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A Galaxy Cluster with Two Pairs of X-Ray Cavities

Optical and X-ray images of the galaxy cluster RBS 797. Four enormous cavities have been found at the center of the galaxy cluster using the Chandra X-ray Observatory, which astronomers think points to the presence of a double supermassive black hole nucleus.

NASA / CXC / University of Bologna / F. Ubertosi / STScl / M. Calzadilla / NSF / NRAO / ALMA

Supermassive black holes (SMBH) at the centers of galaxies sometimes accrete material onto a surrounding torus of gas and dust. When this happens, the material is heated to thousands of degrees, prompting the ejection of powerful jets of charged particles. These jets can produce cavities or bubbles of hot gas that push outward in the galaxies, and which have been seen in radio and X-ray observations. In bright galaxy clusters, the impact of this activity on the intracluster medium can be traced by ripples or filamentary structures that are seen in the X-ray images, and indeed cavities of hot gas have been found that are thought to have been excavated by radio lobes launched from the active nucleus.

CfA astronomers Paul Nulsen, Scott Randall, Will Forman, and Christine Jones were members of a team that obtained a deep Chandra X-ray Observatory image of the X-ray cavities in the galaxy cluster RBS 797. This cluster had been known to host two pronounced X-ray cavities, and follow-up observations in the radio using the Very Large Array uncovered evidence for a second pair of jets and lobes oriented the perpendicular to the first pair but with approximately the same dimensions, about one hundred thousand light-years across. The new X-ray measurements then uncovered two new, corresponding X-ray cavities, making RBS 797 the first galaxy cluster known to have four equidistant, centrally symmetric, radio-emitting X-ray cavities, each roughly between 10-50 million years old and with the differences in their ages being at most 10 million years.

The astronomers investigate two scenarios for the origin of these two giant, perpendicular structures. The first considers the possibility of a binary SMBH nucleus that is simultaneously excavating the cavities in different directions; the second studies whether a single SMBH nucleus could somehow quickly reorient the direction of its jets (in under ten million years) in misaligned outbursts. Because their analysis is consistent with the lobes being coeval, they conclude by preferring the first scenario and its tantalizing evidence for the presence of two active SMBHs simultaneously emitting at the center of this system.

Reference: The Deepest Chandra View of RBS 797: Evidence for Two Pairs of Equidistant X-ray Cavities," F. Ubertosi, M. Gitti, F. Brighenti, G. Brunetti, M. McDonald, P. Nulsen, B. McNamara, S. Randall, W. Forman, M. Donahue, A. Ignesti, M. Gaspari, S. Ettori, L. Feretti, E. L. Blanton, C. Jones, and M. Calzadilla, The Astrophysical Journal 923, L25, 2021.