The limited role of mergers in the black hole to bulge mass relation
  We examine the intrinsic scatter in the correlation between black hole masses and their host bulge masses, and find that it cannot be accounted for by mergers alone. A simple merger scenario of small galaxies leads to a proportionality relation between the late-time black hole and bulge masses, with intrinsic scatter (in linear scale) increasing along the ridge line of the relation as the square root of the mass. By examining a sample of 86 galaxies with well measured black hole masses, we find that the intrinsic scatter increases with mass more rapidly than expected from the merger-only scenario. We discuss the possibility that the feedback mechanism that operated during galaxy formation involved the presence of a cooling flow. 
Carmit Gordon Lahav, Yohai Meiron, Noam Soker
astro-ph.CO