Ultramassive Black Holes in Brightest Cluster Galaxies

We have identified an exceptional brightest cluster galaxy (PKS0745-BCG, z=0.1028) which stands out as being located in one of the most massive and strong cool clusters known, yet has clear rotation of the gas within the central regions of the galaxy to enable modelling of the black hole mass. Our results imply that the black hole mass should be at least 2.5*10^10Msun, but can be as high as 10^11Msun, placing it well above the observed correlation between central black hole mass (M_BH) and bulge stellar velocity dispersion (sigma). It should therefore host the most massive black hole in the Universe, even when compared to other BCGs. We propose to measure its black hole mass using long-slit spectroscopy from STIS onboard HST. There are only four black holes with masses exceeding 10^10Msun known so far. The high-mass end of one of the most fundamental relations in astronomy, the M_BH-sigma relation, therefore remains unconstrained. Careful study of black holes with masses in excess of 10^10Msun is the only way forward to complete our understanding of M_BH-sigma relation. This is the goal of our proposal.