The Host Galaxies of Low-mass Black Holes

TL;DR

This study uses HST data to analyze the structures and scaling relations of 147 host galaxies with low-mass black holes, revealing most have pseudobulges and evolved secularly, challenging the necessity of classical bulges for black hole hosting.

Contribution

It provides the first systematic analysis of low-mass black hole host galaxy structures, highlighting the prevalence of pseudobulges and secular evolution in these systems.

Findings

Most host galaxies have extended disks and pseudobulges.

Low-mass black holes often reside in galaxies without classical bulges.

Galaxies without disks resemble spheroidals more than classical bulges.

Abstract

Using HST observations of 147 host galaxies of low-mass black holes (BHs), we systematically study the structures and scaling relations of these active galaxies. Our sample is selected to have central BHs with virial masses ~10^5-10^6 solar mass. The host galaxies have total I-band magnitudes of -23.2<M_I<-18.8 mag and bulge magnitudes of -22.9<M_I<-16.1 mag. Detailed bulge-disk-bar decompositions with GALFIT show that 93% of the galaxies have extended disks, 39% have bars and 5% have no bulges at all at the limits of our observations. Based on the Sersic index and bulge-to-total ratio, we conclude that the majority of the galaxies with disks are likely to contain pseudobulges and very few of these low-mass BHs live in classical bulges. The fundamental plane of our sample is offset from classical bulges and ellipticals in a way that is consistent with the scaling relations of pseudobulges. The sample has smaller velocity dispersion at fixed luminosity in the Faber-Jackson plane, compared with classical bulges and elliptical galaxies. The galaxies without disks are structurally more similar to spheroidals than to classical bulges according to their positions in the fundamental plane, especially the Faber-Jackson projection. Overall, we suggest that BHs with mass < 10^6 solar mass live in galaxies that have evolved secularly over the majority of their history. A classical bulge is not a prerequisite to host a black hole.