  Observational work conducted over the last few decades indicates that all
massive galaxies have supermassive black holes at their centres. Although the
luminosities and brightness fluctuations of quasars in the early Universe
suggest that some are powered by black holes with masses greater than 10
billion solar masses, the remnants of these objects have not been found in the
nearby Universe. The giant elliptical galaxy Messier 87 hosts the hitherto most
massive known black hole, which has a mass of 6.3 billion solar masses. Here we
report that NGC 3842, the brightest galaxy in a cluster at a distance from
Earth of 98 megaparsecs, has a central black hole with a mass of 9.7 billion
solar masses, and that a black hole of comparable or greater mass is present in
NGC 4889, the brightest galaxy in the Coma cluster (at a distance of 103
megaparsecs). These two black holes are significantly more massive than
predicted by linearly extrapolating the widely-used correlations between black
hole mass and the stellar velocity dispersion or bulge luminosity of the host
galaxy. Although these correlations remain useful for predicting black hole
masses in less massive elliptical galaxies, our measurements suggest that
different evolutionary processes influence the growth of the largest galaxies
and their black holes.
