Newly merged black hole eagerly shreds stars

In this artist's conception, two black holes are about to merge. When they combine, gravitational wave radiation will "kick" the black hole like a rocket engine, sending it rampaging through nearby stars. David A. Aguilar (CfA)

By Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts
Published: April 11, 2011

A galaxy's core is a busy place, crowded with stars swarming around an enormous black hole. When galaxies collide, it gets even messier as the two black holes spiral toward each other, merging to make an even bigger gravitational monster.

Once it is created, the monster goes on a rampage. The merger kicks the black hole into surrounding stars. There it finds a hearty meal, shredding and swallowing stars at a rapid clip. According to new research by Nick Stone and Avi Loeb from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, upcoming sky surveys might offer astronomers a way to catch a gorging black hole "in the act."

Before the merger, as the two black holes whirl around each other, they stir the galactic center like the blade of a blender. Their strong gravity warps space, sending out ripples known as gravitational waves. When the black holes merge, they emit gravitational waves more strongly in one direction. That inequality kicks the black hole in the opposite direction like a rocket.

"That kick is very important. It can shove the black hole toward stars that otherwise would have been at a safe distance," said Stone. "Essentially, the black hole can go from starving to enjoying an all-you-can-eat buffet."

When tidal forces rip a star apart, its remains will spiral around the black hole, smashing and rubbing together, heating up enough to shine in the ultraviolet or X-rays. The black hole will glow as brightly as an exploding star, or supernova, before gradually fading in a distinctive way.

Importantly, a wandering supermassive black hole is expected to swallow many more stars than a black hole in an undisrupted galactic center. A stationary black hole disrupts one star every 100,000 years. In the best-case scenario, a wandering black hole could disrupt a star every decade. This would give astronomers a better opportunity of spotting these events, particularly with new survey facilities like Pan-STARRS and the Large Synoptic Survey Telescope.

Catching the signal from a disrupted star is a good start. However, astronomers really want to combine that information with gravitational wave data from the black hole merger. The Laser Interferometer Space Antenna (LISA), a future mission designed to detect and study gravitational waves, could provide that data.

Gravitational wave measurements yield accurate distances (to better than one part in a hundred, or 1 percent). However, they don't provide precise sky coordinates. A star's tidal disruption will let astronomers pinpoint the galaxy containing the recently merged black-hole binary.

By correlating the galaxy's redshift (a change in its light that's caused by the expanding universe) with an accurate distance, astronomers can infer the equation of state of dark energy. In other words, they can learn more about the force that's accelerating cosmic expansion, and which dominates the cosmic mass/energy budget today.

"Instead of 'standard candles' like supernovae, the black hole binary would be a 'standard siren.' Using it, we could create the most accurate cosmic 'ruler' possible," said Loeb.

Finding a merged black hole also would allow theorists to explore a new regime of Einstein's general theory of relativity.

"We could test general relativity in the regime of strong gravity with unprecedented precision," said Loeb.

Elusive Binary Black Hole System Identified

Artist's conception of the binary supermassive black hole system. Each black hole is surrounded by a disk of material gradually spiraling into its grasp, releasing radiation from x-rays to radio waves. The two black holes complete an orbit around their center of mass every 100 years, traveling with a relative velocity of 6000 kilometers per second.

Credit: p. Marenfeld and NOAO/AURA/NSF

Saturday, March 07, 2009

Finding a needle in a haystack might be easy compared to finding two very similar black holes closely orbiting each other in a distant galaxy.

Astronomers from the National Optical Astronomy Observatory (NOAO) in Tucson have found what looks like two massive black holes orbiting each other in the center of one galaxy. It has been postulated that twin black holes might exist, but it took an innovative, systematic search to find such a rare pair.

The newly identified black holes appear to be separated by only 1/10 of a parsec—a tenth of the distance from Earth to the nearest star. This discovery of the most plausible binary black hole candidate ever found may lead to a greater understanding of how massive black holes form and evolve at the center of galaxies. Their results are published in this week’s edition of the journal Nature.

After a galaxy forms it is likely that a massive black hole can also form at its center. Since many galaxies are found in cluster of galaxies, individual galaxies can collide with each other as they orbit in the cluster. The mystery is what happens to these central black holes when galaxies collide and ultimately merge together. Theory predicts that they will orbit each other and eventually merge into an even larger black hole.

The signature of a black hole in a galaxy has been known for many years. The material falling into a black hole emits light in narrow wavelength regions forming emission lines that can be seen when the light is dispersed into a spectrum. These emission lines carry the information about the speed and direction of the black hole and the material falling into it. If two black holes are present, they would orbit each other before merging and would have a characteristic dual signature in their emission lines. This signature has now been found.

Former NOAO Director Todd Boroson and NOAO Astronomer Tod Lauer used data from the Sloan Digital Sky Survey, a 2.5-meter diameter telescope at Apache Point in southern New Mexico to look for this characteristic dual black hole signature among 17,500 quasars discovered by the survey. More than 100,000 quasars are known, with most being found in the Sloan Digital Sky Survey and at distances that are billions of light-years away.

Quasars are the most luminous versions of the general class of objects known as active galaxies, which can be a hundred times brighter than our Milky Way galaxy, and powered by the accretion of material into supermassive black holes in their nuclei. The matter falling into the black hole doesn’t go directly in, but orbits around the black hole forming a flat accretion disc, much like the soap scum on water orbiting around an open drain.

It has long been thought that all large galaxies must have a massive black hole in their center and that some galaxies must have two or more black holes, at least until the black holes merge. The black holes would be so close together that it would be nearly impossible to see them or their accretion disks separately. However, the light emitted from the accretion disks, and the galaxy containing the black hole, ought to be identifiable.

Boroson and Lauer had to be especially careful to eliminate the possibility that they were seeing two galaxies, each with its own black hole, superimposed on each other. To try to eliminate this superposition possibility, they determined that the quasars were at the same red-shift determined distance and that there was a signature of only one host galaxy.

If the two quasars were independent objects at different distances, the spectral signature of both host galaxies should have been seen and each would have different red shift and thus different distance, even though they would be in the same line of sight. Determining the spectral signature was critical as it would be impossible to see the host galaxies directly against the glare of the quasar.

“The double set of broad emission lines is pretty conclusive evidence of two black holes,” Boroson argues. “If in fact this were a chance superposition, one of the objects must be quite peculiar. One nice thing about this binary black hole system is that we predict that we will see observable velocity changes within a few years at most. We can test our explanation that the binary black hole system is embedded in a galaxy that is itself the result of a merger of two smaller galaxies, each of which contained one of the two black holes.” The smaller black hole has a mass 20 million times that of the sun; the larger one is 50 times bigger, as determined by the their orbital velocities.