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 $M_{\mathrm{BH}}$ and host galaxy velocity dispersion $\ddot{I}{ƒ}_{\mathrm{e}}$. Outside the local Universe, measurements of $M_{\mathrm{BH}}$ 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 $\text{PyAutoLens}$ we reconstruct the distribution of mass along this line of sight. Bayesian model comparison favours a point mass with $M_{\mathrm{BH}}=3.27\pm 2.12\times 10^{10}$M${}_{\odot }$ (3$\ddot{I}ƒ$ 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 $M_{\mathrm{BH}}\le 5.3\times 10^{10}$M${}_{\odot }$, 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 $M_{\mathrm{BH}}$, 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 $M_{\mathrm{BH}}$--$\ddot{I}{ƒ}_{\mathrm{e}}$ relation. Results are available at https://github.com/Jammy2211/autolens_abell_1201.

Accepted in MNRAS, 27 pages, 22 figures