What happens when black holes collide?
What happens when black holes collide?
For more than 20 years, astronomers have considered an intriguing question: What happens when two black holes meet? Inside a galaxy, black holes that formed from dead,
massive stars might encounter each other, especially in double or multiple star systems. No one has yet seen such a collision take place, but the subject is becoming a hot topic of theoretical astrophysics.
Studying the possibilities took a big step forward in 2004, when a team of astronomers — Marc Favata of Cornell University, Scott Hughes of the Massachusetts Institute of Technology, and Daniel Holz of the University of Chicago — authored a study that appeared in Astrophysicol Journal Letters; this kicked off a number of other studies to create a burgeoning field.
It turns out astronomers think a funny thing happens when black holes collide. They spiral toward each other and merge into a single entity. A gravitational "slingshot" effect then violently whips them outside their host galaxies into intergalactic space. The ejection mechanism results from a byproduct of the merger: gravitational waves.The gravitational waves actually shoot the merged black hole far away from the site of its merger.
What role does this process play in building black holes? Could large numbers of black holes exist outside galaxies, where their presence would be extremely difficult to detect? These questions and others are currently on the table, and researchers are looking to build their knowledge of the subject. A real breakthrough would come from observing a binary black hole — a black-hole merger in the making.
"Almost all large galaxies contain black holes,"says Hughes, "and galaxies merge like mad — especially a couple billion years ago." Hughes believes binary black holes could have formed and be forming yet today, but detecting them observationally will be difficult. "We're talking about two
incredibly small bodies separated by a parsec," he says, merely 3.26 light-years in galaxies that span hundreds of thousands of light- years across.
Black holes escaping their parent galaxies would be shot out at high velocities, probably 685,000 mph (1.1 million km/h). Such high-speed objects eventually might join other nomadic black holes in deep space.
Such freeform black holes would prove elusive."If they're not shining [from radiation produced by swallowing nearby bright material], it's hard to know where to look," according to Piero Madau of the University of California, Santz Cruz. The only way to
BABY BLACK HOLE. The feeble light from a red- shift 6.4 quasar (arrow) took roughly 13 billion years to reach our eyes. This image shows the quasar when the universe was just 800 million years old. Physicists are learning how the supermassive black hole that feeds the quasar got so big early in the universe.
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TRAIN WRECK A'COMIN'. Two supermassive black holes sit at the center of the galaxy NGC 6240 after two small galaxies collided. Both black holes are active, shown by their X-ray radiation (inset) imaged by the Chandra X-ray Observatory. While they are roughly 3,000 light-years apart now, in a few hundred million years, the two black holes will merge.
detect intergalactic black holes would be from gravitational-lensing effects, and current telescopes are unable to do that.
Intergalactic black holes could absorb material without radiating, and so, continue along below the radar. "For that reason,"says Mitch Begelman of the University of Colorado, "We can't rule out the possibility that black holes outside galaxies contain more mass than black holes inside galaxies."
The evidence for small black holes gone missing from normal galaxies does hold some potential. According to David Merritt of the Rochester Institute ofTechnology in New York, "As we look at ever smaller galaxies, there's a point where you stop seeing black holes." Merritt and other astronomers wonder if smaller galaxies may have had their small stellar black holes shot out into intergalactic space.
Studying black-hole mergers could pay off big dividends when it comes to understanding large black holes in the early universe. Do they form by the process of mergers or by gradual accretion? Observational tests for determining how big black holes formed in the early cosmos are lacking; perhaps looking at how smaller black holes behave in more recent times will shed light on this question. S3
SIMULATING SMASH-UPS. Astronomers can model black-hole kicks (which may eject them from galaxies) with computer simulations. In these, hundreds of primordial black holes live within dark-matter halos. Each image shows black- hole trajectories over billions of years — from redshift 8.16, when the universe was 600 million years old, to redshift 1, at an age of 6.5 billion years — based on different initial velocities. Yellow corresponds to high-density areas; red to low-density, tomabelvmir
For more than 20 years, astronomers have considered an intriguing question: What happens when two black holes meet? Inside a galaxy, black holes that formed from dead,
massive stars might encounter each other, especially in double or multiple star systems. No one has yet seen such a collision take place, but the subject is becoming a hot topic of theoretical astrophysics.
