2025MNRAS.541.2853R

Supermassive black holes (SMBHs) are found at the centre of every massive galaxy, with their masses tightly connected to their host galaxies through a co-evolution over cosmic time. For massive ellipticals, the SMBH mass ($M_\text{BH}$) strongly correlates with the host central stellar velocity dispersion ($\sigma _e$), via the $M_\text{BH}\!-\!\sigma _e$ relation. However, SMBH mass measurements have traditionally relied on central stellar dynamics in nearby galaxies ($z < 0.1$), limiting our ability to explore the SMBHs across cosmic time. In this work, we present a self-consistent analysis combining 2D stellar dynamics and lens modelling of the Cosmic Horseshoe gravitational lens system ($z_l = 0.44$), one of the most massive lens galaxies ever observed. Using MUSE integral-field spectroscopy and high-resolution Hubble Space Telescope imaging, we simultaneously model the radial arc - sensible to the inner mass structure - with host stellar kinematics to constrain the galaxy's central mass distribution and SMBH mass. Bayesian model comparison yields a $5\sigma$ detection of an ultramassive black hole with $\log _{10}(M_\text{BH}/{\rm M}_{\odot }) = 10.56^{+0.07}_{-0.08} \pm (0.12)^\text{sys}$, consistent across various systematic tests. Our findings place the Cosmic Horseshoe $\sim$$1.5\sigma$ above the $M_\text{BH}\!-\!\sigma _e$ relation, supporting an emerging trend observed in brightest cluster galaxies and other massive galaxies, which suggests a steeper $M_\text{BH}\!-\!\sigma _e$ relationship at the highest masses, potentially driven by a different co-evolution of SMBHs and their host galaxies. Future surveys will uncover more radial arcs, enabling the detection of SMBHs over a broader redshift and mass range. These discoveries will further refine our understanding of the $M_\text{BH}\!-\!\sigma _e$ relation and its evolution across cosmic time.