2023MNRAS.521.3298N

Supermassive black holes (SMBHs) are a key catalyst of galaxy formation and evolution, leading to an observed correlation between SMBH mass M_BH and host galaxy velocity dispersion σ_e. Outside the local Universe, measurements of M_BH are usually only possible for SMBHs in an active state: limiting sample size and introducing selection biases. Gravitational lensing makes it possible to measure the mass of non-active SMBHs. We present models of the $z$ = 0.169 galaxy-scale strong lens Abell 1201. A cD galaxy in a galaxy cluster, it has sufficient 'external shear' that a magnified image of a $z$ = 0.451 background galaxy is projected just ∼1 kpc from the galaxy centre. Using multiband Hubble Space Telescope imaging and the lens modelling software PYAUTOLENS, we reconstruct the distribution of mass along this line of sight. Bayesian model comparison favours a point mass with M_BH = 3.27 ± 2.12 × 10¹⁰ M_⊙ (3σ confidence limit); an ultramassive black hole. One model gives a comparable Bayesian evidence without an SMBH; however, we argue this model is nonphysical given its base assumptions. This model still provides an upper limit of M_BH ≤ 5.3 × 10¹⁰ M_⊙, because an SMBH above this mass deforms the lensed image ∼1 kpc from Abell 1201's centre. This builds on previous work using central images to place upper limits on M_BH, but is the first to also place a lower limit and without a central image being observed. The success of this method suggests that surveys during the next decade could measure thousands more SMBH masses, and any redshift evolution of the M_BH-σ_e relation. Results are available at https://github.com/Jammy2211/autolens_abell_1201.