NASA outlines the curious case of the missing supermassive black hole

NASA has trained the Chandra X-Ray Observatory and Hubble Space Telescope on a galaxy cluster known as Abell 2261. The galaxy cluster is about 2.7 billion light-years from Earth. Astronomers believe that Abell 2261, like most large galaxies in the universe, should have a massive black hole at its center. The black hole expected to be somewhere in the range of 3 billion and 100 billion times the mass of the Sun can't be found.

The image above is a composite made from Hubble's optical data and the Subaru Telescope showing galaxies in the cluster. Chandra X-Ray data shows hot gas, which is colored pink, pervading the cluster in the background. In the center of the image is a large elliptical galaxy that resides in the center of the cluster. NASA scientists say that nearly every large galaxy in the universe has a supermassive black hole in the center, and the mass of the central black hole usually tracks with the mass of the galaxy.

Abell 2261 was expected to contain a supermassive black hole rivaling some of the largest in the known universe. Using Chandra data obtained in 1999 and 2004, the center of the galaxy cluster was searched to find material that was superheated as it fell towards the black hole and produced x-rays. However, no source was detected. Longer Chandra observations in 2018 were used to conduct a more in-depth search for the black hole in the galaxy center.

The team considered an explanation for why the black hole might be missing, and there is a possibility that the black hole was ejected from the center of the host galaxy. Scientists believe the violent event could have resulted from two galaxies merging to form the observed galaxy accompanied by a central black hole in each galaxy merging to create one larger black hole.

Black hole mergers produce ripples in space-time called gravitational waves. If a massive amount of gravitational waves are generated by the merger event where waves are stronger in one direction than the other; the theory is that the new supermassive black hole could have been thrown from the center of the galaxy in the opposite direction. The phenomenon is known as a recoiling black hole.

No definitive evidence for recoiling black holes has been discovered. However, Abell 2261 has two indirect signs of that merger taking place. One sign is data from Hubble and Subaru telescopes revealing a galactic core that is larger than expected for a galaxy of the size. The second is that the densest concentrations of stars in the galaxy is over 2000 light-years away from the center, which is "strikingly distant." Researchers believe that an ejected black hole would explain the large core and the off-center concentration of stars. Research into the galactic cluster is ongoing.