When Their Star-Birthing Days Are Over, Galaxies Retire to Oblivion

This artist's conception depicts an energetic quasar that has cleared the center of the galaxy of gas and dust, and the winds are now pushing it to the outskirts. Michelle Vigeant/University of Kansas

All things must end. Even galaxies.

And when it's time for the Milky Way to go, it will be quite a show — at least, if humans are still clinging to this planet in a few billion years.

Someone may even spot a spectacular blue halo in the sky. That would be a quasar, the incredibly hot gas formed when black holes collide.

And those black holes would be the celestial carnivores at the heart of the Milky Way and neighboring Andromeda galaxy. Their collision — after being locked in a gravitational tango for billions of years — signals that the end is nigh-ish.

The process will take some time. After all, the Milky Way has a lot of things to pack up — the dust and gas that make up as many as 400 billion stars and the countless planets that surround them. Everything will eventually be vented into the gaseous sheath that surrounds galaxies, known as the circumgalactic medium. Without the gas and dust needed to form new stars, a galaxy is considered "red and dead."

But from the nebulous cloud that is the circumgalactic medium, new stars may someday spring, starting the cycle of galactic growth once again.

Of course, that's the story scientists tell us. No one has actually seen the end of a galaxy. But researchers at the University of Maryland have found a group of galaxies in their death throes.

That's where quasars, those super-hot harbingers of doom, have formed, but the galaxies haven't yet come undone. They're keeping it together in the face of the inevitable.

"One of the biggest questions we have in astronomy is: How do galaxies die?" University of Kansas astrophysicist Allison Kirkpatrick, noted in a press release. "We know what they look like once they're dead ... but the rest of it is just pieces that we've guessed at."

An illustration of a bright quasar.
One of the brightest objects in the universe, a quasar typically signals that a galaxy's end is nigh. IgorZh

During a survey of the night sky, Kirkpatrick and her colleagues found 22 quasars in all. As the brightest objects in the universe, these celestial bodies are hard to miss. But an infrared survey revealed that these quasars don't burn quite so hot, likely due to the cool clouds of dust that comprise them.

Kirkpatrick dubs them "cold quasars" — galaxies that teeter on the brink of death but may still be able to birth new stars.

"That in itself is surprising," she noted in her presentation. "These are very compact, blue, luminous sources. They look exactly like you would expect a supermassive black hole to look in the end stages after it has quenched all of the star formation in a galaxy."

Kirkpatrick suggests these "in-betweeners" could shed light on the brief phase between a galaxy's star-birthing glory days — and it's descent into oblivion.

Also known as retirement.

"We've found a population that we can study in detail and map out exactly how these galaxies move from their star formation phase of their life to their retirement phase," she explains.

It's probably not the kind of retirement most of us envision. They won't be playing bridge at the Green Acres Retirement Home for Galaxies.

But when they do finally "retire," these galaxies will be voided of all matter and will become effectively sterile. Along the way, they could show mere Earthlings, how we too fit into the grand picture that is an ever-growing universe.