The Low-Luminosity End of the Radius-Luminosity Relationship for Active
  Galactic Nuclei
  We present an updated and revised analysis of the relationship between the
Hbeta broad-line region (BLR) radius and the luminosity of the active galactic
nucleus (AGN). Specifically, we have carried out two-dimensional surface
brightness decompositions of the host galaxies of 9 new AGNs imaged with the
Hubble Space Telescope Wide Field Camera 3. The surface brightness
decompositions allow us to create "AGN-free" images of the galaxies, from which
we measure the starlight contribution to the optical luminosity measured
through the ground-based spectroscopic aperture. We also incorporate 20 new
reverberation-mapping measurements of the Hbeta time lag, which is assumed to
yield the average Hbeta BLR radius. The final sample includes 41 AGNs covering
four orders of magnitude in luminosity. The additions and updates incorporated
here primarily affect the low-luminosity end of the R-L relationship. The best
fit to the relationship using a Bayesian analysis finds a slope of alpha =
0.533 (+0.035/-0.033), consistent with previous work and with simple
photoionization arguments. Only two AGNs appear to be outliers from the
relationship, but both of them have monitoring light curves that raise doubt
regarding the accuracy of their reported time lags. The scatter around the
relationship is found to be 0.19(+/-0.02) dex, but would be decreased to 0.13
dex by the removal of these two suspect measurements. A large fraction of the
remaining scatter in the relationship is likely due to the inaccurate distances
to the AGN host galaxies. Our results help support the possibility that the R-L
relationship could potentially be used to turn the BLRs of AGNs into
standardizable candles. This would allow the cosmological expansion of the
Universe to be probed by a separate population of objects, and over a larger
range of redshifts.
