Abell 1201: Detection of an Ultramassive Black Hole in a Strong Gravitational Lens

Abstract

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 multi-band Hubble Space Telescope imaging and the lens modeling 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 × 10 10 M (3σ confidence limit); an ultramassive black hole. One model gives a comparable Bayesian evidence without a SMBH, however we argue this model is nonphysical given its base assumptions. This model still provides an upper limit of M BH 5.3×10 10 M , because a 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.