'Monster' galaxy that stopped forming stars stumps astronomers

Looking back 12 billion years ago, astronomers have found a massive galaxy called XMM-2599 that has stopped forming stars, something that they didn't think would happen to a galaxy so young.

XMM-2599 doesn't jibe with what astronomers theorize happens in a galaxy so young or so massive

This is an artist's impression of MAMBO-9, a galaxy in its star-forming period, roughly 970 million years after the big bang. This is what XMM-2599 might have looked like during a similar stage. (NRAO/AUI/NSF, B. Saxton)

An unusual galaxy that formed at the dawn of our universe has left astronomers scratching their heads.

An international team of astronomers has found a massive galaxy that existed just 1.8 billion years after the big bang — called XMM-2599 — that has stopped forming stars.

While it's known that galaxies can eventually stop stellar formation, what's particularly interesting about this galaxy is that it doesn't jibe with what astronomers theorize happens in a galaxy so young or so massive.

"It's the most massive galaxy that's not forming stars any longer in this early time in the universe, and that's something that has not been predicted," said Benjamin Forrest, a post-doctoral researcher at the University of California, Riverside, and also the lead author of the paper published in the Astrophysical Journal.

The nascent universe

Roughly 13.8 billion years ago, in a moment of rapid expansion, the universe as we know it came to be. At first it was hot and opaque, and then, eventually, dark and cold. After about 400,000 years, stars began to form, as did massive galaxies. 

It's believed that stars in young, massive galaxies like XMM-2599 should have stopped forming when the galaxies were roughly three billion years old. Instead, XMM-2599 was just 1.8 billion years old when it slammed the brakes on stellar formation.

A timeline of the universe. (N.R.Fuller, National Science Foundation)

"At some point in time, this was the best place to make stars, and all of a sudden — a snap of the fingers later — it became the worst place in the universe to form stars," said Adam Muzzin, assistant professor at York University's department of physics and astronomy in Toronto. 

"It really just defies the logic of what we think about how galaxies form."

Strange, but true

When astronomers say XMM-2599 is massive, they're not exaggerating: it is roughly 310 billion times more massive than the sun (in terms of stellar mass). For comparison, our Milky Way galaxy is 100 billion times the mass of the sun.

Though it may be massive, finding XMM-2599 was no walk in the park. 

First, astronomers had to identify candidates for these "monster galaxies" in large galaxy surveys, as co-author Cemile Marsan, a postdoctoral science fellow at York explained. She was then part of a team that used the W.M. Keck Observatory in Maunakea, Hawaii, for followup observations. 

This image set shows the possible evolution of XMM-2599, from a massive, dusty, star-forming galaxy (left), to an inactive red galaxy (center), and then perhaps turning into a bright cluster galaxy (right). (NRAO/AUI/NSF, B. Saxton; NASA/ESA/R. Foley; NASA/ESA/STSCI, M. Postman/Clash)

In order to merit further investigation, the galaxy had to tick off three boxes: it had to be bright, massive and very distant, something that XMM-2599 definitely was.

A piece of the puzzle

"What's really exciting for me is, how do these massive things get so monstrous? How is it that they shut off their star formation?" said Cemile Marsan, a post-doctoral science fellow at York and co-author of the study.

"It's like there's some party going on and all of a sudden the party's over."

Muzzin said that some astronomers hypothesize that star formation may come to a stop in massive galaxies due to outflows of gas and dust from a supermassive black hole at the centre. 

"It's like shooting a fire extinguisher all over the galaxy," said Muzzin. 

But, he added, in order for that to happen, you need a supremely massive black hole.

"If that's what's happening with XMM-2599, how do you get a ridiculously big black hole at the centre of that thing? Also, in almost no time at all, since black holes take time to grow as well."

There will be more followup observations with the Atacama Large Millimeter/submillimeter Array in northern Chile. The astronomers are also hoping to use the James Webb Space Telescope — the planned successor to the Hubble Space Telescope due to launch in 2021 — to take even more precise measurements.

In the meantime, Muzzin said he's thrilled to be able to look back in time and find another piece of the puzzle that sheds some light onto the beginning of everything we have come to know.

"I mean, you live in this big, beautiful galaxy — the Milky Way — you go outside, you look at it, you think about the fact that we're these little specks floating around this single star, and you wonder how all this stuff got here," he said.

"We want to figure out how all galaxies got here."


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