Unveiling a 36 billion solar mass black hole at the centre of the Cosmic Horseshoe gravitational lens

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 (MBH) strongly correlates with the host central stellar velocity dispersion (σ ₑ), via the MBH\!-\!σ 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 (zₗ = 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 5σ detection of an ultramassive black hole with log ₁₀(MBH/ M) = 10.56+0.07-0.08 ± (0.12)ˢys, consistent across various systematic tests. Our findings place the Cosmic Horseshoe ~1.5σ$ above the $M_BH\!-\!σ _e$ relation, supporting an emerging trend observed in brightest cluster galaxies and other massive galaxies, which suggests a steeper $M_BH\!-\!σ _e$ 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 MBH\!-\!σ ₑ relation and its evolution across cosmic time.