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Notre Dame faculty, grad students search for answers in the Andromeda galaxy’s halo

| Thursday, September 10, 2020

Every night in September of 2017, graduate student Michelle Berg sat on her laptop in Indiana controlling a telescope in West Virginia. Berg had trained for a week on-site to maneuver the massive single-dish Green Bank Telescope (GBT), the largest steerable radio telescope on the planet.

“One of the days they were actually doing maintenance, so I got to actually go up onto the telescope and get a tour,” she said. “It was really cool to be there at the facility.”

Using the GBT, Berg made observations about the halo of the Andromeda Galaxy as part of a team of Notre Dame students and faculty studying the nearby galaxy. Recently, members of that team discovered new information about the nature of the Andromeda halo by using the Hubble Space Telescope (HST) as well.

One member of the team, J. Christopher Howk, a professor in the physics department, said a galaxy’s halo is in some ways similar to the Earth’s atmosphere: a collection of gas held in place by gravity. He said that as galaxies change, they take in and spew out gas, which can provide insights into the galaxy’s makeup.

“If you look at the atmosphere of gas around the galaxy, you can learn something about its past history and potentially about how it will change in the future,” Howk said.

The team’s lead scientist, Nicolas Lehner, said his work on the subject of galactic halos began long before this particular project got approval. He said Andromeda provides a unique opportunity to study galactic haloes from several different views.

“The problem with our own Milky Way is that we live in it now, so we don’t have an external view,” Lehner said. “You don’t have any depth information, so you don’t know how far the halo may extend.”

The team learned about Andromeda’s halo by conducting “chemistry in the sky,” as Howk called it. He said elements in the halo absorb light from distant sources, and by testing which wavelengths of light never reached this galaxy, the team could understand which elements those were and how they were organized.

Howk said a 2007 addition to the HST made the instrument 10 times more sensitive, allowing for the kind of views that their team used. Time on the HST is coveted — NASA receives requests for seven to 10 times more access than they can provide — and requires thorough research in advance.

“About once a year, there’s a call for proposals, and it’s very competitive,” Howk said. “You’re responsible for putting together all of the base level technical information, like ‘here’s where I want to look, here’s the settings on the instrument we should use.’”

Lehner said this new information might spawn more studies and that he has several ideas already. One of these potential projects involves a deeper look into the heart of the Andromeda Galaxy.

“What we’d like to do now is study even closer to the disc of [the Andromeda Galaxy] and compare what’s happening in the halo to what’s happening in the disc,” he said.

Berg’s research on hydrogen in Andromeda’s halo did not end up in this most recent study, but she said that the experience of working with the GBT was one of her favorite parts of researching. She said any student with an interest in astronomy can have amazing experiences by being curious.

“Just talk to the people that are there,” Berg said. “I know it’s intimidating to talk to the professors, so the grad students would be a great way to learn a little more. Grad students are very honest.”

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