be done, and only with Hubble. Only with an ultraviolet-sensitive future space telescope can scientists routinely conduct such experiments outside the approximately 30 galaxies of the Local Group.

“That Project AMIGA has also given us a glimpse of the future,” Lehner said.

The team’s findings appear in the August 27 edition of The Astrophysical Journal.