Tatsuya Akiba (Unversity of Colorado Boulder)
On the Hunt for Recoiling Supermassive Black Holes Using Tidal Disruption Events

Following the merger of two supermassive black holes (SMBHs), a gravitational wave recoil kick is imparted on the merger remnant due to the anisotropic emission of gravitational waves. These recoil kicks can be as high as ~10^3 km/s which exceeds the escape velocity of most galaxies, and these super-kicks can lead to ejected or rogue SMBHs. While there are several candidates, recoiling SMBHs are difficult to uniquely identify since observations so far cannot rule out alternate possibilities such as dual active galactic nuclei. Here, we present a novel observational signature: off-nuclear or extragalactic tidal disruption events of stars by recoiling SMBHs. When a super-kick is imparted on a SMBH, there is a tightly bound cluster of stars recoiling with it. We show that these bound stars should be in an eccentric, apse-aligned disk where stars are strongly torqued to extremely high eccentricities. The corresponding rate of tidal disruption events in an eccentric disk is expected to be of order ~0.1 yr^-1 gal^-1, several orders of magnitude higher than in an isotropic cluster. We show that this high expected rate of tidal disruption events in an eccentric disk implies off-nuclear or extragalactic tidal disruption events as viable and likely observables of a recoiling or rogue SMBH.

Sangjun Cha (Yonsei University)
Probing Galaxy Clusters by Combining Strong and Weak Gravitational Lensing in the JWST Era

Gravitational lensing is one of the most powerful methods for mapping the mass of astronomical objects. In galaxy clusters, two types of lensing are generally used: strong lensing (SL) and weak lensing (WL). SL focuses on the central regions of galaxy clusters, while WL can cover a wider area. By combining SL and WL, we can achieve comprehensive mass reconstructions that leverage both strengths. Thanks to deep JWST observations of the galaxy cluster Abell 2744, we can use the largest datasets ever employed for cluster mass reconstruction, enabling the reconstruction of a high-resolution, model-independent mass map. Our mass map aligns well with BCGs and the reported directions of large-scale filamentary structures. We propose merger axes of Abell 2744 based on “mass bridges” from our mass map and radio relic observations. In addition, magnification predictions from our models are consistent with observational data, further validating the robustness of our approach. Our results demonstrate that combining deep SL and WL observations can provide robust and detailed mass distributions of galaxy clusters from their cores to the outskirts. In the JWST era, combining SL and WL with extensive datasets will enhance our understanding of galaxy cluster evolution, dark matter properties, and large-scale structures.