The violent history of the big galaxy next door

The violent history of the big galaxy next door
Andromeda has eaten several smaller galaxies, likely within the last few billion years, with left-overs found in large streams of stars. Credit: Dougal Mackey, ANU

Astronomers have pieced together the cannibalistic past of our neighbouring large galaxy Andromeda, which has now set its sights on the Milky Way as its next main course.   

The galactic detective work found that Andromeda has eaten several smaller , likely within the last few billion years, with left-overs found in large streams of stars.

ANU researcher Dr. Dougal Mackey, who co-led the study with Professor Geraint Lewis from the University of Sydney, said the international research team also found very faint traces of more small galaxies that Andromeda gobbled up even earlier, perhaps as far back as 10 billion years when it was first forming.

"The Milky Way is on a collision course with Andromeda in about four billion years. So knowing what kind of a monster our galaxy is up against is useful in finding out the Milky Way's ultimate fate," said Dr. Mackey from the ANU Research School of Astronomy and Astrophysics.

"Andromeda has a much bigger and more complex stellar halo than the Milky Way, which indicates that it has cannibalised many more galaxies, possibly larger ones."

The signs of ancient feasting are written in the stars orbiting Andromeda, with the team studying dense groups of stars, known as globular clusters, to reveal the ancient mealtimes. 

"By tracing the faint remains of these smaller galaxies with embedded star clusters, we've been able to recreate the way Andromeda drew them in and ultimately enveloped them at the different times," Dr. Mackey said.    

The discovery presents several new mysteries, with the two bouts of galactic feeding coming from completely different directions.

"This is very weird and suggests that the extragalactic meals are fed from what's known as the '' of matter that threads the universe," said Professor Lewis from the Sydney Institute for Astronomy and University of Sydney School of Physics.

"More surprising is the discovery that the direction of the ancient feeding is the same as the bizarre 'plane of satellites', an unexpected alignment of dwarf galaxies orbiting Andromeda."

Dr. Mackey and Professor Lewis were part of a team that previously discovered such planes were fragile and rapidly destroyed by Andromeda's gravity within a few billion years.

"This deepens the mystery as the plane must be young, but it appears to be aligned with ancient feeding of dwarf galaxies. Maybe this is because of the cosmic web, but really, this is only speculation," Professor Lewis said.

"We're going to have to think quite hard to unravel what this is telling us."

Dr. Mackey said studying Andromeda also informed understanding about the way our galaxy has grown and evolved over many billions of years.  

"One of our main motivations in studying astronomy is to understand our place in the Universe. A way of learning about our galaxy is to study others that are similar to it, and try to understand how these systems formed and evolved. 

"Sometimes this can actually be easier than looking at the Milky Way, because we live inside it and that can make certain types of observations quite difficult."

The study, published in Nature, analysed data from the Pan-Andromeda Archaeological Survey, known as PAndAS.

"We are cosmic archaeologists, except we are digging through the fossils of long-dead galaxies rather than human history," said Professor Lewis, who is a leading member of the survey.

Explore further

The Milky Way's long-lost sibling finally found

More information: Two major accretion epochs in M31 from two distinct populations of globular clusters, Nature (2019). DOI: 10.1038/s41586-019-1597-1 ,
Journal information: Nature

Citation: The violent history of the big galaxy next door (2019, October 2) retrieved 20 October 2019 from
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Oct 02, 2019
Our earth will have vanished millions of years before Andromeda begins absorbing our Milky Way galaxy.

Oct 02, 2019
Our earth will have vanished millions of years before Andromeda begins absorbing our Milky Way galaxy.

Billions, more likely. Point taken, though.

Oct 02, 2019
Not to worry - you won't feel a thing when it happens.

Oct 02, 2019
On second thought, the Earth will still be orbiting even after the Sun has gone nova and then becomes a white dwarf. The Earth will BE a burnt out cinder, possibly smaller than its former self, since much of the geological formations such as sand melted into glass, and soil will have shrunken and be far more compacted. But it may still orbit the white dwarf with far less H2O, if any at all. By then, humans would have gone off-world already to the Stars and found new homes. Only the information technology of the time will be available to tell the story of the place where humans (and animals) had originated.
That is why pictures of present-day Earth is important. Its history in pictures and all its artifacts, and the pictures of its humans, animals and plants to preserve for those that will come after, who will want to know where they have come from.
As Andromeda nears, perhaps there will be SOME brave humans who will journey to somewhere in Andromeda as settlers on a nice green planet

Oct 02, 2019
trying to make Andromeda sound like a looming monster at this scale of SpaceTime is clickbait hyperbole

it is not as if our Milky Way galaxy is a dewey-eyed virgin
or even a shrinking violet

by now, all of the larger galaxies & probably many of the smaller
are amalgamated congregations of mutual assimilation & congelation

plenty of evidence
observed by sane people
that our MW has devoured at least two or three other galaxies, probably more

within the next four billion years the MW should pull in dozens of satellite galaxies

by the SpaceTime the MW & A get close enough to visibly share tidal filaments?
the two galaxies may be a closer match in the same weight-class

what is most interesting about the process?
is that each absorption of satellites will reignite their core BH accretion disks
producing renewed jets blazing & violent clouds of dusts & gases

rejuvenating the intertwined galaxies with new stars & nebulae & star systems

for hundreds of billions of years

Oct 02, 2019
You don't mess with Milky Way and come out unscathed.

Oct 02, 2019
When I as a PHD candidate, one of my professors (GW Collins) posed a problem like this to his PHD students (me included) in the late 1970s. We concluded that the likelihood that a star would be "destroyed or disrupted" in the merge was extremely unlikely. There would be a disturbance in the structure of the interacting galaxies. Some stellar systems would have their "velocity vectors" change (some staying in the original galaxy, some captured by the invading galaxy, and some ejected from the galaxies. Depending on the interaction of the galaxies, the stability of the galaxy centers (generally super-massive black-holes), could be disrupted resulting in galactic stability changes.

Oct 03, 2019
The filament hypothesis for the satellite planes gets a healthy boost, but the problem is not the observations but the model understanding of it [ https://astrobite...n-a-row/ ]. "... three possible solutions that have been put forth to address this outstanding question, but concluded that all three have their own problems ... This suggests work for theoreticians, simulation authors, and observers: theoreticians can start working on modeling these planar structures, simulation authors can figure out how to incorporate them into simulations, and observational astronomers can search for more such structures around other galaxies to better understand just how common these things are in the universe. Definitely an exciting problem for astronomers to solve!"

Oct 03, 2019
Our earth will have vanished millions of years before Andromeda begins absorbing our Milky Way galaxy.

- "The Andromeda–Milky Way collision is a galactic collision predicted to occur in about 4.5 billion years ..." [ https://en.wikipe...ollision ].

- "Once the Sun changes from burning hydrogen ... Most of Earth's atmosphere will be lost to space and its surface will consist of a lava ocean ... If Earth is not destroyed by the expanding red giant Sun in 7.6 billion years, then on a time scale of 10^19 (10 quintillion) years the remaining planets in the Solar System will be ejected from the system by violent relaxation. If this does not occur to the Earth, the ultimate fate of the planet will be that it collides with the black dwarf Sun due to the decay of its orbit via gravitational radiation, in 1020 (Short Scale: 100 quintillion, Long Scale: 100 trillion) years.[101]" [ https://en.wikipe...nt_stage ]

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