Speaker
Description
Supermassive black holes (SMBHs) residing at the centres of massive galaxies are intricately linked to their hosts through a co-evolutionary process. For massive elliptical galaxies, this connection is typically described by the $M_\text{BH}-\sigma_e$ relation, which ties the SMBH mass ($M_\text{BH}$) to the host’s central stellar velocity dispersion ($\sigma_e$). However, traditional methods for measuring SMBH masses rely on stellar dynamics in nearby galaxies (z<0.1), leaving the evolution of SMBHs at higher redshifts largely unexplored. In this work, we report the detection of an SMBH with $\log_{10}(M_\text{BH}/M_\odot) = 10.56^{+0.07}_{-0.08} \pm 0.12^\text{sys}$ at the centre of the Cosmic Horseshoe gravitational lens, a massive elliptical galaxy at z = 0.44. Using MUSE integral-field spectroscopy and HST imaging, we jointly model the two-dimensional stellar kinematics and radial lensing features to constrain the galaxy’s central mass distribution and SMBH mass. Bayesian model comparison yields a $5\sigma$ detection, with results robust against systematics. The Cosmic Horseshoe SMBH lies $\sim 1.5\sigma$ above the local $M_\text{BH}-\sigma_e$ relation, consistent with an emerging trend among BCGs and other massive galaxies. This offset suggests a possible steepening of the $M_\text{BH}-\sigma_e$ relation at the highest mass scales, pointing towards a distinct co-evolutionary pathway for SMBHs and their hosts. 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.
