(Update: Although this news blog was posted six weeks ago, the research described has just been published in Nature's March 5th issue.)
Astronomers sifting though the massive data from the Sloan Digital Sky Survey (SDSS) have apparently turned up a remarkable find: a fast-orbiting, tight binary quasar. Two supermassive black holes seem to be revolving around each other at high speed less than a light-year apart.
If this is what the object really is — and its expected orbital motion should settle the question soon — it's an important find. Its existence would boost the case for building LISA, an ambitious, space-based gravitational-wave detector currently being developed at the European Space Agency and NASA. LISA would be able to sense the effects on spacetime of the spiraling-together and merger of supermassive black holes anywhere in the visible universe.
How often these events actually happen is unknown, though astronomers have come up with estimates. The discovery of supermassive black-hole binaries on their way to this violent fate would boost the case that building LISA will be worthwhile.
A Double Spectrum
The object is a 17th-magnitude speck in Serpens Caput known as SDSS J153636.22+044127.0, or J1536+0441 for short. (Names like this are made from an object’s celestial coordinates in the J2000.0 coordinate system.) It was fished up from the deeps when Todd A. Boroson and Tod R. Lauer (National Optical Astronomy Observatory) went sifting through Sloan's spectra of 17,500 quasars, looking for oddballs. This one stood out as unique. It shows the superposed spectra of two active galactic nuclei — supermassive black holes accreting hot matter — at two separate redshifts: z values 0.3727 and 0.3889. That amounts to a difference of 3,500 kilometers per second in their velocities.
In addition, a third set of spectral lines indicates a much larger region of thin, cool, quiet gas surrounding the pair. One of the black holes is moving away from us by 240 km/sec with respect to this gas, and the other is approaching us by 3,300 km/sec with respect to the gas.
Boroson and Lauer estimate that the holes have masses of 20 million and 800 million Suns, are separated by roughly 1/3 of a light-year, and have an orbital period of roughly 100 years. Picture a scale model: the black holes are the size of a pea and a basketball and are 600 feet (200 meters) apart. The hot, bright accretion disk around each is about a couple hundred feet wide.
The previous clear record-holder for tightest binary supermassive black hole involves two holes about 24 light-years apart, with an orbital period of about 150,000 years. There are indirect signs that the famous blazar OJ 287 is a tight binary in just a 12-year orbit, but many researchers are skeptical of this.
The new find is an unresolved pinpoint of light. Could it be two unrelated quasars at different distances, aligned by luck on the same line of sight accurately enough that Sloan can’t resolve them? It's not impossible. The authors estimate that there’s a 1 in 300 chance of this happening somewhere among the quasars they surveyed. And that assumes that quasars are scattered randomly on the sky, when in fact they tend to cluster (like galaxies).
We should have an answer soon. If the two objects really are orbiting each other as fast as Boroson and Lauer expect, their relative redshifts should change by 100 km/sec in only about 1 year. Spectra taken just a couple years apart should show the change.
A binary with those approximate orbital characteristics should take anywhere from several billion to several hundred billion years to spiral together and merge, if gravitational radiation is the only thing causing it to lose orbital energy. Clearly, this particular object isn’t about to perform for LISA anytime soon.
“This timescale is interesting,” write Boroson and Lauer, “as it implies that the binary has evolved past the ‘final parsec’ scale at which [orbital] decay due to energy exchange with stars becomes inefficient, but where gravitational radiation decay remains too weak to carry the evolution further.”
Astrophysicist Cole Miller (University of Maryland) comments, “Unless something else can bring [the pair] together a lot faster than that, one is led to wonder why we don't see more systems like this” — that is, hung up around 1 light-year separation and barely able to spiral any closer.
One way that a close supermassive binary could lose energy and creep together much faster is by interacting with, and flinging away, interstellar gas instead of stars. Whether this mechanism should produce lots of supermassive black-hole mergers or not is being studied.
Such mergers, and their literally cosmos-shaking effects, are explored in a feature article in the April Sky & Telescope, available in early March.
Here’s a preprint of the researchers’ paper, which will appear in Nature.