A supermassive black hole in the early universe is at least 10 times too heavy for its host galaxy, raising questions about galaxy and black hole coevolution.
The temperature dial had been hovering dangerously low when it finally dropped to the red zone. Megan Urry was nervous. She was observing on one of the world’s largest telescopes — the 10-meter Keck I in Hawai‘i — and didn’t want to lose precious telescope time.
At 14,000 feet above the Pacific Ocean, snow isn’t uncommon on Mauna Kea’s summit. But snow wasn’t her concern. The sky was crystal clear. It was so clear in fact that she could see over halfway across the observable universe. No, she was concerned that once the temperature reached a certain threshold, telescope operators would have to shut down or risk the dome freezing.
The dial stayed in the red zone for a breathtaking five minutes before it inched back up, encouraged by the Sun’s approaching rays. Urry let out a sigh of relief, thrilled that her hard work wouldn’t be lost. But she didn’t realize just how lucky she was on that cold February night. The object under scrutiny would soon defy all expectations.
Monstrous Black Hole
CID-947 is a supermassive black hole that’s visible across the observable universe because it’s actively gobbling down gas and spitting out radiation. It’s a classic quasar in every way except one: its supermassive black hole is too heavy for its galaxy.
Over the past decade, astronomers have discovered that supermassive black holes have a scaling relationship with their host galaxies: the more massive the black hole, the more massive the galaxy. On average, a supermassive black hole has a mass 1/100 to 1/1,000 times that of its host galaxy. Seemingly consistent relationships like this one have led astronomers to suspect that a black hole’s growth and its host galaxy’s growth are intertwined.
But CID-947 doesn’t lie anywhere near this relation. At 2 billion years after the Big Bang, this quasar only weighs eight times less than its host galaxy. “We found a black hole that's too big for its breeches,” says co-author Martin Elvis (Harvard-Smithsonian Center for Astrophysics).
Learn more about mysterious and supermassive black holes in our FREE black holes ebook. Read four articles from the experts on everything from how supermassive black holes shaped the universe we live in today, to how astronomers plan to image the silhouette of Milky Way's black hole within the decade!
Do Galaxies go it Alone?
Such an odd black hole raises a number of questions and every team member seems to have his or her favorite. Elvis, for example, wants to know how this black hole grew to be so massive so quickly. (It’s a question that’s growing increasingly common.)
Urry, on the other hand, wants to know what type of galaxy it looks like today. “The only way that it becomes something that looks familiar to us today is if its galaxy grows a whole lot in the next 12 billion years and its black hole doesn't grow at all,” says Urry. “It's a little strange.”
But it’s possible. Gas continuously spools toward galaxies along thin filaments of dark matter. “You can think of all the matter in the universe as always on the move toward the more dense areas and away from the less dense areas,” says Urry. Eventually so much matter will rain onto galaxies that they will grow steadily heavier.
Meanwhile their supermassive black holes would remain unchanged. Although black holes are often perceived as bloodthirsty monsters that suck down any nearby gas “it’s actually hard for matter to fall into a black hole,” says Urry. “The typical trajectory of a particle moving toward a black hole would just be to orbit the black hole kind of like we orbit the Sun.”
If this is the classic evolutionary path that supermassive black holes and their galaxies follow, then the two are not in lockstep after all. Instead black holes evolve quickly and their host galaxies follow. John Kormendy (University of Texas at Austin), an outside expert who was not involved in the research, agrees that it might be time to step off the lockstep bandwagon.
But that’s a strong statement based on one object alone. That said, Urry is pretty convinced these objects are common. Unlike large surveys like the Sloan Digital Sky Survey (SDSS), her team only looked at a tiny portion of the sky (one square degree compared to SDSS’s 10,000 square degrees). And when you look at such a small patch of the universe you don’t expect to see any outliers. It’s like searching for a four-leaf clover. You have a far better chance of finding it if you’re looking at a field’s worth of clovers as opposed to a handful. And if you did find it in a handful of clovers, then you might wonder whether it’s actually lucky or just commonplace.
Still, Kormendy has his doubts about the accuracy of the measurements. Measuring a supermassive black hole’s mass, for example, is tricky. The team calculated it by measuring the speed of clouds that orbit the black hole, but without a clear view of those clouds they had to make a few estimates.
And Elvis agrees. The mass of the black hole is likely a large source of error. “But this quasar is just so far off the normal relation that even those fairly large errors don't make a difference,” says Elvis. “It's very hard to find our way out of this problem.”
“The object tells us dramatically (but with some uncertainty) a story that other objects have told us, too (also with a lot of uncertainty),” says Kormendy. “Life at the frontier is tricky. Even when you do good work.”
Benny Trakhtenbrot et al. “An over-massive black hole in a typical star-forming galaxy, 2 billion years after the Big Bang.” Science. July 10, 2015.