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

TL;DR

This study investigates two galaxies with unusually high black hole-to-bulge mass ratios, revealing that their black holes grew independently of their stellar bulges and are linked to their dark matter halos.

Contribution

The paper demonstrates that these galaxies' black holes are disproportionately large relative to their bulges and that dark matter halos influence black hole growth, challenging traditional co-evolution models.

Findings

Both galaxies have massive dark matter halos extending beyond stellar regions.

Tidal stripping cannot account for the high black hole-to-bulge ratios.

Black hole mass correlates with dark matter halo properties.

Abstract

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.