Intermediate-mass black holes “eat” stars, then expel the leftovers

Intermediate-mass black holes, if they exist, probably gobble up rogue stars by taking a few ‘bites’ and then spewing the remains across the galaxy, say the authors of a new study led by Northwestern University and published in The AstrophysicalJournal.

In new 3D computer simulations, astrophysicists modeled black holes of various masses and hurled stars (the size of our Sun) past them to see what would happen. They found that when a star approaches an intermediate-mass black hole, it becomes trapped in its orbit. Afterwards, the black hole begins its long and violent meal. Every time the star goes around, the black hole takes a bite out of it, further cannibalizing the star with each pass.

In the end, only the misshapen and incredibly dense core of the star remains. At that moment, the black hole ejects the debris. The remnants of the star fly to safety across the galaxy. These new simulations not only point to unknown behaviors of intermediate-mass black holes, but also provide astronomers with new clues to help finally locate these hidden giants in our night sky.

“Obviously, we can’t observe black holes directly because they don’t emit light,” explains Fulya Kiroglu of Northwestern, who led the study. So instead we need to look at the interactions between black holes and their environments.” “We discovered that stars undergo multiple passages before being ejected,” he continues. After each step, they lose more mass, causing a flare of light as they tear apart. Each flare is brighter than the last, creating a signature that could help astronomers find them.” Kiroglu, an astrophysics graduate student at Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), presented the study at the American Physical Society (APS) meeting.

Although astrophysicists have shown the existence of lower-mass and higher-mass block holes, intermediate-mass black holes have remained elusive. Created when supernovae collapse, stellar remnant black holes are between 3 and 10 times the mass of our sun.

Although these intermediate-mass black holes should exist in theory, astrophysicists have yet to find irrefutable observational proof. “Their presence is still under debate,” Kiroglu says. Astrophysicists have discovered evidence that they exist, but that evidence can often be explained by other mechanisms. For example, what appears to be an intermediate-mass black hole could actually be the accumulation of stellar-mass black holes.” To explore the behavior of these elusive objects, Kiroglu, Frederic Rasio, Joseph Cummings Professor of Physics and Astronomy at Weinberg and CIERA member and co-author of the paper, and their team developed new hydrodynamic simulations.

First, they created a model of a star, made up of many particles. They then sent the star towards the black hole and calculated the gravitational force acting on the particles during the approach of the star. “We can specifically calculate which particle is bound to the star and which particle is perturbed (or no longer bound to the star),” Kiroglu says. Using these simulations, Kiroglu and his team found that stars could orbit an intermediate-mass black hole up to five times before finally being ejected. With each revolution around the black hole, the star loses more and more mass as it tears apart. The black hole then ejects the debris, moving at blistering speeds, back into the galaxy. This repeating pattern would create an impressive light show that would help astronomers recognize—and prove—the existence of intermediate-mass black holes.