THE LOW-LUMINOSITY END OF THE RADIUS-LUMINOSITY RELATIONSHIP FOR ACTIVE GALACTIC NUCLEI

Abstract

We present an updated and revised analysis of the relationship between the\nHbeta broad-line region (BLR) radius and the luminosity of the active galactic\nnucleus (AGN). Specifically, we have carried out two-dimensional surface\nbrightness decompositions of the host galaxies of 9 new AGNs imaged with the\nHubble Space Telescope Wide Field Camera 3. The surface brightness\ndecompositions allow us to create "AGN-free" images of the galaxies, from which\nwe measure the starlight contribution to the optical luminosity measured\nthrough the ground-based spectroscopic aperture. We also incorporate 20 new\nreverberation-mapping measurements of the Hbeta time lag, which is assumed to\nyield the average Hbeta BLR radius. The final sample includes 41 AGNs covering\nfour orders of magnitude in luminosity. The additions and updates incorporated\nhere primarily affect the low-luminosity end of the R-L relationship. The best\nfit to the relationship using a Bayesian analysis finds a slope of alpha =\n0.533 (+0.035/-0.033), consistent with previous work and with simple\nphotoionization arguments. Only two AGNs appear to be outliers from the\nrelationship, but both of them have monitoring light curves that raise doubt\nregarding the accuracy of their reported time lags. The scatter around the\nrelationship is found to be 0.19(+/-0.02) dex, but would be decreased to 0.13\ndex by the removal of these two suspect measurements. A large fraction of the\nremaining scatter in the relationship is likely due to the inaccurate distances\nto the AGN host galaxies. Our results help support the possibility that the R-L\nrelationship could potentially be used to turn the BLRs of AGNs into\nstandardizable candles. This would allow the cosmological expansion of the\nUniverse to be probed by a separate population of objects, and over a larger\nrange of redshifts.\n