How do you catch the movement of something that absorbs light itself? Scientists have been trying to answer this question for a long time but to no avail. While the idea of moving black holes has been around for centuries, empirical evidence about their structure and existence has eluded astronomers so far.
Now, in a surprising turn of events, scientists at the Harvard & Smithsonian Center for Astrophysics have identified the clearest case to date of a supermassive black hole in motion. Located 230 million light-years away from Earth, it is present at the centre of a galaxy named J0437+2456. The mass of this supermassive black hole is a whopping three million times that of our Sun.
Supermassive Black Holes
The idea of a black hole originates from Einstein’s theory of general relativity which predicted that when a massive star dies, a small core is left behind that is more than three times the mass of the Sun. The gravitational force of this core is so strong that it absorbs everything surrounding it, including itself, in turn producing a black hole.
Broadly, these black holes have been divided into four types—supermassive blackholes being the largest among them with masses of millions to billions of times that of the Sun. “We don’t expect the majority of supermassive black holes to be moving; they’re usually content to just sit around,” says Dominic Pesce, an astronomer at the Center for Astrophysics who led the study.
While scientists cannot directly observe these supermassive black holes with telescopes, they can infer the presence of these cosmic entities by studying the impact they have on their surrounding stars and gases.
“They’re just so heavy that it’s tough to get them going. Consider how much more difficult it is to kick a bowling ball into motion than it is to kick a soccer ball—realizing that in this case, the ‘bowling ball’ is several million times the mass of our Sun. That’s going to require a pretty mighty kick” said Pesce.
Is this Supermassive Black Hole going somewhere?
Pesce and his collaborators have been trying to understand the movement of these supermassive black holes for the last five years by comparing their velocities with that of galaxies. “We asked: Are the velocities of the black holes the same as the velocities of the galaxies they reside in?” he explains. “We expect them to have the same velocity. If they don’t, that implies the black hole has been disturbed.”
Initially, the team studied 10 distant galaxies and the supermassive black holes present at their cores. Specifically, they examined black holes that contained water within their accretion disk—the spiral structure that spins inward towards the black hole.
As the water spins around the black hole, it creates a laser-like beam of radio light known as a maser. When studied through a combined network of radio antennas using a technique known as very long baseline interferometry (VLBI), masers can assist in measuring a black hole’s velocity very precisely, Pesce reports in a statement from the Harvard & Smithsonian Center for Astrophysics.
Through this technique, the team found that while nine out of the ten supermassive black holes were at rest, one seemed to be in motion. To confirm these initial findings, the researchers conducted follow-up observations with the Arecibo and Gemini Observatories. They found that the supermassive black hole is, in fact, moving at a speed of about 110,000 miles per hour inside the galaxy J0437+2456.
Possible reasons for the unlikely motion
Astronomers are not yet certain about the possible reasons for the motion of the supermassive black hole. “We may be observing the aftermath of two supermassive black holes merging,” says Jim Condon, a radio astronomer at the National Radio Astronomy Observatory who was involved in the study. “The result of such a merger can cause the newborn black hole to recoil, and we may be watching it in the act of recoiling or as it settles down again.”
Yet, an even more exciting possibility that explains this phenomenon is that the black hole may be part of a binary system. A binary system refers to a system of two cosmic bodies that are in such proximity that their gravitational force leads them to orbit each other around their centre of mass.
“Despite every expectation that they really ought to be out there in some abundance, scientists have had a hard time identifying clear examples of binary supermassive black holes,” Pesce says. “What we could be seeing in the galaxy J0437+2456 is one of the black holes in such a pair, with the other remaining hidden to our radio observations because of its lack of maser emission.”
Their results were published in the Astrophysical Journal last week and can be accessed here.
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