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

Supermassive black holes (SMBHs) are a key catalyst of galaxy formation and evolution, leading to an observed correlation between SMBH mass \(M_{\rm BH}\) and host galaxy velocity dispersion \(\sigma_{\rm e}\). Outside the local Universe, measurements of \(M_{\rm BH}\) 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 \(\sim 1\) kpc from the galaxy centre. Using multi-band Hubble Space Telescope imaging and the lens modeling software \(\texttt{PyAutoLens}\) we reconstruct the distribution of mass along this line of sight. Bayesian model comparison favours a point mass with \(M_{\rm BH} = 3.27 \pm 2.12\times10^{10}\,\)M\(_{\rm \odot}\) (3\(\sigma\) 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 \(M_{\rm BH} \leq 5.3 \times 10^{10}\,\)M\(_{\rm \odot}\), because a SMBH above this mass deforms the lensed image \(\sim 1\) kpc from Abell 1201's centre. This builds on previous work using central images to place upper limits on \(M_{\rm BH}\), 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 \(M_{\rm BH}\)--\(\sigma_{\rm e}\) relation. Results are available at https://github.com/Jammy2211/autolens_abell_1201.