Authors: Alister W. Graham
While dry mergers can produce considerable scatter in the (black hole mass, Mbh)-(spheroid stellar mass, M*,sph) and Mbh-(spheroid half-light radius, Re,sph) diagrams, the virial theorem is used here to explain why the scatter about the Mbh-(velocity dispersion, σ) relation remains low in the face of such mergers. Its small scatter has been claimed as evidence of feedback from active galactic nuclei (AGNs). However, it is shown that galaxy mergers also play a significant role. The major merger of two S0 galaxies with M*,sph ~1011 Msolar advances a system along a slope of ~5 in the Mbh-σ diagram. However, a major E+E galaxy merger moves a system (slightly) along a trajectory with a slope of ~9, while mergers of lower-mass S0 galaxies with M*,sph~1010 Msolar move (slightly) along a trajectory with a slope of ~3. This produces a steeper distribution for the E (and Es,e) galaxies in the Mbh-σ diagram, reported here to have a slope of 7.27±0.91, compared to the S0 galaxies which have a slope of 5.68±0.60. This result forms an important complement to the AGN feedback models like that from Silk and Rees, providing a more complete picture of galaxy/(black hole) coevolution. It also has important implications for nanohertz gravitational wave research. Abridged.