  Quasars are associated with and powered by the accretion of material onto
massive black holes; the detection of highly luminous quasars with redshifts
greater than z = 6 suggests that black holes of up to ten billion solar masses
already existed 13 billion years ago. Two possible present-day dormant
descendants of this population of active black holes have been found in the
galaxies NGC 3842 and NGC 4889 at the centres of the Leo and Coma galaxy
clusters, which together form the central region of the Great Wall - the
largest local structure of galaxies. The most luminous quasars, however, are
not confined to such high-density regions of the early Universe; yet dormant
black holes of this high mass have not yet been found outside of modern-day
rich clusters. Here we report observations of the stellar velocity distribution
in the galaxy NGC 1600 - a relatively isolated elliptical galaxy near the
centre of a galaxy group at a distance of 64 Mpc from Earth. We use orbit
superposition models to determine that the black hole at the centre of NGC 1600
has a mass of 17 billion solar masses. The spatial distribution of stars near
the centre of NGC 1600 is rather diffuse. We find that the region of depleted
stellar density in the cores of massive elliptical galaxies extends over the
same radius as the gravitational sphere of influence of the central black
holes, and interpret this as the dynamical imprint of the black holes.
