Galaxy may hold hundreds of rogue black holes

January 9, 2008
Galaxy may hold hundreds of rogue black holes
The arrow points to a candidate intermediate mass black hole in a Hubble space telescope image of a globular cluster in the M20 galaxy. Credit: Space Telescope Science Institute

If the latest simulation of what happens when black holes merge is correct, there could be hundreds of rogue black holes, each weighing several thousand times the mass of the sun, roaming around the Milky Way galaxy.

“Rogue black holes like this would be very difficult to spot,” says Vanderbilt astronomer Kelly Holley-Bockelmann, who is presenting the results of the supercomputer simulation at a meeting of the American Astronomical Society on Jan. 9 in Austin, Texas. Much of the research was done at Penn State University in collaboration with Deirdre Shoemaker and Nicolas Yunes before Holley-Bockelmann moved to Vanderbilt. Kayhan Gultekin at the University of Michigan also participated in the study.

“Unless it's swallowing a lot of gas, about the only way to detect the approach of such a black hole would be to observe the way in which its super-strength gravitational field bends the light that passes nearby. This produces an effect called gravitational lensing that would make background stars appear to shift and brighten momentarily,” she says.

The research focused on modeling "intermediate mass" black holes, whose very existence is controversial. Astronomers have ample evidence that small black holes less than 100 solar masses are produced when giant stars explode. There is similar evidence that “super-massive” black holes weighing the equivalent of millions to billions of solar masses sit at the heart of many galaxies, including the Milky Way. In addition, theoreticians have predicted that globular clusters – ancient, gravitationally bound groups of 100,000 to a million stars – should contain a third class of black holes, called intermediate mass black holes, that weigh a few thousand solar masses. But so far there have only been two tentative observations of objects of this sort.

In the past two years, scientists have succeeded in numerically simulating black hole mergers that incorporate Einstein’s theory of relativity. One of the big surprises to come from this effort is the prediction that when two black holes that are rotating at different speeds or are different sizes combine, the newly merged black hole receives a big kick due to conservation of momentum, pushing it away in an arbitrary direction at velocities as high as 4,000 kilometers per second.

“This is much higher than anyone predicted. Even the average kick velocity of 200 kilometers per second is extremely high when compared to the escape velocities of typical astronomical objects,” says Holley-Bockelmann. “We realized that basically any black hole merger would kick the new remnant out of a globular cluster, because the escape velocity is less than 100 kilometers per second.”

Using the facilities of Vanderbilt’s Advanced Center for Computation, Research and Education, Holley-Bockelmann’s team ran a number of simulations of the growth of intermediate mass black holes as they combine with a number of stellar-sized black holes, which are plentiful in globular clusters, paying close attention to the kick they received after each merger.

“We used different assumptions for the initial black hole mass, for the range of stellar black hole masses within a globular cluster, and assumed that the spins and spin orientations were distributed randomly. With our most conservative assumptions, we found that, even if every globular cluster started out with an intermediate-sized black hole, only about 30 percent retain them through the merger epoch. With our least conservative assumptions, less than 2 percent of the globular clusters should contain intermediate mass black holes today,” she says.

If the roughly 200 globular clusters in the Milky Way have indeed spawned intermediate-sized black holes, this means that hundreds of them are probably wandering invisibly around the Milky Way, waiting to engulf the nebulae, stars and planets that are unfortunate enough to cross their paths.

Fortunately, the existence of a few rogue black holes in the neighborhood does not present a major danger. “These rogue black holes are extremely unlikely to do any damage to us in the lifetime of the universe,” Holley-Bockelmann stresses. “Their danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometers. There are far more dangerous things in our neighborhood!”

Source: Vanderbilt University

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5 / 5 (2) Jan 09, 2008
%u201CTheir danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometers. There are far more dangerous things in our neighborhood!%u201D

That has got to be a mis-quote. The schwarzchild radius may be where the gravity over powers light itself, but an "intermediate mass" black hole would have a thousand to several thousand solar masses.

Last time I checked standard model, the sun supposedly gravitationally holds earth from a distances of 93 million miles, which is a HELL of a lot more than "a few hundred kilometers". In fact, if an "intermediate" black hole with 1000 solar masses passed within 32 astronomical units of the sun, it would capture and destroy the earth, all planets outside the earth, and possibly the sun itself, as its gravitational effects on the solar system would still be greater than or equal to that of the sun.

Even At 1000 AU, the 1000 solar mass black hole would have more gravitational force on pluto, than the sun itself does, if the black hole approached from the pluto side of the solar system! Pluto ~30-50AU depending on the timing of the flyby. Mass = 1000 solar. So sqrt 1000 < 32. Which means that even if the black hole were 1000 AU from the sun, it would still have stronger gravity on anything which was 32 AU away from the sun (which means it would capture pluto and all the dwarf planets/comets, etc). Even at that distance it would seriously perturb planetary and lunar orbits, and any closer it would start to wreck havoc on the inner solar system.

You can do the math yourself and see. Gravitational acceleration is proportional to mass and inversely proportional to distance squared. If A 1000 solar mass black hole passes anywhere even remotely near the solar system, its gravity WILL destroy the solar system, whether or not it sucks it into the event horizon.
3.5 / 5 (2) Jan 09, 2008
^Definitely a point that needed to be mentioned.

Sounds like the black holes can achieve velocities great enough to escape entire galaxies. Maybe an explanation for dark matter?
1 / 5 (4) Jan 09, 2008
2.7 / 5 (3) Jan 09, 2008

%u201CThese rogue black holes are extremely unlikely to do any damage to us in the lifetime of the universe,%u201D Holley-Bockelmann stresses. %u201CTheir danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometers. There are far more dangerous things in our neighborhood!%u201D

Exactly as QC said, the gravitational attraction of a black hole doesn't just suddenly stop at the Schwartzchild Radius; heck, if anything the accretion disk is also a 'danger zone', and that extends much further out than the Schwartzchild Radius, and there is still powerful gravitation much, much further out than that... what an odd thing to have said... However, one can still perhaps agree that it is unlikely they will cause us any serious damage any time soon, at least compared to nearby asteroids.

Or maybe he was trying really hard not to sound alarmist. Thats something which people today seem quite fond of calling others.
4 / 5 (2) Jan 10, 2008
I third what QC said.
I've done enough gravity simulations to know that if something with 1000 solar masses passes near the solar system you're going to notice it in a really bad way.
4 / 5 (3) Jan 10, 2008
Lessee... 100 to 150 intermediate black holes, OUTSIDE OF OUR GALAXY in the galactic halo, flying around at significant speed. By my estimate one of these 100 black holes would have to come well within 1 light year of Earth to have even the slightest direct effect on us.

Again, keep in mind that these are Halo objects, not things within our galaxy. Also keep in mind that they can be fired in any direction, and that they have escape velocity from our galaxy. That means that even if they should happen to be fired in the direction of our galactic disc they would only make one pass through, quite probably not interacting with any stars along the way, and then whoosh on out the other side. Every one of these black holes probably got booted out of our halo billions of years ago and are long gone by now.

This article isn't alarmist, it is interesting. But I've seen articles on this same story on other sites that say "EARTH COULD BE SWALLOWED BY ROGUE BLACK HOLES AT ANY MOMENT SAY SCIENTISTS!!!111one11!!"

If that isn't alarmist, I don't know what is.
not rated yet Jan 11, 2008
Well the chances of it being swallowed are extremely low from what we know, but the chances of our solar system being tampered with? Who knows?

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