"Cold Quasars" Are the Harbingers of Galactic Death, Or Are They?

What are "cold quasars?" What do they do, how do they work with galaxies?
Jaime Trosper
Rendering of a cold quasarNASA/Daniel Rutter

Quasars are fascinating objects. In the simplest possible terms, quasars are bright accretion disks that form when intense heat and light is emitted from the accretion disk. This is caused by friction produced from the material swirling around, and eventually into, the black hole. The material gathers around the event horizon — the point at which gravity becomes so strong that no matter — not even light — is able to escape. As the material around the black hole spins, friction heats up the gases and other material in the accretion disk, making them visible in optical light, x-ray, and infrared wavelengths on the electromagnetic spectrum.

These objects are so luminous, they can outshine all of the stars in their galaxies combined. That's pretty impressive, however, they are also believed to signal one of the final stages of galactic star formation. Then, the discovery of so-called "cold quasars" came along and challenged our theory of end-stage galaxy evolution. 

Looking back in time to the earliest days of the universe, some 670 million years after the big bang, astronomers found the oldest supermassive black hole yet, which lends credence to the theory that black holes proliferated the primordial universe. Dubbed J0313-1806, this supermassive black hole is located a little over 13 billion light-years from Earth and contains a mass of approximately 1.6 billion Suns. So much gas and dust orbit the black hole, it has formed a bright accretion disk, shining over 1,000 times brighter than all of the stars in the Milky Way combined.

All good things must come to an end, and eventually, galaxies run out of fuel to produce more stars, as all of the hydrogen is either locked up in existing nebulae, stars, and planets, or gobbled up by a supermassive black hole. However, using spectral data, astronomers have speculated that the J0313-1806 supermassive black hole is still gobbling up the equivalent of 25 suns every year — which means it's still growing.

This is where our cold quasars come in. 

What Exactly Are Cold Quasars? 

The author of an article on cold quasars, Allison Kirkpatrick from the University of Kansas notes: 

“All the gas that is accreting on the black hole is being heated and giving off X-rays,” The wavelength of light that you give off directly corresponds to how hot you are. For example, you and I give off infrared light. But something that’s giving off X-rays is one of the hottest things in the universe. This gas starts accreting onto the black hole and starts moving at relativistic speeds; you also have a magnetic field around this gas, and it can get twisted up. In the same way that you get solar flares, you can have jets of material go up through these magnetic field lines and be shot away from the black hole. These jets essentially choke off the gas supply of the galaxy, so no more gas can fall onto the galaxy and form new stars. After a galaxy has stopped forming stars, we say it’s a passive dead galaxy.”

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Makes sense that the black hole in the center of the galaxy would eat up all the gas and dust it can get its singularity on, right? Well, turns out, quasars aren't an automatic death sentence for galaxies, at least for some galaxies, so yes and no. Approximately 10 percent of older galaxies studied and observed with quasars in their center have stockpiles of gas and are thus still creating new stars. 

“We already knew quasars go through a dust-obscured phase,” Kirkpatrick noted. “We knew they go through a heavily shrouded phase where dust is surrounding the supermassive black hole. We call that the red quasar phase. But now, we’ve found this unique transition regime that we didn’t know before. Before, if you told someone you had found a luminous quasar that had a blue optical color — but it still had a lot of dust and gas in it, and a lot of star formation — people would say, ‘No, that’s not the way these things should look.’”

Blue Cold Quasar
This is an optical blue quasar looking back 7 billion years, This distant object is strange because ordinarily, they don't give off infrared emission. Source: Dark Energy Camera Legacy Survey DR7/NOAO

Except that appears to be how it does work. Objects in space tend to act in pretty unpredictable ways. We also don't have a firm grasp on the evolution of galaxies. We can view them from billions of light-years away — stretching back to some of the earliest stars and galaxies ever born — but that only goes so far because galaxies come in all shapes, sizes, and colors.

“We thought the way these things proceed was you have a growing black hole, it’s enshrouded by dust and gas, it begins to blow that material out,” she said. “Then it becomes a luminous blue object. We assumed when it blew out its own gas, it would blow out its host gas as well. But it seems with these objects, that’s not the case. These have blown out their own dust — so we see it as a blue object — but they haven’t yet blown out all of the dust and gas in the host galaxies. This is a transition phase, let’s say of 10 million years. In universal timescales, that’s really short — and it’s hard to catch this thing. We’re doing what we call a blind survey to find objects we weren’t looking for. And by finding these objects, yes, it could imply that this happens to every galaxy.”

An Interesting Discovery: 

Astronomist from NASA wrote, "SOFIA, a joint project of NASA and the German Aerospace Center, DLR, studied an extremely distant galaxy, located more than 5.25 billion light-years away called CQ4479. At its core is a special type of quasar that was recently discovered by Kirkpatrick called a “cold quasar.” In this kind of quasar, the active black hole is still feasting on material from its host galaxy, but the quasar’s intense energy has not ravaged all of the cold gas, so stars can keep forming and the galaxy lives on. This is the first time researchers have a detailed look at a cold quasar, directly measuring the black hole’s growth, star birth rate, and how much cold gas remains to fuel the galaxy." It currently births approximately 100 Sun-like stars every year.

“SOFIA lets us see into this brief window of time where the two processes can co-exist,” said one researcher. “It’s the only telescope capable of studying star birth in this galaxy without being overwhelmed by the intensely luminous quasar.”

Artist Rendering of a Quasar
Quasar rendering. Look how bright and powerful these objects are Source: Salome et al. 2015; Mahoro et al. 2017, Perna

“If this tandem growth continues both the black hole and the stars surrounding it would triple in mass before the galaxy reaches the end of its life.”  

That's some pretty impressive growth, Eventually, the quasar at the center of the galaxy will most likely exhaust all the gas and dust needed for star formation. The real question is: is this a rare occurrence, or are we seeing a phase all galaxies with quasars go through?

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