Andromeda Was Just Hungry

[Recusant]

The pop-science articles on this are running with "ate it" so I'll play along. Scientists have proposed that there is evidence of a merger between the Andromeda galaxy and another fairly large galaxy in the past.

"The Milky Way Had a Big Sibling Long Ago — And Andromeda Ate It" | Space.com

The Milky Way had a previously unknown big sibling that was torn apart by the neighboring Andromeda galaxy long ago, a new study suggests.

Andromeda and the Milky Way are the two largest members of the Local Group, a collection of more than 50 galaxies packed into a dumbbell-shaped region of space about 10 million light-years across. Andromeda was not kind to the onetime third-biggest member of this family, devouring it about 2 billion years ago, according to the new research.

"Astronomers have been studying the Local Group — the Milky Way, Andromeda and their companions — for so long," study co-author Eric Bell, a professor of astronomy at the University of Michigan (UM), said in a statement. "It was shocking to realize that the Milky Way had a large sibling, and we never knew about it." [When Galaxies Collide: Photos of Great Galactic Crashes]

Andromeda, also known as M31, is a prolific cannibal; the huge spiral galaxy is thought to have shredded hundreds of its smaller kin over the eons. The number and complexity of these mergers makes it tough to tease out the details of any particular one — but Bell and study lead author Richard D'Souza, a postdoctoral researcher at UM, were able to do just that.

Using computer simulations, the duo determined that most of the stars in the faint outer reaches of Andromeda's "halo" — the roughly spherical region surrounding the galaxy's disk — came from a single smashup.

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[xSilverPhinx]

Normal bickering between galactic siblings, I suppose? 

[Recusant]

It is fairly certain that in about 4 billion years the Milky Way will merge with Andromeda. I wonder whether there's any consensus on who will be "eating" who when that occurs.    Really, the prevalent titling of this story is pop-science clickbait.

[Dave]

https://youtu.be/4disyKG7XtU

[Tank]

I would love to see an imagining of what it would be like to be on a planet a little outside a galaxy with the void on on side and the galaxy on the other.

[Recusant]

Andromeda isn't the only galaxy that has merged with other galaxies in the past. According to the paper described in the article below, there's good evidence of the Milky Way merging with a smaller galaxy.

If you've watched the video of merging galaxies posted by Dave above, you'll have noted that they perform a sort of dance, meshing then pulling apart, then coming together again. The cycle can repeat a few times, depending on the relative velocities of the galaxies. The authors of the paper say that they've observed evidence of such a cycle in the Milky Way. 

The paper is behind a paywall, but there's a link to the abstract in the portion of the article quoted below.

"Astronomers Find the Hollowed-Out Shell of a Dwarf Galaxy that Collided With the Milky Way Billions of Years Ago" | Universe Today

In 2005 astronomers found a dense grouping of stars in the Virgo constellation. It looked like a star cluster, except further surveys showed that some of the stars are moving towards us, and some are moving away. That finding was unexpected and suggested the Stream was no simple star cluster.

A 2019 study showed that the grouping of stars is no star cluster at all; instead, it's the hollowed-out shell of a dwarf spheroidal galaxy that merged with the Milky Way. It's called the Virgo Overdensity (VOD) or the Virgo Stellar Stream.

A new study involving some of the same researchers shows how and when the merger occurred and identifies other shells from the same merger.

The new paper's title is "The Milky Way's Shell Structure Reveals the Time of a Radial Collision." The first author is Thomas Donlon II, a Rensselaer grad student, who was also the first author of the 2019 study. The paper is published in The Astrophysical Journal.

In their paper, the authors write "In this work, we identify shell substructure in the Milky Way for the first time, and we argue that these shells are indeed associated with the VRM [Virgo Radial Merger] and therefore a radial merger event."

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