Abstract
Our limited ability to determine the masses of central supermassive black holes (SMBHs) constrains our understanding of their growth over cosmic times. Direct and hence the most reliable measurements using stellar and gas dynamics are limited to distances up to the Virgo cluster. Here, we employ gravitational lensing to directly determine, for the first time, the mass of a SMBH at z = 0.54, located in the BCG of the cluster MACS1149. One of the background galaxies multiply-lensed by this cluster is that of a highly magnified spiral galaxy at z = 1.49. In the lensed image closest to the BCG, a feature in this galaxy is lensed across the critical curve of a central point mass. We deduce a mass for this SMBH of 
～5 X 109Msolar, offset from the centre of the BCG by 3.8 ± 0.8 kpc 
. The mass of this SMBH is in rough accord with the local relationship between the masses of central SMBHs and their host galaxies. The offset of the SMBH from the centre of the BCG can be explained by the “rocket effect” due to the coalescence of two SMBHs about 
～3 X 107 yrs ago, imparting a kick with a velocity of 300 km/s on the resulting SMBH. Our work demonstrates the potential of using gravitational lensing to study SMBHs at intermediate redshifts, especially when even deeper lensing images at higher angular resolutions become available.
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