An ancient galaxy grew massive — then oddly stopped making stars

Even galaxies burn out if they work too hard.

A galactic monster in the early universe quit forming stars after ferociously churning them out for hundreds of millions of years, researchers report. Why it slacked off is unclear, but the answer might teach astronomers a thing or two about how the earliest galaxies grew and evolved into the stellar metropolises that surround us today.

The light from this galaxy, designated XMM-2599, took nearly 12 billion years to reach Earth. So astronomers see the galaxy as it was just 1.8 billion years after the Big Bang. By that time, the galaxy had bulked up to a mass of about 300 billion suns, new observations show, making it three times as massive as similar known galaxies from that epoch.

The galaxy appears to have gotten so hefty by cranking out stars at a rate of over 1,000 solar masses per year for several hundred million years. But then, the star making suddenly stopped, astronomer Benjamin Forrest at the University of California, Riverside and colleagues report in the Feb. 10 Astrophysical Journal Letters. By comparison, the Milky Way produces a paltry two to three stars per year.

“This paper is telling us that star formation can be very, very efficient in the early universe and equally efficiently be quenched,” says Mauro Giavalisco, an astronomer at the University of Massachusetts Amherst who was not involved with this study.

Galaxies make stars for as long as they have a supply of cold gas — the Milky Way has been at it for nearly the entire 13.8-billion-year history of the universe. The only ways to shut down the star-forming factories are to prevent cold gas from raining down on the galaxy or to physically remove the gas that’s already there, says Giavalisco. “Theory doesn’t quite fully explain how to stop so quickly and efficiently star formation in a galaxy which, only a little bit earlier, was forming stars with prodigious efficiency.”

Other prolific star-forming galaxies were known to exist in the early universe (SN: 8/7/19), including a few that also had retired. But none is as massive as XMM-2599, Forrest says. Computer simulations of this early epoch don’t produce similarly gargantuan deadbeats, so XMM-2599 is an enigma.

A supermassive black hole in the center of the galaxy may be to blame. Such a black hole would likely have attracted a superheated whirlpool of cosmic detritus, which blazed with light that energized gas around the galaxy (SN: 12/5/18), preventing it from coalescing into clumps needed to birth new stars.

Alternatively, the stars themselves may have been their own undoing. Lots of star formation eventually leads to lots of supernovas, Giavalisco says, which could have ejected gas on a galactic scale.

“We don’t have any confirmation for a reason, mostly because it happened in the past, and there’s only so many clues we can pick up,” Forrest says. By scanning the skies for similarly quiet monsters, researchers could “confirm whether this thing is just a one-off weird galaxy … or if it’s part of a larger population,” he says.

As for what becomes of such a galaxy, that’s hard to say. But Forrest and Giavalisco speculate that, over time, its gravity may attract other galaxies, possibly making XMM-2599 and its ilk the seeds around which galactic clusters grow.