Globular Star Clusters May Foster Repeated Merging of Multiple Black Holes

Stellar-mass black holes that reside in globular clusters — small regions of space, usually at the edges of a galaxy, that are packed with hundreds of thousands to millions of stars — could combine repeatedly to form objects bigger than anything a single star could produce.

A snapshot of a simulation showing a binary black hole formed in the center of a globular cluster. Image credit: Northwestern Visualization / Carl Rodriguez.

A snapshot of a simulation showing a binary black hole formed in the center of a globular cluster. Image credit: Northwestern Visualization / Carl Rodriguez.

Stellar binary black holes are formed when two black holes, created out of the remnants of massive stars, begin to orbit each other. Eventually, the black holes merge in a collision that, according to Einstein’s theory of general relativity, should release a huge amount of energy in the form of gravitational waves.

Now, MIT astrophysicist Carl Rodriguez and co-authors suggest that black holes may partner up and merge multiple times, producing black holes more massive than those that form from single stars. These ‘second-generation mergers’ should come from globular clusters.

“We think these clusters formed with hundreds to thousands of black holes that rapidly sank down in the center,” Dr. Rodriguez said.

“These kinds of clusters are essentially factories for black hole binaries, where you’ve got so many black holes hanging out in a small region of space that two black holes could merge and produce a more massive black hole. Then that new black hole can find another companion and merge again.”

In the study, Dr. Rodriguez and colleagues used Northwestern University’s supercomputer called Quest to simulate the complex, dynamical interactions within 24 stellar clusters, ranging in size from 200,000 to 2 million stars, and covering a range of different densities and metallic compositions.

The simulations model the evolution of individual stars within these clusters over 12 billion years, following their interactions with other stars and, ultimately, the formation and evolution of the black holes. The simulations also model the trajectories of black holes once they form.

“The neat thing is, because black holes are the most massive objects in these clusters, they sink to the center, where you get a high enough density of black holes to form binaries,” Dr. Rodriguez said.

“Binary black holes are basically like giant targets hanging out in the cluster, and as you throw other black holes or stars at them, they undergo these crazy chaotic encounters.”

When running their simulations, the researchers added a key ingredient that was missing in previous efforts to simulate globular clusters.

“What people had done in the past was to treat this as a purely Newtonian problem,” Dr. Rodriguez noted.

“Newton’s theory of gravity works in 99.9% of all cases. The few cases in which it doesn’t work might be when you have two black holes whizzing by each other very closely, which normally doesn’t happen in most galaxies.”

Newton’s theory of relativity assumes that, if the black holes were unbound to begin with, neither one would affect the other, and they would simply pass each other by, unchanged.

This line of reasoning stems from the fact that Newton failed to recognize the existence of gravitational waves — which Einstein much later predicted would arise from massive orbiting objects, such as two black holes in close proximity.

“In Einstein’s theory of general relativity, where I can emit gravitational waves, then when one black hole passes near another, it can actually emit a tiny pulse of gravitational waves,” Dr. Rodriguez said.

“This can subtract enough energy from the system that the two black holes actually become bound, and then they will rapidly merge.”

The study authors decided to add Einstein’s relativistic effects into their simulations of globular clusters.

After running the simulations, they observed black holes merging with each other to create new black holes, inside the stellar clusters themselves.

Without relativistic effects, Newtonian gravity predicts that most binary black holes would be kicked out of the cluster by other black holes before they could merge. But by taking relativistic effects into account, the team found that nearly half of the binary black holes merged inside their stellar clusters, creating a new generation of black holes more massive than those formed from the stars. What happens to those new black holes inside the cluster is a matter of spin.

“If the two black holes are spinning when they merge, the black hole they create will emit gravitational waves in a single preferred direction, like a rocket, creating a new black hole that can shoot out as fast as 5,000 km per second — so, insanely fast,” Dr. Rodriguez said.

“It only takes a kick of maybe a few tens to a hundred km per second to escape one of these clusters.”

Because of this effect, scientists have largely figured that the product of any black hole merger would get kicked out of the cluster, since it was assumed that most black holes are rapidly spinning.

This assumption, however, seems to contradict the measurements from LIGO’s twin detectors, which has so far only detected binary black holes with low spins.

To test the implications of this, the researchers dialed down the spins of the black holes in their simulations and found that in this scenario, nearly 20% of binary black holes from clusters had at least one black hole that was formed in a previous merger.

Because they were formed from other black holes, some of these second-generation black holes can be in the range of 50 to 130 solar masses. Scientists believe black holes of this mass cannot form from a single star.

“If gravitational-wave telescopes such as LIGO detect an object with a mass within this range, there is a good chance that it came not from a single collapsing star, but from a dense stellar cluster,” Dr. Rodriguez said.

The team’s results appear in the journal Physical Review Letters.

_____

Carl L. Rodriguez et al. 2018. Post-Newtonian Dynamics in Dense Star Clusters: Highly Eccentric, Highly Spinning, and Repeated Binary Black Hole Mergers. Phys. Rev. Lett 120 (15): 151101; doi: 10.1103/PhysRevLett.120.151101

About Skype

Check Also

, Quantum Chromodynamics, #Bizwhiznetwork.com Innovation ΛI

Quantum Chromodynamics

In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons …

Leave a Reply

Your email address will not be published. Required fields are marked *

Bizwhiznetwork Consultation