A supermassive black hole 9 billion light-years away appears to have a companion black hole orbiting around it. As the orbit shrinks, the pair gets closer to merging.
Supermassive black holes millions to billions of times the mass of our Sun lie at the heart of most galaxies, and astronomers are eager to know how these behemoths came to be. While they think most resulted from at least one merger between two smaller supermassive black holes, scientists lacked the observations that could give insight, since only one pair of supermassive black holes on the way to a merger had been found.
A new study may change that: Researchers observing a supermassive black hole report signs that it has a closely orbiting companion. The enormous duo – called a binary – circle one another about every two years.
If the team is correct, the diameter of the binary’s orbit is 10 to 100 times smaller than the only other known supermassive binary, and the pair will merge in roughly 10,000 years. That might seem like a long time, but it would take a total of about 100 million years for black holes of this size to begin orbiting one another and finally come together. So this pair is more than 99% of the way to a collision.
Joseph Lazio and Michele Vallisneri, at NASA’s Jet Propulsion Laboratory in Southern California, provided insight into how supermassive black holes behave in a binary system and how to interpret the radio data.
Evidence that this supermassive black hole may have a companion comes from observations by radio telescopes on Earth. Black holes don’t emit light, but their gravity can gather disks of hot gas around them and eject some of that material into space. These jets can stretch for millions of light-years. A jet pointed toward Earth appears far brighter than a jet pointed away from Earth. Astronomers call supermassive black holes with jets oriented toward Earth blazars, and a blazar named PKS 2131-021 is at the heart of this recent paper.
Located about 9 billion light-years from Earth, PKS 2131-021 is one of 1,800 blazars that a group of researchers at Caltech in Pasadena has been monitoring with the Owens Valley Radio Observatory in Northern California for 13 years as part of a general study of blazar behavior. But this particular blazar exhibits a strange behavior: Its brightness shows regular ups and downs as predictably as the ticking of a clock.
Researchers now think this regular variation is the result of a second black hole tugging on the first as they orbit each other about every two years. Each of the two black holes in PKS 2131-021 is estimated to be a few hundred million times the mass of our Sun. To confirm the finding, scientists will try to detect gravitational waves – ripples in space – coming from the system. The first detection of gravitational waves from black hole binaries was announced in 2016.
To confirm that the oscillations weren’t random or the cause of a temporary effect around the black hole, the team had to look beyond the decade (2008 to 2019) of data from the Owens Valley Observatory. After learning that two other radio telescopes had also studied this system – the University of Michigan Radio Observatory (1980 to 2012) and the Haystack Observatory (1975 to 1983) – they dug into the additional data and found that it matched predictions for how the blazar’s brightness should change over time.
“This work is a testament to the importance of perseverance,” said Lazio. “It took 45 years of radio observations to produce this result. Small teams, at different observatories across the country, took data week in and week out, month in and month out, to make this possible.”
For more on this study, see Colossal Black Holes Locked in an Epic Cosmic Dance at Heart of Galaxy.
Reference: “The Unanticipated Phenomenology of the Blazar PKS 2131–021: A Unique Supermassive Black Hole Binary Candidate” by S. O’Neill, S. Kiehlmann, A. C. S. Readhead, M. F. Aller, R. D. Blandford, I. Liodakis, M. L. Lister, P. Mróz, C. P. O’Dea, T. J. Pearson, V. Ravi, M. Vallisneri, K. A. Cleary, M. J. Graham, K. J. B. Grainge, M. W. Hodges, T. Hovatta, A. Lähteenmäki, J. W. Lamb, T. J. W. Lazio, W. Max-Moerbeck, V. Pavlidou, T. A. Prince, R. A. Reeves, M. Tornikoski, P. Vergara de la Parra and J. A. Zensus, 23 February 2022, The Astrophysical Journal Letters.
The really incredible thing is we are watching something that already happened 9 billion years ago!
The pair “will merge” in 10,000 years or the pair has merged 8,999.990.000 years ago?
Jordi, if their time estimate is correct the pair merged in the distant past but since the light from the jets takes 9 billion years to arrive here it will be seen to occur in our future.
Fascinating, but I have a question. If the black hole is wobbling (precesing) wouldn’t that produce the same regular brightening and dimming? If so is there any way to determine which is actually occurring? Just a thought born of curiosity.
That’s the funny thing about light-cones and relativity; there is no absolute reference frame by which you can say two things with space-like separation occurred “simultaneously”. To say something occurred 9 billion years ago because it took light that long to reach us is a misleading statement; It’s even weirder than that. “Simultaneous” events in space-time are events which fall directly along the edge of a light-cone. So if you see a star which exploded 9 billion years ago, that literally DEFINES the simultaneity between the star exploding and you seeing it. I know… I sound like a lunatic. But that’s just the way it is… there’s no “background” clock in the universe.
I’d argue this statement is somewhat misleading also. While it is certainly true you can’t definitely say the collision happened 9 billion years ago because that is the apparent “current” location of the object, this is because the objects have receded significantly since the light/radiation/gravity waves of the collision were emitted.
We’d have to make some assumptions regarding the cosmological constant value to pin it down, but from our perspective, the collision definitely happened billions of years ago, possibly before our sun was even born. It seems even more misleading to suggest it is going to happen in the future.
Some approximate numbers from a cosmological calculator:
Light from an object we see as being 9 billion light-years away was emitted ~6.5 billion years ago. (This corresponds to a redshift of ~1.0 and a current recession velocity of ~0.6c)
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