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Written by Dennis Overbye
Astronomers are looking at the Cosmic Lost and Found Office for one of the largest and worst black holes believed to exist. So far they haven’t found it.
In the last decades it has become part of the astronomical tradition that in the center of every galaxy a huge black hole lurks, in which the equivalent of millions or even billions of suns has disappeared. The larger the galaxy, the more massive the black hole is in its center.
So it was a surprise a decade ago when Marc Postman of the Space Telescope Science Institute found an oversized galaxy with no evidence of a black hole in its center using the Hubble Space Telescope to study galaxy clusters. Normally the core of the galaxy would have a kink of extra light in its center, a kind of sparkling cloak created by stars gathered there by the gravity of a huge black hole.
On the contrary, right in the middle of the galaxy’s broad core, where there should have been a slight bump in the starlight, there was a slight slope. In addition, the entire core, a cloud of stars about 20,000 light years across, was not even in the middle of the galaxy.
“Oh my god, this is really unusual,” recalled Tod Lauer, an expert on galactic nuclei at the National Optical Astronomy Observatory in Tucson, Ariz., And author of the paper when Postman showed him the finding.
That was in 2012. In recent years, the two researchers and their colleagues have been looking for X-ray or radio waves from the missing black hole.
The galaxy is the brightest in a cluster called Abell 2261. From here it is about 2.7 billion light years, in the constellation Hercules in the northern sky, not far from the prominent star Vega. According to the standard rule of thumb, the black hole missing from the center of the 2261 galaxy should be 10 billion solar masses or more. In comparison, the black hole in the center of the Milky Way has only about 4 million solar masses.
Where has nature hidden the equivalent of 10 billion suns?
One possibility is that the black hole is there but has fallen silent and temporarily has nothing to eat. Another provocative possibility, say Lauer and his colleagues, is that the black hole has been thrown out of the galaxy altogether.
“A pit in every peach”
Evidence of the latter could provide insight into some of the most violent and dynamic processes in the evolution of galaxies and the cosmos that astronomers have theorized but never seen – a dance of titanium forces and swirling worlds that can toss stars and planets through the void.
“It’s a fascinating puzzle, and we’re into the case,” Postman said in an email. He added that the upcoming James Webb space telescope would have the ability to shed light on the case, so to speak.
“What happens when you eject a supermassive black hole out of a galaxy?” Asked Lauer.
Lauer is part of an informal group called the Nukers. The group first met under Sandra Faber of the University of California at Santa Cruz in the early days of the Hubble Space Telescope. For the past four decades, they have attempted to unravel the nature of galactic nuclei by peering into the intimate hearts of distant galaxies with the keen eye of Hubble and other new facilities.
“The story of A2261-BCG,” he said, referring to the galaxy’s formal name in the literature, “is what happens to the most massive galaxies in the universe, the giant elliptical galaxies, at the end point of galaxy evolution.”
Black holes are objects that are so dense that not even light can escape from their gravitational couplings. They’re invisible by definition, but the turmoil – x-rays and radio screams – caused by material falling into its grip can be seen throughout the universe. The discovery of quasars in the centers of galaxies in the 1960s initially led astronomers to believe that supermassive black holes are responsible for such fireworks.
By the turn of the century, astronomers had concluded that every galaxy had a supermassive black hole in its bosom that was millions to billions of times more massive than the sun. Where they came from – whether they grew out of smaller black holes formed by the collapse of stars or some other process early in the universe – no one is sure. “There’s a pit in every peach,” said Lauer.
But how do these entities affect their environment?
In 1980 three astronomers, Mitchell Begelman, Martin Rees, and Roger Blandford, wrote about how these black holes would change the evolution of the galaxies they inhabit. When two galaxies collided and merged – a particularly common occurrence in the earlier universe – their central black holes met and formed a binary system, with two black holes orbiting each other.
