Black Holes in Pseudobulges and Spheroidals: A Change in the Black Hole-Bulge Scaling Relations at Low Mass

TL;DR

This study explores the relationship between black hole mass and host galaxy properties in low-mass active galaxies, revealing that black hole scaling relations vary with galaxy structure and that classical bulges are not necessary for black hole presence.

Contribution

It demonstrates that black hole-galaxy scaling relations depend on galaxy morphology and that black holes can exist in pseudobulges and spheroidals, challenging previous assumptions.

Findings

Black holes are present in low-mass, disk-dominated, and spheroidal galaxies.

The black hole to bulge mass ratio is lower than in classical bulges.

Black hole scaling relations vary with galaxy structure.

Abstract

We investigate the relationship between black hole mass and host galaxy properties for active galaxies with the lowest black hole masses currently known in galaxy nuclei. Hubble Space Telescope imaging confirms that the host galaxies have correspondingly low luminosity; they are ~1 mag below L*. In terms of morphology, ~60% of the members of the sample are disk-dominated, and all of these are consistent with containing a bulge or (more likely) pseudobulge, while the remainder are compact systems with no discernible disk component. In general the compact components of the galaxies do not obey the fundamental plane of giant elliptical galaxies and classical bulges, but rather are less centrally concentrated at a given luminosity, much like spheroidal galaxies. Our results strongly confirm that a classical bulge is not a requirement for a nuclear black hole. At the same time, the observed ratio of black hole to bulge mass is nearly an order of magnitude lower in this sample than that seen for classical bulges. While the M-sigma relation appears to continue to low mass, it seems that black hole-galaxy scaling relations do depend on galaxy structure.