First direct mass measurement of a distant supermassive black hole through gravitational lensing

Abstract

Our limited ability to determine the masses of central supermassive black holes (SMBHs) constrains our understanding of their growth over cosmic times. Direct determinations of SMBH masses are restricted to the local Universe, where stars and gas orbiting close to the SMBH can be resolved (Kormendy, 2004). Beyond the local Universe, measurements of SMBH masses rely on reverberation mapping (Kaspi et al., 2007), the accuracy of which remains in questions. At intermediate and high redshifts, scaling relations are used to infer SMBH masses from AGN emission lines, the veracity of which is even more questionable (Graham, 2016). SMBH mass measurements in the local Universe suggest a co-evolution history of SMBHs and their host galaxies. Some high-redshift quasars, however, are inferred, through the aforementioned scaling relations, to harbour the most massive SMBHs known to date, hence challenging the co-evolution theory. To understand the co-evolution, or lack thereof, between SMBHs and their host galaxies, reliable measurements of SMBH masses are needed over cosmic history.

At cosmological distances, gravitational lensing can provide a direct determination of SMBH masses for lensed light passing centrally through the Einstein ring of the host galaxy (Hezaveh et al., 2015). In my thesis, I report the first such direct detection, in the galaxy cluster MACSJ1149.5+2223 with the deep Hubble Frontier Fields data, where the curvature of a lensed background source (at z = 1.49) betrays the presence of a dark point mass near the center of the brightest cluster galaxy (BCG), at z = 0.54, when the Universe was half of its present age. Based on an accurate non-parametric cluster lens model for MACS 1149, I fine tune the lens model for the BCG. A lensed feature, referred to as L1, is predicted to be straight by the lens model, while in data it possesses a curved shape with a small radius of curvature of 0.′′6. This curvature can be most simply and robustly reproduced by adding a point mass to the lens model.

I derive a point mass of 〖8.4〗_(-1.8)^(+4.3)×〖10〗^9 M⊙, similar to SMBH masses in local galaxies of comparable luminosities. This object is noticeably offset by 0.′′69 ± 0.′′05, or 4.4 ± 0.3 kpc, from the projected light center of the BCG, placing it just beyond the stellar core of this galaxy, plausibly as a consequence of galaxy or black hole merging. A similar effect could be produced by a compact galaxy having a sharply truncated profile of < 4kpc and mass of ∼ 2 × 10^10M⊙ within or near the cluster, but it perturbs nearby lensed images in an unsatisfactory manner. In addition, this galaxy requires an extreme mass-to-light ratio exceeding > 50(M/L)⊙, the combination of high mass and high mass-to-light ratio for which is not known for any object.