Studying the possibilities took a big step forward in 2004, when a team of astronomers — Marc Favata of Cornell University, Scott Hughes of the Massachusetts Institute of Technology, and Daniel Holz of the University of Chicago — authored a study that appeared in Astrophysicol Journal Letters; this kicked off a number of other studies to create a burgeoning field.
It turns out astronomers think a funny thing happens when black holes collide. They spiral toward each other and merge into a single entity. A gravitational "slingshot" effect then violently whips them outside their host galaxies into intergalactic space. The ejection mechanism results from a byproduct of the merger: gravitational waves.The gravitational waves actually shoot the merged black hole far away from the site of its merger.
What role does this process play in building black holes? Could large numbers of black holes exist outside galaxies, where their presence would be extremely difficult to detect? These questions and others are currently on the table, and researchers are looking to build their knowledge of the subject. A real breakthrough would come from observing a binary black hole — a black-hole merger in the making.
"Almost all large galaxies contain black holes,"says Hughes, "and galaxies merge like mad — especially a couple billion years ago." Hughes believes binary black holes could have formed and be forming yet today, but detecting them observationally will be difficult. "We're talking about two
incredibly small bodies separated by a parsec," he says, merely 3.26 light-years in galaxies that span hundreds of thousands of light- years across.
Black holes escaping their parent galaxies would be shot out at high velocities, probably 685,000 mph (1.1 million km/h). Such high-speed objects eventually might join other nomadic black holes in deep space.
Such freeform black holes would prove elusive."If they're not shining [from radiation produced by swallowing nearby bright material], it's hard to know where to look," according to Piero Madau of the University of California, Santz Cruz. The only way to
BABY BLACK HOLE. The feeble light from a red- shift 6.4 quasar (arrow) took roughly 13 billion years to reach our eyes. This image shows the quasar when the universe was just 800 million years old. Physicists are learning how the supermassive black hole that feeds the quasar got so big early in the universe.
WWW.ASTRONOMY.COM 55
5DSS
TRAIN WRECK A'COMIN'. Two supermassive black holes sit at the center of the galaxy NGC 6240 after two small galaxies collided. Both black holes are active, shown by their X-ray radiation (inset) imaged by the Chandra X-ray Observatory. While they are roughly 3,000 light-years apart now, in a few hundred million years, the two black holes will merge.
detect intergalactic black holes would be from gravitational-lensing effects, and current telescopes are unable to do that.
Intergalactic black holes could absorb material without radiating, and so, continue along below the radar. "For that reason,"says Mitch Begelman of the University of Colorado, "We can't rule out the possibility that black holes outside galaxies contain more mass than black holes inside galaxies."
The evidence for small black holes gone missing from normal galaxies does hold some potential. According to David Merritt of the Rochester Institute ofTechnology in New York, "As we look at ever smaller galaxies, there's a point where you stop seeing black holes." Merritt and other astronomers wonder if smaller galaxies may have had their small stellar black holes shot out into intergalactic space.
Studying black-hole mergers could pay off big dividends when it comes to understanding large black holes in the early universe. Do they form by the process of mergers or by gradual accretion? Observational tests for determining how big black holes formed in the early cosmos are lacking; perhaps looking at how smaller black holes behave in more recent times will shed light on this question. S3
SIMULATING SMASH-UPS. Astronomers can model black-hole kicks (which may eject them from galaxies) with computer simulations. In these, hundreds of primordial black holes live within dark-matter halos. Each image shows black- hole trajectories over billions of years — from redshift 8.16, when the universe was 600 million years old, to redshift 1, at an age of 6.5 billion years — based on different initial velocities. Yellow corresponds to high-density areas; red to low-density, tomabelvmir
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