Central Masses and Broad‐Line Region Sizes of Active Galactic Nuclei. I. Comparing the Photoionization and Reverberation Techniques

Abstract

The masses and emission-line region sizes of AGNs can be measured by "reverberation-mapping" techniques and we use these results to calibrate similar determinations made by photoionization models of the AGN line-emitting regions. Reverberation mapping uses the light travel-time delayed emission-line response to continuum variations to determine the size and kinematics of the emission-line region. We compile a sample of 17 Seyfert 1 galaxies and 2 quasars with reliable reverberation and spectroscopy data, twice the number available previously. The data provide strong evidence that the BLR size (as measured by the lag of the emission-line luminosity after changes in the continuum) and the emission-line width measure directly the central mass: the virial assumption is tested with long-term UV and optical monitoring data on NGC 5548. Two methods are used to estimate the distance of the broad emission-line region (BLR) from the ionizing source: the photoionization method (which is available for many AGNs but has large intrinsic uncertainties), and the reverberation method (which gives very reliable distances, but is available for only a few objects). The distance estimate is combined with the velocity dispersion, derived from the broad Hβ line width (in the photoionization method) or from the variable part (RMS) of the line profile, in the reverberation -RMS method, to estimate the virial mass. Comparing the central masses calculated with the reverberation -RMS method to those calculated using a photoionization model, we find a highly significant, nearly linear correlation. This provides a calibration of the photoionization method on the objects with presently available reverberation data, which should enable mass estimates for all AGNs with measured Hβ line width. We find that the correlation between the masses is significantly better than the correlation between the corresponding BLR sizes calculated by the two methods, which further supports the conclusion that both methods measure the mass of the central black hole. Comparing the BLR sizes given by the two methods also enables us to estimate the ionizing EUV luminosity L ion which is directly unobservable. Typically it is ten times the monochromatic luminosity at 5100Å(L v ). The Eddington ratio for the objects in our sample is in the range L v /L Edd ∼ 0.001 − 0.03 and L ion /L Edd ≈ 0.01 − 0.3.

Evolution of active galaxies: black-hole mass–bulge relations for narrow line objects

“…We find that IC 4329A is an outlier in all the relations discussed in Wandel, Peterson & Malkan 1999 as well as in Peterson et al 2000. This leads us to suspect the virial BH mass of this object.…”

“…For NAB 0205+024, PG 1402+261 and PG 1444+407 we take the host galaxy absolute mag-2 The virial mass for IC 4329A is adopted from Wandel 1999 who has adopted it from Wandel, Peterson & Malkan 1999. We find that IC 4329A is an outlier in all the relations discussed in Wandel, Peterson & Malkan 1999 as well as in Peterson et al 2000.…”

Black hole masses in active galaxies

“…One important application for such measurements is as a consistency check on the black hole masses determined from reverberation mapping of nearby Seyfert 1 galaxies (Wandel, Peterson, & Malkan 1999;Kaspi et al 2000). Gebhardt et al (2000b) and Ferrarese et al (2001) have shown that the reverberation masses of Seyfert 1 galaxies are in good agreement with the M −σ relation of nearby galaxies; this is a reassuring sign that the reverberation measurements give reasonable estimates of the black hole masses.…”

The Fueling and Evolution of AGN: Internal and External Triggers

“…Consequently, BH masses in local AGN are commonly mapped with alternative techniques such as reverberation mapping (Blandford & Mc Kee 1982;Peterson 1993; see Peterson these proceedings) where one estimates the virial mass inside the broad-line region (BLR) by combining the velocity of the BLR with an estimate of the size of the BLR based on time delay measurements. Reverberation mapping can typically probe scales ∼ 600 R s−bh and has yielded BH masses for several tens of AGN (Peterson 1993; Wandel, Peterson, & Malkan 1999;Kaspi 2000). Earlier controversies existed on the reliability of the method due to purported systematic differences in the BH-to-bulge mass ratio between AGN with reverberation mapping data and quiescent galaxies or QSOs.…”