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 ^ { +3.8 } _ { -2.3 } \% and 1.9 \% \pm 0.6 \% , respectively , which significantly exceed the typical observed ratio of \sim 0.2 \% .
As a consequence of the exceedingly large black hole-to-bulge mass ratios , NGC4342 and NGC4291 are \approx 5.1 \sigma and \approx 3.4 \sigma outliers from the M _ { \bullet } - 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 \gtrsim 90 \% of the stellar mass was lost , can not 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 .