Overmassive black holes in the $M_{\rm BH} - \sigma$ diagram do not
  belong to over (dry) merged galaxies
  Semi-analytical models in a $\Lambda$CDM cosmology have predicted the
presence of outlying, "overmassive" black holes at the high-mass end of the
(black hole mass -- galaxy velocity dispersion) $M_{\rm BH} - \sigma$ diagram
(which we update here with a sample of 89 galaxies). They are a consequence of
having experienced more dry mergers -- thought not to increase a galaxy's
velocity dispersion -- than the "main-sequence" population. Wet mergers and
gas-rich processes, on the other hand, preserve the main correlation. Due to
the scouring action of binary supermassive black holes, the extent of these dry
mergers (since the last significant wet merger) can be traced by the ratio
between the central stellar mass deficit and the black hole mass ($M_{\rm
def,*} / M_{\rm BH}$). However, in a sample of 23 galaxies with partially
depleted cores, including central cluster galaxies, we show that the
"overmassive" black holes are actually hosted by galaxies that appear to have
undergone the lowest degree of such merging. In addition, the rotational
kinematics of 37 galaxies in the $M_{\rm BH} - \sigma$ diagram reveals that
fast and slow rotators are not significantly offset from each other, also
contrary to what is expected if these two populations were the product of wet
and dry mergers respectively. The observations are thus not in accordance with
model predictions and further investigation is required.
