A Supermassive Double Black Hole?

(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.

LISA illustration
In this symbolic illustration, LISA (the Laser Interferometer Space Antenna) measures slight ripples in spacetime from a black-hole merger billions of light-years away. LISA will consist of three stations 5 million km apart in solar orbit, linked by laser beams monitoring changes in their separations much smaller than the width of an atom.



That's assuming LISA gets built. A forerunner mission, LISA Pathfinder, is scheduled to launch at the end of 2009 to test the technology. If LISA itself gets fully funded, which remains in doubt, it would launch no sooner than 2018.
ESA / NASA
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.

Gravitational waves from merging black holes
Gravitational waves thrown off by a closely orbiting pair of black holes are shown in red in this frame from a simulation by NASA's Columbia supercomputer.
HENZE / NASA
The Inspiral

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.

12 thoughts on “A Supermassive Double Black Hole?

  1. Peter WilsonPeterr W

    There has to be a tidal interaction, causing the pair to spiral inwards. Tides on Earth cause its rotation to slow. Tidal interaction between the pair and their galzxy would cause them to spiral in. This isn’t mentioned in the article.

  2. Gill

    As a side issue to the double black hole article, I’ve often wondered about LIGO and LISA, and the fact that these projects are trying to measure changes in space. Since we are living in what is known as “space/time”, wouldn’t it be easier to measure changes in time? I would think it could even be set up on Earth, and not have to waste money on mucking about in space. Set up 4 atomic clocks in a tetrahedral configuration around the planet to be able to triangulate the signal’s direction and speed. Each station would have a recorder to record the signal. The signal would then be transmitted at a later time, and in this way the originating signal from the fluctuations in time would not get entangled in the transmitter’s signal to the receiving station. Would that not be a feasible project? And cheaper?

  3. Peter WilsonPeter Wilson

    Gill:
    The Earth moves…that is the problem with ground-based systems already. Everything from trucks rolling by to waves hitting the coasts causes vibrations, which must be filtered out. Putting ground-based stations farther apart only increases the problem. Space eliminates the shaking problem altogether…for a price.

  4. Peter WilsonPeter Wilson

    Gill:
    The Earth moves…that is the problem with ground-based systems already. Everything from trucks rolling by to waves hitting the coasts causes vibrations, which must be filtered out. Putting ground-based stations farther apart only increases the problem. Space eliminates the shaking problem altogether…for a price.

  5. Robert Sheaffer

    Not so sure about any “tidal interaction” involving two black holes. Tidal forces occur because gravitationally-interacting bodies are extended objects, and the differing gravitational forces acting at different places on the objects result in stretching and sloshing about. But black holes are supposed to be point zources, objects that have collapsed into a singularity. And if two gravitationally-interacting bodies are so tiny as to be represented as point sources, there will be no tidal forces.

  6. Gill

    As I was talking about measuring changes in time, and not space, would rumbling trucks cause changes in time? And if the equipment needs to be put in space with sufficient distances between each receiving station to accurately triangulate, I think an atomic clock, recorder and transmitter is still less expensive than the equipment and logistics needed for LISA. I’m not opposed to LISA in the least bit. But I also know that it may not even get off of the ground because of its cost, and our science projects dwindling budgets.

  7. Gill

    As I was talking about measuring changes in time, and not space, would rumbling trucks cause changes in time? And if the equipment needs to be put in space with sufficient distances between each receiving station to accurately triangulate, I think an atomic clock, recorder and transmitter is still less expensive than the equipment and logistics needed for LISA. I’m not opposed to LISA in the least bit. But I also know that it may not even get off of the ground because of its cost, and our science projects dwindling budgets.

  8. Peter WilsonPeter Wilson

    The “tidal” interaction I am referring to is 20 and 800 million solar masses orbiting a common center of gravity, verse a single, spinning, 820 million solar mass object. From a distance, the two orbiting masses are equivalent to a single rotating object with an extreme tidal bulge. The rest of the mass in the galaxy will feel a much stronger interaction from the two orbiting masses than they would from a single, combined, object. How much stronger, I do not know, but I doubt it can be ignored.

  9. Peter WilsonPeter Wilson

    The “tidal” interaction I am referring to is 20 and 800 million solar masses orbiting a common center of gravity, verse a single, spinning, 820 million solar mass object. From a distance, the two orbiting masses are equivalent to a single rotating object with an extreme tidal bulge. The rest of the mass in the galaxy will feel a much stronger interaction from the two orbiting masses than they would from a single, combined, object. How much stronger, I do not know, but I doubt it can be ignored.

  10. Pingback: Black Holes Tango in Distant Galaxy

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