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

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. HST 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 MBH-σ* relation appears to continue to low mass, it seems that black hole-galaxy scaling relations do depend on galaxy structure.