Monster Galaxies Grow By Cannibalism

Monster galaxies gain weight by eating smaller neighbors, according to Australian astronomers.

AsianScientist (Sep. 22, 2014) – Massive galaxies in the universe have stopped making their own stars and are instead snacking on nearby galaxies, according to research by Australian scientists. The findings have been published in Monthly Notices of the Royal Astronomical Society.

Astronomers looked at more than 22,000 galaxies and found that while smaller galaxies were very efficient at creating stars from gas, the most massive galaxies were much less efficient at star formation, producing hardly any new stars themselves, and instead grew by eating other galaxies.

The study was led by Dr. Aaron Robotham, based at the University of Western Australia’s node of the International Center for Radio Astronomy Research (ICRAR). According to Dr. Robotham, smaller ‘dwarf’ galaxies are being eaten by their larger counterparts.

“All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars,” he said. “Then every now and then they get completely cannibalized by some much larger galaxy.”

Dr. Robotham highlighted that the Milky Way—the galaxy Earth is located in—is at a tipping point, and expected to now grow mainly by eating smaller galaxies, rather than by collecting gas.

“The Milky Way hasn’t merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we’ve cannibalized,” he said.

“We’re also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years.”

But Dr. Robotham said the Milky Way would eventually get its comeuppance when it merged with the nearby Andromeda Galaxy in about five billion years.

“Technically, Andromeda will eat us because it’s the more massive one,” he added.

Dr. Robotham added that as galaxies grow, they have more gravity and can therefore more easily pull in their neighbors. The slowing of star formation in the more massive galaxies is thought to be due extreme feedback events in a very bright region at the center of a galaxy known as an active galactic nucleus.

“The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars,” Dr. Robotham explained.

Ultimately, gravity is expected to cause all the galaxies in bound groups and clusters to merge into a few super-giant galaxies, although we will have to wait many billions of years before that happens.

The article can be found at: Robotham et al. (2014) Galaxy And Mass Assembly (GAMA): galaxy close pairs, mergers and the future fate of stellar mass.

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Source: University of Western Australia; Photo: Hubble Heritage/Flickr/CC.
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