“It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” said Eric BellUniversity of Michigan Professor of Astronomy (2018) about his discovery of the Andromeda Galaxy (also known as M31), which was our closest large galaxy neighbor two billion years back.
A halo of stars left that is larger than the Andromeda Galaxy itself
Even though it was mostly destroyed, this huge galaxy left behind a rich trail: an almost invisible halo full of stars that is larger than Andromeda’s, an obscure stream of stars, and M32, an elusive compact galaxy. Astronomers will benefit from the discovery and study of this decimated galaxy to better understand how disk galaxies such as the Milky Way develop and survive large mergers.
Andromeda’s Orbiting Dwarf Galaxies – “Point to Something Profound”
Long-lost sibling of the Milky Way
This galaxy was named after a disrupted star M32pAfter the Milky Way galaxies and Andromeda,,, was the third largest member of the Local Group. Using computer models, Richard D’Souza and Eric Bell of the University of Michigan’s Department of Astronomy were able to piece together this evidence, revealing this long-lost sibling of the Milky Way. Nature Astronomy published their findings.
M32 is the surviving center of the Milky Way’s long-lost sibling, like the indestructible pit of a plum.”
Scientists know for a long time that the large halo of stars around galaxies is made up of remnants from smaller, cannibalized galaxies. One galaxy like Andromeda is expected to have consumed hundreds more of its smaller companions. Researchers thought that this would make learning about each one difficult.
Using new computer simulations, the scientists were able to understand that even though many companion galaxies were consumed by Andromeda, most of the stars in the Andromeda’s outer faint halo were mostly contributed by shredding a single large galaxy.
The ‘Eureka’ Moment
“It was a ‘eureka’ moment. We realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies,” said lead author D’Souza, a postdoctoral researcher at U-M.
“Astronomers have been studying the Local Group—the Milky Way, Andromeda and their companions—for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” said co-author Bell.
“The Monster” –Andromeda Galaxy Foreshadows the Milky Way’s Fate
M32p was the name of the galaxy that was destroyed by the Andromeda galaxies. It was 20 times bigger than any galaxy that merged with the Milky Way in its lifetime. M32p would be massive and make it the third largest galaxies in the Local Group, behind the Andromeda or the Milky Way.
M32 is a Weirdo–“There isn’t another galaxy like it”
This work might also solve a long-standing mystery: the formation of Andromeda’s enigmatic M32 satellite galaxy, the scientists say. They suggest that the compact and dense M32 is the surviving center of the Milky Way’s long-lost sibling, like the indestructible pit of a plum.
“M32 is a weirdo,” Bell said. “While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the universe. There isn’t another galaxy like it.”
“We don’t think that it was an elliptical,” Bell told The Daily Galaxy, “rather we think that it was likely to be a disk galaxy (because it had gas – as evidenced by the ongoing star formation during the merger, and most galaxies with gas in the local Universe are disks/spiral galaxies).”
Researchers believe their research could alter our understanding of how galaxies form. They realized that the Andromeda’s disk survived an impact with a massive galaxy, which would question the common wisdom that such large interactions would destroy disks and form an elliptical galaxy.
The timing of the merger could also explain thickening in the Andromeda Galaxy’s disk and a burst star formation two billion year ago. This discovery was independently made by French researchers earlier this month.
“Andromeda would have looked so different 2 Billion Years Ago”
“The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago,” Bell said. “When I was at graduate school, I was told that understanding how the Andromeda Galaxy and its satellite galaxy M32 formed would go a long way towards unraveling the mysteries of galaxy formation.”
“Using models of galaxy formation,” Bell wrote in reply to an email from The Daily Galaxy “we realized that if a satellite galaxy has gas and is forming stars, that it stops forming stars when it is finally ripped apart by the gravitational forces of the big galaxy – in this case, Andromeda. So, we were able to use other folks’ measurements of the history of star formation in M32, and in M31’s diffuse stellar halo, realized that star formation stopped around 2 billion years ago, and used that to age-date when M32p was destroyed.”
“The Monster” –Andromeda Galaxy Foreshadows the Milky Way’s Fate
“There is other research – Ivanna Escala (at Carnegie Observatory) has been measuring the chemical composition of stars in Andromeda’s diffuse stellar halo; her results were a little of a surprise to Richard D’Souza and I, and we are thinking about what they tell us about the merger process,” Bell wrote in reply to an email from The Daily Galaxy asking about subsequent research about M32 and M32p. “Richard D’Souza and I actually turned our attention to thinking about satellite galaxies of Andromeda, and are arguing that most of Andromeda’s current satellite galaxies were actually M32p companionsThey were originally given to Andromeda in the merger.
Source: Richard D’Souza et al, The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitorNature Astronomy (2018).
Avi Shporer MIT Kavli Institute for Astrophysics and Space Research via University of Michigan, NatureEric Bell.
Image credit at the top of this page: Shutterstock Licence
Avi Shporer Research Scientist, MIT Kavli Institute for Astrophysics and Space Research. A Google ScholarAvi was previously a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL). His motto, not surprisingly, is a quote from Carl Sagan: “Somewhere, something incredible is waiting to be known.”
