Galaxy may hold hundreds of rogue black holes
Pastor Dale Morgan
Galaxy may hold hundreds of rogue black holes*
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