Astronomers using NASA’s Chandra X-ray Observatory are searching for signs of a recoiling black hole in the core of A2261-BCG, the most massive and brightest galaxy in the giant galaxy cluster Abell 2261.
This image of Abell 2261, a galaxy cluster located 3 billion light-years away, contain X-ray data from Chandra showing hot gas pervading the cluster as well as optical data from Hubble and the Subaru Telescope that show galaxies in the cluster and in the background. A2261-BCG (center), also known as 2MASX J17222717+3207571, is the most massive and brightest galaxy in the cluster. Image credit: NASA / CXC / University of Michigan / K. Gültekin / STScI / NAOJ / Subaru / NSF / NOAO / KPNO.
When black holes merge, they produce ripples in spacetime called gravitational waves.
If the huge amount of gravitational waves generated by such an event was stronger in one direction than another, the theory predicts that the new, even more massive black hole would have been sent careening away from the center of the galaxy in the opposite direction. This is called a recoiling black hole.
Astronomers have not found definitive evidence for recoiling black holes and it is not known whether supermassive black holes even get close enough to each other to produce gravitational waves and merge.
A2261-BCG is an excellent galaxy to search for a recoiling black hole because there are two indirect signs that a merger between two massive black holes might have taken place.
First, optical data from the NASA/ESA Hubble Space Telescope and the Subaru Telescope revealed a galactic core that is much larger than expected for a galaxy of its size.
The second sign is that the densest concentration of stars in A2261-BCG is over 2,000 light-years away from the galaxy’s center, which is strikingly distant.
These features were first identified by Space Telescope Science Institute’s Dr. Marc Postman and colleagues in their earlier Hubble and Subaru images, and led them to suggest the idea of a merged black hole in Abell 2261.
During a merger, the supermassive black hole in each galaxy sinks toward the center of the newly coalesced galaxy.
If they become bound to each other by gravity and their orbit begins to shrink, the black holes are expected to interact with surrounding stars and eject them from the center of the galaxy. This would explain Abell 2261’s large core.
The off-center concentration of stars may also have been caused by a violent event such as the merger of two supermassive black holes and subsequent recoil of a single, larger black hole.
Even though there are clues that a black hole merger took place, neither Chandra nor Hubble data showed evidence for the black hole itself.
A research team led by West Virginia University’s Dr. Sarah Burke-Spolaor had previously used Hubble to look for a clump of stars that might have been carried off by a recoiling black hole.
They studied three clumps near the center of the galaxy, and examined whether the motions of stars in these clumps are high enough to suggest they contain a ten billion solar mass black hole.
No clear evidence for a black hole was found in two of the clumps and the stars in the other one were too faint to produce useful conclusions.
The astronomers also previously studied observations of Abell 2261 with NSF’s Karl G. Jansky Very Large Array.
Radio emission detected near the center of A2261-BCG showed evidence that supermassive black hole activity had occurred there 50 million years ago, but does not indicate that the center of the galaxy currently contains such a black hole.
They then turned to Chandra to look for material that had been superheated and produced X-rays as it fell towards the black hole.
While the Chandra data did reveal that the densest hot gas was not in the center of the galaxy, they did not reveal any possible X-ray signatures of a growing supermassive black hole — no X-ray source was found in the center of the cluster, or in any of the clumps of stars, or at the site of the radio emission.
“Either there is no black hole at any of these locations, or that it is pulling material in too slowly to produce a detectable X-ray signal,” the researchers said.
The team’s paper will be published in a journal of the American Astronomical Society.
_____
Kayhan Gultekin et al. 2020. Chandra Observations of Abell 2261 Brightest Cluster Galaxy, a Candidate Host to a Recoiling Black Hole. AAS Journals, in press; arXiv: 2010.13980
