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 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.

• The paper refines the AGN radius–luminosity relationship at low luminosity using HST imaging and 20 new reverberation measurements, achieving a best-fit slope of 0.533.
• It employs Bayesian analysis and two-dimensional surface brightness decompositions to minimize AGN contamination, reducing scatter from 0.19 dex to 0.13 dex when accounting for outliers.
• The study advances the use of AGNs as standardizable candles by highlighting the need for improved distance estimates and reverberation techniques for robust cosmological applications.

The Low-Luminosity End of the Radius–Luminosity Relationship for Active Galactic Nuclei

The paper "The Low-Luminosity End of the Radius–Luminosity Relationship for Active Galactic Nuclei" by Bentz et al. presents a comprehensive analysis of the relationship between the broad-line region (BLR) radius and the luminosity in active galactic nuclei (AGN). This study addresses the low-luminosity range of AGNs to refine the established radius-luminosity (R-L) relationship and explore its potential implications in cosmological research.

The research introduces significant advancements by including nine newly imaged AGNs captured through the Hubble Space Telescope Wide Field Camera 3, allowing for precise two-dimensional surface brightness decompositions. This method effectively removes the AGN's influence from the images, facilitating accurate measurements of the host galaxies' starlight contributions. Additionally, twenty new reverberation mapping measurements are incorporated, broadening the luminosity scale to four orders of magnitude.

The study employs a Bayesian analysis to fit the R-L relationship, finding a best-fit slope of $\alpha = 0.533^{+0.035}_{-0.033}$, which is consistent with previous analyses and supports photoionization models. The scatter around the relationship, quantified as 0.19 dex, is noted but can be reduced to 0.13 dex if the outlying data points are reconsidered or discrepancies resolved.

Key findings include:

• Only two AGN appear as outliers in the R-L relationship. However, these have suspect measurements of their Hβ time lags, suggesting potential errors rather than intrinsic anomalies.
• A suggestion that part of the scatter in the R-L relationship is attributable to the imprecision in distance estimates to the AGN host galaxies.

The implications of refining the R-L relationship are significant. With improved precision, the BLR of AGNs could be utilized as standardizable candles for cosmological investigations, allowing probing into the expansion of the Universe with a distinct class of celestial objects across varied redshifts. This capability would extend beyond those traditionally employed, such as Type Ia supernovae. Realizing this potential requires improved accuracy in both distance measurements and reverberation techniques.

This paper also highlights challenges and future directions, emphasizing the need for accurate distance measurements of AGN host galaxies, additional reverberation mapping efforts to confirm the results, and exploring other emission lines like C IV to establish corresponding R-L relationships. The paper thereby sets the groundwork for future developments and applications in astrophysics, particularly in enhancing our understanding of galaxy development and the Universe's expansion.