Exploring the unusually high black hole-to-bulge mass ratios in NGC4342
  and NGC4291: the asynchronous growth of bulges and black holes
  We study two nearby, early-type galaxies, NGC4342 and NGC4291, that host
unusually massive black holes relative to their low stellar mass. The observed
black hole-to-bulge mass ratios of NGC4342 and NGC4291 are ~6.9% and ~1.9%,
respectively, which significantly exceed the typical observed ratio of ~0.2%.
As a consequence of the exceedingly large black hole-to-bulge mass ratios,
NGC4342 and NGC4291 are ~5.1 sigma and ~3.4 sigma outliers from the M_BH -
M_bulge scaling relation, respectively. In this paper, we explore the origin of
the unusually high black hole-to-bulge mass ratio. Based on Chandra X-ray
observations of the hot gas content of NGC4342 and NGC4291, we compute
gravitating mass profiles, and conclude that both galaxies reside in massive
dark matter halos, which extend well beyond the stellar light. The presence of
dark matter halos around NGC4342 and NGC4291 and a deep optical image of the
environment of NGC4342 indicate that tidal stripping, in which >90% of the
stellar mass was lost, cannot explain the observed high black hole-to-bulge
mass ratios. Therefore, we conclude that these galaxies formed with low stellar
masses, implying that the bulge and black hole did not grow in tandem. We also
find that the black hole mass correlates well with the properties of the dark
matter halo, suggesting that dark matter halos may play a major role in
regulating the growth of the supermassive black holes.
