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We study two nearby, early-type galaxies, NGC4342 and NGC4291, that hostunusually massive black holes relative to their low stellar mass. The observedblack 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 ofthe unusually high black hole-to-bulge mass ratio. Based on Chandra X-rayobservations of the hot gas content of NGC4342 and NGC4291, we computegravitating mass profiles, and conclude that both galaxies reside in massivedark matter halos, which extend well beyond the stellar light. The presence ofdark matter halos around NGC4342 and NGC4291 and a deep optical image of theenvironment of NGC4342 indicate that tidal stripping, in which >90% of thestellar mass was lost, cannot explain the observed high black hole-to-bulgemass ratios. Therefore, we conclude that these galaxies formed with low stellarmasses, implying that the bulge and black hole did not grow in tandem. We alsofind that the black hole mass correlates well with the properties of the darkmatter halo, suggesting that dark matter halos may play a major role inregulating the growth of the supermassive black holes.

Exploring the unusually high black hole-to-bulge mass ratios in NGC4342 and NGC4291: the asynchronous growth of bulges and black holes