A low-luminosity type-1 QSO sample: I. Overluminous host spheroidals or
  undermassive black holes?
  Recognizing the properties of the host galaxies of quasi-stellar objects
(QSOs) is essential to understand the suspected coevolution of central
supermassive black holes (BHs) and their host galaxies. We selected a subsample
of the Hamburg/ESO survey for bright UV-excess QSOs, containing only the 99
nearest QSOs with redshift z<=0.06, that are close enough to allow detailed
structural analysis. From this "low-luminosity type-1 QSO sample", we observed
20 galaxies and performed aperture photometry and
bulge-disk-bar-AGN-decomposition with BUDDA on near-infrared J, H, K band
images. From the photometric decomposition of these 20 objects and visual
inspection of images of another 26, we find that ~50% of the hosts are disk
galaxies and most of them (86%) are barred. Stellar masses, calculated from
parametric models based on inactive galaxy colors, range from 2x10^9 M_sun to
2x10^11 M_sun. Black hole masses measured from single epoch spectroscopy range
from 1x10^6 M_sun to 5x10^8 M_sun. In comparison to higher luminosity QSO
samples, LLQSOs tend to have lower stellar and BH masses. Also, in the
effective radius vs. mean surface-brightness projection of the fundamental
plane, they lie in the transition area between luminous QSOs and "normal"
galaxies. This can be seen as further evidence that they can be pictured as a
"bridge" between the local Seyfert population and luminous QSOs at higher
redshift. Eleven low-luminosity QSOs for which we have reliable morphological
decompositions and BH mass estimations lie below the published BH mass vs.
bulge luminosity relations for inactive galaxies. This could be partially
explained by bulges of active galaxies containing much younger stellar
populations than bulges of inactive galaxies. Also, one could suspect that
their BHs are undermassive. This might hint at the growth of the host spheroid
to precede that of the BH.
