Correlated variations in the line and continuum emission from active galactic nuclei (AGNs) can be used to determine the size and geometry of the broad emission-line regions (BLRs). We have spectrophotometrically monitored a well-deÐned sample of 28 Palomar-Green quasars in order to obtain measurements of their BLRs and to investigate the relationships between quasar luminosity, central black hole mass, and BLR size in AGNs. Spectrophotometry was obtained every 1È4 months for 7.5 yr, yielding 20È70 observing epochs per object. Both the continuum and emission-line Ñuxes of all of the quasars were observed to change during the duration of the observing program. Seventeen of the 28 objects were observed with adequate sampling independent observing epochs) to search for correlated variations (Z20 between the Balmer emission lines and the continuum Ñux. For each of these 17 objects, a signiÐcant correlation was observed, with the Balmer-line variations lagging those of the continuum by D100 days (rest frame). Our work increases the available luminosity range for studying the size-mass-luminosity relations in AGNs by 2 orders of magnitude and doubles the number of objects suitable for such studies. Combining our results with comparable published data available for Seyfert 1 galaxies, we Ðnd the BLR size scales with the rest-frame 5100 luminosity as L0.70B0.03. This determination of the scaling of the A size of the BLR as a function of luminosity is signiÐcantly di †erent from those previously published and suggests that the e †ective ionization parameter in AGNs may be a decreasing function of luminosity. We are also able to constrain, subject to our assumption that gravity dominates the motions of the BLR gas, the scaling relationship between the mass of the central black holes and the luminosity in AGNs. We Ðnd that the central mass scales with 5100 luminosity as M P L0.5B0.1. This is inconsistent with all A AGNs having optical luminosity that is a constant fraction of the Eddington luminosity.

We present improved black hole masses for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis of broad emission-line reverberation-mapping data. From objects with multiple line measurements, we find that the highest precision measure of the virial product cτ ∆V 2 /G, where τ is the emission-line lag relative to continuum variations and ∆V is the emission-line width, is obtained by using the crosscorrelation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to full width half maximum) for the line width and by measuring the line width in the variable part of the spectrum. Accurate line-width measurement depends critically on avoiding contaminating features, in particular the narrow components of the emission lines. We find that the precision (or random component of the error) of reverberation-based black hole mass measurements is typically around 30%, comparable to the precision attained in measurement of black hole masses in quiescent galaxies by gas or stellar dynamical methods. Based on results

We reinvestigate the relationship between the characteristic broad-line region size (R BLR ) and the Balmer emissionline, X-ray, UV, and optical continuum luminosities. Our study makes use of the best available determinations of R BLR for a large number of active galactic nuclei (AGNs) from Peterson et al. Using their determinations of R BLR for a large sample of AGNs and two different regression methods, we investigate the robustness of our correlation results as a function of data subsample and regression technique. Although small systematic differences were found depending on the method of analysis, our results are generally consistent. Assuming a power-law relation R BLR / L , we find that the mean bestfitting is about 0:67 AE 0:05 for the optical continuum and the broad H luminosity, about 0:56 AE 0:05 for the UV continuum luminosity, and about 0:70 AE 0:14 for the X-ray luminosity. We also find an intrinsic scatter of $40% in these relations. The disagreement of our results with the theoretical expected slope of 0.5 indicates that the simple assumption of all AGNs having on average the same ionization parameter, BLR density, column density, and ionizing spectral energy distribution is not valid and there is likely some evolution of a few of these characteristics along the luminosity scale.

We present high-resolution Hubble Space Telescope images of all 35 active galactic nuclei (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 5100 Å. After correcting the luminosities of the AGNs for the contribution from starlight, we re-examine the Hβ R BLR -L relationship. Our best fit for the relationship gives a power-law 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-scaled versions of each other. We discuss various consistency checks relating to the galaxy modeling and starlight contributions, as well as possible systematic errors in the current set of reverberation measurements from which we determine the form of the R BLR -L relationship.

This review describes recent developments related to the unified model of active galactic nuclei (AGN). It focuses on new ideas about the origin and properties of the central obscurer (torus), and the connection with its surrounding. The review does not address radio unification. AGN tori must be clumpy but the uncertainties about their properties are still large. Todays most promising models involve disk winds of various types and hydrodynamical simulations that link the large scale galactic disk to the inner accretion flow. IR studies greatly improved the understanding of the spectral energy distribution of AGNs but they are hindered by various selection effects. X-ray samples are more complete. A basic relationship which is still unexplained is the dependence of the torus covering factor on luminosity. There is also much confusion regarding "real type-II AGNs" that do not fit into a simple unification scheme. The most impressive recent results are due to IR interferometry, which is not in accord with most torus models, and the accurate mapping of central ionization cones. AGN unification may not apply to merging systems and is possibly restricted to secularly evolving galaxies.