# A Kiloparsec-Scale Stellar Cavity in the Center of Abell402-BCG May be Caused by Dynamic Interactions with an Ultramassive Black Hole

**Authors:** Michael McDonald, Gourav Khullar, David Lagattuta, Guillaume Mahler, Shashank Dattathri, Jose M. Diego, Alastair C. Edge, Benjamin Floyd, Michael D. Gladders, Scott A. Hughes, Mathilde Jauzac, Nader Khonji, Gavin Leroy, Richard Massey, Mireia Montes, Priyamvada Natarajan, Michael Reefe, Keren Sharon, Frank van den Bosch, Stepane Werner  
**arXiv:** [2603.10104](https://arxiv.org/abs/2603.10104)  
**Date:** 2026-03-10  
**Summary page:** https://www.summarizepaper.com/en/paper/2603.10104v1/

## Key points

- Recent observations from the James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) and Hubble Space Telescope (HST) have revealed a kiloparsec-wide stellar cavity within the central galaxy of the Abell402 cluster.
- The stellar cavity indicates a localized depletion in the stellar density field equivalent to approximately 2x10^9 solar masses of missing stars within a confined volume of 0.5kpc^3.
- Both JWST and HST data show evidence of a flattened core spanning 2.2kpc in the stellar distribution, suggesting the presence of an ultra-massive black hole at the center with an estimated mass of M_BH = 6 +/- 4 x10^10 solar masses.
- An infrared-bright point source at one edge of the cavity indicates active accretion by this black hole, identified as a Low-Ionization Nuclear Emission-line Region (LINER) Active Galactic Nucleus (AGN).
- Another candidate AGN has been detected on the opposite side of the cavity with a relative velocity of 370 km/s, potentially indicating a dual AGN system separated by kiloparsecs with a combined binary mass estimated to be 6 +/- 2 x10^10 solar masses.
- The researchers propose that this unique stellar cavity is likely formed as a result of transient dynamical interactions involving at least one supermassive black hole and surrounding stellar density field, possibly triggered by three-body scattering or dipole instability.
- These findings shed light on rare astrophysical phenomena and provide valuable insights into galaxy evolution mechanisms within dense cluster environments.

## Summary

Recent observations from the James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) have revealed a remarkable kiloparsec-wide stellar cavity within the central galaxy of the Abell402 cluster. This cavity, detected in conjunction with data from the Hubble Space Telescope (HST), is distinct from extinction caused by dust and indicates a localized depletion in the stellar density field equivalent to approximately 2x10^9 solar masses of missing stars within a confined volume of 0.5kpc^3. On a larger scale, both JWST and HST data show evidence of a flattened core spanning 2.2kpc in the stellar distribution, upon which the smaller-scale cavity is overlaid. This flattened core suggests the presence of an ultra-massive black hole at the center with an estimated mass of M_BH = 6 +/- 4 x10^10 solar masses. Notably, an infrared-bright point source situated at one edge of the cavity indicates active accretion by this black hole. Subsequent spectroscopic analysis using MUSE has identified this source as a Low-Ionization Nuclear Emission-line Region (LINER) Active Galactic Nucleus (AGN). Additionally, another candidate AGN has been detected on the opposite side of the cavity with a relative velocity of 370 km/s. If confirmed, this finding implies the existence of a dual AGN system separated by kiloparsecs, with a combined binary mass estimated to be 6 +/- 2 x10^10 solar masses – potentially making it one of the most massive binary black hole systems known to date. The researchers propose that this unique stellar cavity is likely formed as a result of transient dynamical interactions involving at least one supermassive black hole and the surrounding stellar density field. These interactions could have been triggered by three-body scattering during binary hardening or through inducing a dipole instability within the stellar density field itself. Overall, these findings shed light on rare astrophysical phenomena and provide valuable insights into galaxy evolution mechanisms within dense cluster environments.