New spectrographic data from the Hubble Space Telescope suggests that the halo of the Andromeda galaxy is already overlapping with that of the Milky Way.

The research team behind Project AMIGA — Absorption Maps In the Gas of Andromeda — has just unveiled the most comprehensive survey of Andromeda galaxy’s circumgalactic medium, or CGM, to date. The CGM is the region of diffuse gases extending beyond the inner disk of a galaxy. The study, published in The Astrophysical Journal on Aug. 27, used ultraviolet data from Hubble’s Cosmic Origins Spectrograph to shed light on the complexities of the galactic structure. Samantha Berek 20 co-authored the study at the University of Notre Dame during the summer after her junior year at Yale as part of the Research Experiences for Undergraduates, or REU, program.

Project AMIGA is the first project that’s been able to really understand diffuse gas and how it interacts with the other components of galaxies and influences the galaxies’ evolution,” Berek said. “We now know that the CGM is huge. Andromeda’s extends about halfway to the Milky Way, which means that it probably bumps up against our own CGM. We knew that our two galaxies were going to merge in billions of years, but we didn’t know that they’re already interacting this much.”

Berek, who worked on the study as an undergrad, shared about how she got involved through the REU program funded by the National Science Foundation, which allows students to apply to do research over the summer at different universities. She was matched with the lead author of this study, astrophysics research professor Nicolas Lehner of the University of Notre Dame. Berek spent the summer working as a co-author on the publication, as well as engaging in academic, professional and social opportunities at Notre Dame. 

 The team used a technique called quasar absorption line spectroscopy. A quasar is a bright celestial body that functions as a lightbulb in the background of the galaxy, allowing astronomers to study the properties of the gases of the galaxy in the foreground.

Lehner explained that for galaxies that are farther away, there are typically only a couple of quasar sightlines available, resulting in a lack of information about how the properties of the gases change across the galaxy’s radius. With the Andromeda’s relative proximity and larger area in the sky, the researchers were able to collect data from 43 sightlines at a multitude of perspectives, providing an in-depth look into the characteristics of its gaseous halo.  

“We think that gas at a large radius away from a galaxy can be a fairly significant component of mass,”astronomy professor Marla Geha said.  “We often call this missing matter or missing baryons, and so understanding both how much gas is in galaxies and what its chemical composition is, is quite important.” 

Co-investigator Bart Wakker of the University of Wisconsin-Madison postulated that the gas in the surrounding halo of the galaxy could represent twice as much material in the universe as the matter within the galaxy. This provides support for the emerging paradigm in astronomy that most of the matter in the universe is not within galaxies, but beyond them. The future direction of the project is to look beyond the halo and explore processes occurring in the inner region of the Andromeda. 

“It has become much clearer as time passes that galaxies are not just the light that’s come from the stars,” said Lehner. “It’s the light that comes from the stars, but also all the things that we don’t see around the galaxy. So if you want to understand galaxies, you should not just study what’s happening in the disk of galaxies, where all the stars and planets are, but you need to really understand its halo. 

The Andromeda galaxy is our closest galactic neighbor at approximately 2.5 million light-years away and is comparable in size to the Milky Way.

Alexa Jeanne Loste | alexa.loste@yale.edu