Astronomers searching for an intermediate mass black hole at the heart of Omega Centauri have encountered an unexpected result—nothing. This dense globular cluster, located in the southern sky, is the largest and brightest of its kind in the Milky Way, containing approximately ten million stars. Earlier this year, researchers suggested that a black hole with a mass ranging from 8,200 to 47,000 solar masses could be hiding at its core, based on the rapid movement of seven stars that appear gravitationally bound to a massive object.
In a bid to confirm the existence of this black hole, a team led by Angiraben Mahida conducted an extensive observation campaign using the Australia Telescope Compact Array. Over the course of approximately 170 hours, the team focused on the central region of Omega Centauri, achieving the most sensitive radio imaging of the cluster to date, with a sensitivity of 1.1 microjanskys at a frequency of 7.25 gigahertz.
Despite their efforts, the researchers found no evidence of radio emissions that would indicate the presence of a black hole. The absence of detection imposes significant constraints on the black hole’s characteristics. By employing the fundamental plane of black hole activity, which links a black hole’s mass with its radio and X-ray luminosity, the team deduced that any intermediate mass black hole in Omega Centauri must exhibit remarkably low accretion efficiency. This efficiency is estimated to be less than 0.004, meaning that less than half a percent of the energy from infalling material is converted into detectable radiation.
The lack of activity is not entirely surprising. Omega Centauri is believed to be the remnant core of a dwarf galaxy that was absorbed by the Milky Way billions of years ago. As a result, the central region of the cluster may not contain sufficient gas for the black hole to feed on. Unlike the gas-rich environments surrounding supermassive black holes found in active galaxies, this intermediate mass black hole appears to exist in a relatively barren region, lacking the fuel necessary to generate the radiation typically associated with black hole activity.
This study underscores the ongoing challenge in identifying and confirming the existence of intermediate mass black holes, which remain a poorly understood aspect of black hole evolution. The findings are detailed in the recent publication titled “No evidence for accretion around the intermediate-mass black hole in Omega Centauri.” The continued exploration of Omega Centauri and similar clusters will be essential in unraveling the mysteries surrounding these elusive cosmic entities.
