Study Led by BMCC Professors Sheds Light on Black Hole Mergers

A team of scientists led by two Borough of Manhattan Community College (BMCC/CUNY) professors have published a study that suggests researchers might be able to see light from black hole mergers in space if the collisions happen in the presence of gas, something previously thought to be impossible.

Science Professors Barry McKernan and K.E. Saavik Ford, both research associates at the American Museum of Natural History’s Department of Astrophysics, collaborated with researchers from the California Institute of Technology, Jet Propulsion Laboratory, University of Edinburgh, Columbia University and University of Florida on the study published in The Astrophysical Journal Letters.

Gravitational wave detectors are detecting the mergers of black holes, up to a few tens of times the mass of the Sun, that merge at a rate of about one per week. If these mergers are happening ‘in the dark,’ in empty space, then there is  no way to see any light from the merger.

“Our work is saying that big bright disks of gas in the centers of galaxies, around a central supermassive black hole have the potential to merge lots of these small black holes together,” said McKernan. “This is because there’s so much gas in the disks that it dominates the behavior of the smaller mass black holes.”

Formed when massive stars die, black holes tend to sink into regions of galaxies where gravity is strongest, especially, it’s thought, into the centers of galaxies near the vastly larger, single, supermassive black holes lurking there.

“Once black holes merge with each other, they usually experience a kick. If the merger happened in empty space, the merged new black hole will fly off at high speed and you’ll have no way of knowing that it’s happening if you’re looking at it with telescopes that collect light, since no light is being emitted,” said McKernan.

“If the merger happens in a bright gas disk, the kicked merged black hole flies through the gas and tries to bring nearby gas with it.”

McKernan offers this analogy: “Imagine someone holding onto a whole bunch of Helium balloons trying to carry them with them.  If they run fast, the balloons hit air and get pushed backwards. If the person then gets into a car and leaves the balloons tied to the window as they drive off at high speed, the air resistance will start ripping those balloons from the car.”

“It’s a similar situation with the kicked black holes in disks,” said McKernan. “The kicked, merged black hole tries to carry a bunch of disk gas with them, but the gas runs into the rest of the disk, the black hole carries on and lets go of the gas. The result is a high speed gas collision which can show up in light as a temporary flare.”

The resulting flare is detectable in sky surveys if the gas disk is thin enough to let light escape and may help astronomers distinguish black hole mergers from random variations in the disk gas.

“It’ll be super-exciting if we find something on the right time scale. If we don’t find anything, it’ll also tell us something about how thick the gas disk blanket must be to cover up such a sign,” said Ford.