It’s a Galaxy-Eat-Galaxy Cosmos Out There

Monster Galaxies Nibble On Smaller Ones to Get  Bigger

Galaxies grow by eating other galaxies — that’s a given in the cosmos. It’s also true that galaxies spend much of their productive lives making stars from the gas they contain. These two galactic activities began with the first proto-galactic “shreds” that began to combine to grow today’s galaxies. It has continued ever since.

The Milky Way is a good example of this. It formed some 13 billion years ago and grew larger by consuming stars and gas from other, smaller galaxies. It is, in fact, still cannibalizing some dwarf galaxies and may eventually gobble up the Large and Small Magellanic Clouds in a few billion years.

In the future, the Milky Way and the Andromeda Galaxy — currently some 2.5 billion light-years apart — will collide. It’s very likely that the  much-more-massive Andromeda will cannibalize the Milky Way. Ultimately the new galaxy that’s formed will undergo a superburst of star formation, using up the gases from the two combined galaxies. Astronomers have seen massive starburst knots at other galaxy collisions, so it’s very likely a feature of most galaxy collisions.

Researchers are now working to pin down some of the complexities of this galaxy-evolution-by-cannibalism mechanism. A team  in Australia is using spectroscopic observations 0f light and radio waves from distant galaxies to find many sites where galaxies are merging, larger ones pigging out on smaller ones. Interestingly, although the galaxies undergo starburst events during the mergers, eventually many of the most massive galaxies created from these collisions eventually stop making stars. The reasons may be related to events occurring within their active central cores (where many galaxies have supermassive black holes that could be sending jets out to space and somehow disrupting the normal course of events).

The group studied many merging galaxies and characterized the shapes they saw. Each of the galaxies in the image at the left from their survey captures a galaxy collision in a snapshot of time. You can see some are about to merge and others have passed by each other, interacted, and are about to go for a second pass. Ultimately each of these mergers will result in a monster galaxy.

The group also created an animation from a computer model showing the interaction of the Milky Way and Andromeda, which will commence in about five billion years. Ultimately, both galaxies will lose their separate identities and become a new, more massive version of the originals.

Andromeda and the Milky Way Collide! from ICRAR on Vimeo. In about five billion years time, nearby massive galaxy Andromeda will merge with our own galaxy, the Milky Way, in an act of galactic cannibalism (technically Andromeda will be eating us, as it’s the bigger of the two galaxies.). There haven’t been any large mergers with our galaxy recently, but we can see the remnants of galaxies that have previously been snacked on by the Milky Way. We’re also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds sometime in the future.
This simulation shows what will happen when the Milky Way and Andromeda get closer together and then collide, and then finally come together once more to merge into an even bigger galaxy.
Simulation Credit: Prof Chris power (ICRAR-UWA), Dr Alex Hobbs (ETH Zurich), Prof Justin Reid (University of Surrey), Dr Dave Cole (University of Central Lancashire) and the Theoretical Astrophysics Group at the University of Leicester.Video Production Credit: Pete Wheeler, ICRAR.

In the far distant figure, many billions of years from now, galaxies in clusters and groups will likely have merged into a few supergiant monster galaxies. This will markedly change the look of the cosmos in ways that astronomers can now only speculate about. Stay tuned!