Begelman and colleagues argued that these two massive black holes swinging around would interact with the sea of stars they were immersed in. Occasionally one of these stars would have a close encounter with the binary, and gravitational forces pushed the star out of center, leaving the black holes bound even tighter.
Over time, more stars would be thrown away from the center. Gradually, the starlight that was once concentrated in the middle spread out into a wider, diffuse core, with a small kink in the middle where the black hole binary did its mating dance. The process is known as “scrubbing”.
“They were way ahead of the game,” said Lauer of the three astronomers.
A gnarly problem
A thinned core was the kind of situation Lauer and Postman thought they’d experienced with Abell 2261. But instead of a peak in the middle of the core, there was a slope, as if the supermassive black hole and its associated stars had simply been removed.
This opened up the more dramatic possibility that the scenario envisaged by Begelman and his colleagues had played out: the two black holes had merged into one giant gulp of nothing. The merger would have been accompanied by a catastrophic burst of gravitational waves, space-time waves that Einstein predicted in 1916 and finally seen a century later, in 2016, by the LIGO instruments.
If this eruption had been crooked, it would have caused the resulting supermassive black hole to fly through the galaxy or even out of it, which astronomers had never observed. Finding the faulty black hole was therefore of paramount importance.
Another examination of A2261-BCG revealed four small nodes of light within the diffuse core. Could any of them host the black hole?
A team led by Sarah Burke-Spolaor of West Virginia University took to the skies with Hubble and the Very Large Array radio telescope in Socorro, New Mexico. Spectroscopic measurements from Hubble were able to determine how fast the stars were moving in the nodes and therefore whether a massive object was required to hold them together.
Two of the nodes, they concluded, were likely small galaxies with small internal motions that cannibalized from the large galaxy. Measurements of the third node had error bars so large that it could not yet be excluded or excluded as the location of the black hole.
The fourth, very compact knot near the bottom edge of the core was too weak for Hubble, Burke-Spolaor reported. “Observing this node would have required excessive time (hundreds of hours) to observe with the Hubble Space Telescope,” she said in an email, and so it remains a candidate for hiding.
The galaxy’s core also sends out radio waves, but they didn’t help the search, Burke-Spolaor said.
“We had originally hoped the radio emission would be some sort of literal smoke gun showing an active jet pointing straight back at the location of the black hole,” she said. But the radio relic was at least 50 million years old by its spectral properties, which meant that the great black hole would have had enough time to move to another location since the jet was switched off.
The next stop was NASA’s Orbiting Chandra X-ray Observatory. Kayhan Gultekin of the University of Michigan, another veteran Nuker who was not on the original discovery team, pointed the telescope at the cluster core and those suspicious nodes. No dice. The suspected black hole would have to feed at a millionth of its potential rate if it were there at all, Gultekin said.
“Either a black hole in the middle is very weak or it isn’t there,” he wrote in an email. The same is true in the case of a binary black hole system, he said; it would have to eat very little gasoline to stay hidden.
Meanwhile, Imran Nasim of the University of Surrey, who was not part of Postman’s team, has published a detailed analysis of how the fusion of two supermassive black holes could transform the galaxy into what astronomers found.
“The recoil of the gravitational wave” simply throws the supermassive black hole out of the galaxy, “Nasim explained in an email. After losing its supermassive anchor, the star cloud spreads around the black hole binary and becomes more diffuse. The density of stars in this region – the densest part of the entire giant galaxy – is only one-tenth the density of stars in our own neighborhood of the Milky Way, resulting in a night sky that appears anemic compared to our own.
All of this is another reason astronomers are eagerly awaiting the launch of the James Webb Space Telescope, the long-awaited successor to Hubble, now slated for late October. This telescope can examine all four nodes at the same time and determine whether any of them are a supermassive black hole.
“Here you can see our great sophistication,” said Lauer. “Hey, maybe it’s in the knot! Hey maybe it isn’t! Better look for everything! “