Abstract: At cosmological distances, gravitational lensing can in principle provide direct mass measurements of supermassive black holes (SMBHs). Here, we directly estimate the mass of a SMBH in the brightest cluster galaxy (BCG) of MACS J1149.5+2223 at z = 0.54 using one of the multiply lensed images of a background spiral galaxy at z = 1.49 projected close to the BCG. A lensed arc is curved toward the BCG center, corresponding to an intrinsically compact region in one of the spiral arms. This arc has a radius of curvature of only ?0farcs6, betraying the presence of a local compact deflector. Its curvature is most simply reproduced by a point-like object with a mass of, similar to SMBH masses in local elliptical galaxies having comparable luminosities. The SMBH is noticeably offset by 4.4 ± 0.3 kpc from the BCG light center, which is plausibly the result of a kick imparted ?2.0 × 107 years ago during the merger of two SMBHs, placing it just beyond the stellar core. A similar curvature can be produced by replacing the offset SMBH with a compact galaxy having a mass of ?2 × 1010 M? within a cutoff radius of <4 kpc, and an unusually large to make it undetectable in the deep Hubble Frontiers Fields image, at or close to the cluster redshift. However, such a lensing galaxy perturbs the adjacent lensed images in an undesirable way.

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