Brilliant quasar from early cosmos houses supermassive black hole
Scientists nearly missed the shining behemoth because they never conceived that such a distant quasar could be so bright.
Astronomers say they have detected one of the brightest quasars in the cosmos, pegged to a time when the universe was less than 1 billion years old. The quasar houses a black hole tipping the cosmic scales at 12 billion times the mass of the sun.
The find was unexpected. Quasars from this period tend to be much dimmer – and their black holes less hefty. This raises questions about how such a powerful quasar with such a massive black hole could appear so early in the universe's history.
These questions touch on the processes guiding the early evolution of galaxies and of the supermassive black holes at their centers. Early quasars also are of interest because the prodigious amounts of energy they release are thought to have played a key role in lifting the cosmos out of its dark ages – a period when the universe was filled with a light-damping fog of neutral hydrogen gas.
Data from such early quasars provide a rigorous test of theories that researchers have proposed to describe the processes governing galaxy and supermassive black hole evolution, notes Anton Koekemoer, an astrophysicist at the Space Telescope Science Institute in Baltimore.
"We know locally that we have [supermassive] black holes. The question is: How and when did they get that massive?" says Dr. Koekemoer, who was not part of the international team reporting the discovery.
Quasars are gas-rich galaxies where star formation is intense and supermassive black holes at their centers are capturing vast quantities of gas and dust. As material orbits inward toward a supermassive black hole, the black hole's gravity compresses and heats it, producing radiation.
The black hole's magnetic fields concentrate much of that radiation into jets that stretch from each pole deep into space. Seen end-on, these jets appear as bright points – far outshining the galaxies where they originated. Indeed, quasars are the brightest objects in the universe.
Researchers involved with the study, as well as others who weren't, describe a heavyweight among early galaxies.
The discovery, described in a paper appearing in Thursday's issue of the journal Nature, involves a quasar known by its Sloan Digital Sky Survey catalog number, J0100+2802.
Light from the quasar originated at a time when the universe was about 875 million years old.
It's up to seven times more luminous than other quasars detected from this period. It's roughly six times as massive as its nearest competitors from the period. And if the heft of the black hole at this early epoch is any indication of the mass of the galaxy's stars, which is the case for more-local galaxies, J0100+2802 would host from 4 trillion to 9 trillion solar masses' worth of stars, according to Bram Venemans, a researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany. That's comparable to the largest galaxies in the nearby universe, he notes.
Indeed, the quasar is close to being the brightest one with the most massive central black hole of any quasar yet detected from any period in cosmic history, according to the research team, led by Chinese astrophysicists Xue-Bing Wu and Feige Wang with Peking University in Beijing, and Xiaohui Fan, an astronomer at the University of Arizona in Tucson.
Ironically, the team initially overlooked the quasar, acknowledges Dr. Fan. It first cropped up as the team sifted through data from the Sloan Digital Sky Survey to find likely quasars from this early period – targets for more-detailed study.
While the object had some traits that suggested it might be a quasar, it was too bright. So it sat on a list of "borderline" candidates. Only after the researchers used a second set of sky-survey data, this time from NASA's Wide-field Infrared Survey Explorer mission, which provided additional insights into the object, did the team tag it for a closer look.
"We didn't expect a quasar to be that bright at that kind of distance," Fan says, adding that it was a case of being too sure about what they were looking for.
Once the team gathered additional information on the quasar using a relatively modest telescope in China, astronomers at several major observatories around the globe gathered yet more data on the quasar to help tease out its traits.
Having found the brilliantly shining behemoth at such an early epoch, the next task is to explain it.
Supermassive black holes can grow through galaxy mergers, Koekemoer explains. But that requires far more time than would have been available.
Another possibility: The supermassive black holes grew from black holes 10 to perhaps 100 times the mass of the sun. These would have formed as the early generations of very massive, quick-burning stars exploded as supernovae and collapsed into black holes. If the black hole has enough gas feeding it at a high rate for enough time, it could grow into a black hole several billion times the mass of the sun, Koekemoer says.
But, he adds, black-hole growth rates have a natural regulator, determined by the black hole's gravity and the intensity of the radiation emitted by the oblivion-bound material as it approaches the black hole and heats up. This radiation exerts outward pressure, which counteracts the gravity. As the pressure increasingly counterbalances the gravity, the black hole's consumption rate slows, curbing its growth.
This process also would take too long.
Within the past decade, researchers have revisited a variant that could explain J0100+2802's rapid growth. The black hole could continue to grow at an increasing pace if enough gas kept flowing toward the black hole so that the gas's high density absorbed the increases in radiation that might otherwise put the brakes on growth.
Yet another alternative, suggests Fan, is a two-stage process where larger black-hole seeds emit their radiation in all directions as they gather up gas, diffusing the radiation enough to allow for rapid growth until the black hole reaches about 1 million solar masses. At that point, more-typical growth mechanisms kick in.