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Register a new userThe Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements II. The Full Sample of Reverberation-Mapped AGNs
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Misty C. Bentz
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We present high-resolution HST images of all 35 AGNs with optical reverberation-mapping results, which we have modeled to create a nucleus-free image of each AGN host galaxy. From the nucleus-free images, we determine the host-galaxy contribution to ground-based spectroscopic luminosity measurements at 5100A. After correcting the luminosities of the AGNs for the contribution from starlight, we re-examine the Hbeta R-L relationship. Our best fit for the relationship gives a powerlaw slope of 0.52 with a range of 0.45 - 0.59 allowed by the uncertainties. This is consistent with our previous findings, and thus still consistent with the naive assumption that all AGNs are simply luminosity-scal
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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.
Black hole masses for samples of active galactic nuclei (AGN) are currently estimated from single-epoch optical spectra. In particular, the size of the broad-line emitting region needed to compute the black hole mass is derived from the optical or ultraviolet continuum luminosity. Here we consider the relationship between the broad-line region size, R, and the near-infrared (near-IR) AGN continuum luminosity, L, as the near-IR continuum suffers less dust extinction than at shorter wavelengths and the prospects for separating the AGN continuum from host-galaxy starlight are better in the near-IR than in the optical. For a relationship of the form R propto L^alpha, we obtain for a sample of 14 reverberation-mapped AGN a best-fit slope of alpha=0.5+/-0.1, which is consistent with the slope of the relationship in the optical band and with the value of 0.5 naively expected from photoionisation theory. Black hole masses can then be estimated from the near-IR virial product, which is calculated using the strong and unblended Paschen broad emission lines (Pa alpha or Pa beta).
The radius-luminosity (R-L) relationship of active galactic nuclei (AGNs) established by the reverberation mapping (RM) observations has been widely used as a single-epoch black hole mass estimator in the research of large AGN samples. However, the recent RM campaigns discovered that the AGNs with high accretion rates show shorter time lags by factors of a few comparing with the predictions from the R-L relationship. The explanation of the shortened time lags has not been finalized yet. We collect 8 different single-epoch spectral properties to investigate how the shortening of the time lags correlate with those properties and to understand what is the origin of the shortened lags. We find that the flux ratio between Fe II and H$beta$ emission lines shows the most prominent correlation, thus confirm that accretion rate is the main driver for the shortened lags. In addition, we establish a new scaling relation including the relative strength of Fe II emission. This new scaling relation can provide less biased estimates of the black hole mass and accretion rate from the single-epoch spectra of AGNs.
We present HI 21 cm spectroscopy from the GBT for the host galaxies of 31 nearby AGNs with direct M$_{textrm{BH}}$ measurements from reverberation mapping. These are the first published HI detections for 12 galaxies, and the spectral quality is generally an improvement over archival data for the remainder of the sample. We present measurements of emission-line fluxes, velocity widths, and recessional velocities from which we derive HI mass, total gas mass, and redshifts. Combining M$_{textrm{GAS}}$ with constraints on M$_{textrm{STARS}}$ allows exploration of the baryonic content of these galaxies. We find a typical M$_{textrm{GAS}}$/M$_{textrm{STARS}}$ fraction of 10%, with a few reaching $sim$30-50%. We also examined several relationships between M$_{textrm{STARS}}$, M$_{textrm{GAS}}$, M$_{textrm{BH}}$, baryonic mass, and morphological type. We find a weak preference for galaxies with larger M$_{textrm{GAS}}$ to host more massive black holes. We also find gas-to-stellar fractions to weakly correlate with later types in unbarred spirals, with an approximately constant fraction for barred spirals. Consistent with previous studies, we find declining M$_{textrm{GAS}}$/M$_{textrm{STARS}}$ with increasing M$_{textrm{STARS}}$, with a slope suggesting the gas reservoirs have been replenished. Finally, we find a clear relationship for M$_{textrm{BH}}$-M$_{textrm{BARY}}$ with a similar slope as M$_{textrm{BH}}$-M$_{textrm{STARS}}$ reported by Bentz & Manne-Nicholas (2018). The dwarf Seyfert NGC 4395 appears to follow this relationship as well, even though it has a significantly higher gas fraction and smaller M$_{textrm{BH}}$ than the remainder of our sample.
For a compiled sample of 120 reverberation-mapped AGNs, the bivariate correlations of the broad-line regions (BLRs) size ($R_{rm BLR}$) with the continuum luminosity at 5100 AA ($L_{5100}$) and the dimensionless accretion rates ($dot{mathscr{M}}$) are investigated. Using our recently calibrated virial factor $f$, and the velocity tracer from the H$beta$ Full-width at half-maximum (FWHM(H$beta$)) or the line dispersion ($sigma_{rm Hbeta}$) measured in the mean spectra, three kinds of SMBH masses and $dot{mathscr{M}}$ are calculated. An extended RL relation including $dot{mathscr{M}}$ is found to be stronger than the canonical $R_{rm BLR}({rm Hbeta}) - L_{rm 5100}$ relation, showing smaller scatters. The observational parameters, $R_{rm Fe}$ (the ratio of optical Fe II to H$beta$ line flux) and the line profile parameter $D_{rm Hbeta}$ ($D_{rm Hbeta}=rm FWHM(Hbeta)/sigma_{rm Hbeta}$), have relations with three kinds of $dot{mathscr{M}}$. Using $R_{rm Fe}$ and $D_{rm Hbeta}$ to substitute $dot{mathscr{M}}$, extended empirical $R_{rm BLR}({rm Hbeta}) - L_{rm 5100}$ relations are presented. $R_{rm Fe}$ is a better fix for the $R_{rm BLR}({rm Hbeta}) - L_{rm 5100}$ offset than the H$beta$ shape $D_{rm Hbeta}$. The extended empirical $R_{rm BLR}({rm Hbeta}) - L_{rm 5100}$ relation including $R_{rm Fe}$ can be used to calculate $R_{rm BLR}$, and thus the single-epoch SMBH mass $M_{rm BH}$. Our measured accretion rate dependence is not consistent with the simple model of the accretion disk instability leading the BLRs formation. The BLR may instead form from the inner edge of the torus, or from some other means in which BLR size is positively correlated with accretion rate and the SMBH mass.
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