Massive elliptical galaxy merger a ‘train wreck’ in early universe

Astronomers have pulled back curtains of space dust to find a star-spangled show: a massive pair of galaxies smashing into each other to form, eventually, an even bigger elliptical galaxy, one that’s 10 times the size of the Milky Way, in which stars are forming at the breakneck speed of 2,000 suns per year.

The colliding galaxies known as HXMM01, described Wednesday by an article in Nature, reveal a brief, shining moment in the creation of a massive elliptical galaxy, said lead author Hai Fu, a UC Irvine astronomer.

Elliptical galaxies don’t look like our own disc-like, spiral Milky Way; they’re big blobby clusters, often filled with very old, red stars. Astronomers have wondered whether massive elliptical galaxies from the early universe formed over time with smaller galaxies getting tossed into the mix, or primarily by a single giant collision of already massive galaxies.

The two galaxies colliding just around 3 billion years after the Big Bang seem to answer that question, Fu said.

“It’s like a train wreck,” Fu said.

Scientists spotted the distant collision, which happened 11 billion years ago, using data from the Herschel Space Observatory. Herschel, which recently ran out of the coolant needed to keep its infrared instruments working, was capable of observing “cold” corners of the cosmos.

This super-luminous blob stood out from the crowd, Fu said — so much so that scientists initially thought the light source had been magnified by gravitational lensing (which happens when a massive object magnifies light from a more distant object, making it seem brighter).

The scientists soon ruled that option out, realizing that they were looking at two massive, bright galaxies churning out stars at eye-popping rates, a thousand times faster than the roughly two per year in our own Milky Way.

This was a very rare find, Fu said. For one thing, the galaxy mash-up looked pretty big.

“In humans the tall guys are really rare, like [retired basketball player] Yao Ming,” Fu said. “So that’s another reason why it’s very hard to find objects like this one, because it is at the most massive end of galaxies that we know.”

It’s also rare to catch a galaxy merger in the act. This crashing and melding phase is very brief by galactic standards -- a mere 200 million years or so, Fu said.

“For a galaxy’s life, it’s like an instant,” Fu said. “It’s like snap our fingers.”

The hot, “blue” stars coming out of this stellar furnace should have mostly died off well within a billion years from this snapshot, leaving it still glowing with dim red stars -- and looking much more like typical elliptical galaxies.

Capturing such a fleeing but important moment in the development of a massive elliptical galaxy will help scientists better understand their evolution, Fu said.

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