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 \sim 10 ^ { 5 } -10 ^ { 6 } M _ { \odot } .
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 Sérsic 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 \lesssim 10 ^ { 6 } M _ { \odot } 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 .