The Host Galaxies of Low-mass Black Holes
  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.
