A regular brightening and dimming of a quasar 9 billion light years away is most easily explained by the presence of two supermassive black holes on a path to merger. If so, their orbits are decyaing at a rate that should lead to them combining in around 10,000 years, an event that will produce a gravitational wave of epic proportions.
Most galaxies have supermassive black holes hundreds of millions, or even billions, of times the mass of the Sun at their core. When galaxies merge, these vast masses fall towards each other, swinging into an orbit that gradually tightens as it loses energy until the two objects collide.
At least that is the theory. Seeing it in practice is a different matter. A paper in The Astrophysical Journal Letters claims to have found the closest such pair ever identified, and only the second time such giant masses have been found closer than the distance from the Sun to the nearest star.
PKS 2131-021 is a quasar that has attracted astronomers’ attention for almost 50 years. Like other quasars, it is spitting out a jet of material at more than 99 percent of the speed of light, but unlike most others the jet is heading straight towards Earth, making it a blazar.
Uniquely, PKS 2131-021’s jet’s brightness shows a near-perfect sinusoidal curve in data collected with Caltech’s Owens Valley Radio Observatory radio telescope. “That means that there is a pattern we can trace continuously over time.” Professor Tony Readhead of Caltech said in a statement.
Readhead and colleagues looked at old records from other telescopes. Data from 1981 and 2005 showed a matching pattern, but observations in between did not. Then Sandra O’Neill, a chemistry major investigating blazers as a pandemic project, learned even earlier archival records of PKS 2131-021 existed, made with the Haystack Observatory.
“When we realized that the peaks and troughs of the light curve detected from recent times matched the peaks and troughs observed between 1975 and 1983, we knew something very special was going on,” O’Neill said.
Modelling indicated the curve could be explained by an immense black hole orbiting an even heavier one every two years, although relativistic effects stretch the timescales so the cycle takes 4.7 years from our perspective. The paper acknowledges less straightforward explanations exist, but the authors consider them less likely by Occam’s Razor and have not considered them in depth.
The pair’s enormous masses mean they circle each other far faster than a planet orbiting the Sun at a similar distance. O’Neill and co-authors estimate they are separated by no more than 2,000 astronomical units – 2,000 times the Earth-sun distance or fifty times the orbit of Pluto – perhaps a tenth of that.
The only previous supermassive black hole pair thought to be remotely as close are in OJ 287, but even the components of that are 10-100 times further apart. Consequently, PKS 2131-021 allows us to witness a much more interesting stage of the process where slow orbital decay leads to a merger. Perhaps because of this closeness, the light curve for PKS 2131-021 is much more regular than OJ 287. Other “close” examples have separations thousands of times wider.
Since the first detection of a gravitational wave produced by merging black holes scientists have confirmed such events are quite common. However, these are stellar black holes, the remnants of supernovas with masses 20 times the Sun or more. The black holes at the centers of galaxies are to stellar black holes as the Sun is to a large asteroid. They are also vastly rarer, something which must also apply to their mergers.
In 10,000 years or so when the authors anticipate PKS 2131-021 will merge, the gravitational wave produced will be immense. Even at their current distance, however, the pair’s interaction must be warping spacetime to such an extent there is a chance the gravitational wave they are producing could alter the timing of pulsars in a detectable way.