5. THE ACCURACY OF AGN BLACK HOLE MASS DETERMINATIONS

The component of velocity perpendicular to the equatorial plane is vital for AGN black hole mass determinations! Without this strong turbulent velocity component, variations in sini would introduce substantial scatter into AGN black hole mass estimates, especially since type-1 AGNs are observed close to face-on. There is recent evidence that there is remarkably little scatter in AGN mass estimates. Firstly, it has become apparent (Bochkarev & Gaskell 2009) that the two main methods of estimating black hole masses from the BLR agree surprisingly well. The Dibai single-epoch-spectrum method (Dibai 1977) and reverberation mapping methods agree to within the expected errors. Gaskell (2009b) has shown furthermore that a simple refinement of the method produces even better agreement. The agreements mean that such methods are estimating the effective radii of the BLR correctly. As Bochkarev & Gaskell (2009) discuss, the success of the Dibai method means that the inner regions of AGNs are very similar. In particular:

1. The spectral energy distribution (SED) from the optical to the far UV must be very similar in all type-1 AGNs because the optical region where the flux is measured is far removed in energy from the far UV which is photoionizing the gas. Although AGN SEDs look different, Gaskell et al. (2004) and Gaskell & Benker (2007) have already argued that the apparent variation is not real but is primarily caused by reddening.

2. There is a simple scaling relationship between the luminosity and the effective radius. This is supported by reverberation mapping estimates of the effective radii of BLRs (Koratkar & Gaskell 1991c, Kaspi et al. 2000, 2005, Bentz et al. 2006, 2009a).

Both the Dibai and reverberation-mapping methods of estimating black hole masses depend on observed BLR line widths, so geometric differences and orientation effects will affect both methods. An important external check on the accuracy of AGN black hole mass estimates is provided by the tightness of the relationship between black hole mass, M, and luminosity, L_host, of the bulge of the host galaxy. Gaskell & Kormendy (2009) have recently shown that estimating M by the Dibai method and L_host from the fraction of starlight in SDSS spectra gives a scatter of ± 0.23 dex in logM (see Fig. 14). Bentz et al. (2009b) have estimated L_host completely independently for a different set of AGNs using HST photometry and published reverberation mapping mass estimates. They get a scatter in logM of ± 0.33 dex. Both of these scatters in the AGN M - logL_host relationships are smaller than the ± 0.38 dex scatter Gultekin et al. (2009) and others find when M is determined by stellar dynamical methods, but they are still greater than the ± 0.17 dex scatter in the M - _* relationship for pure bulge (i.e., barless) galaxies (Graham 2008). The Dibai method and the method proposed by Gaskell (2009b) seems to give particularly tight M - _* and M - L_bulge relationships for the most massive elliptical galaxies (Gaskell 2009a). This is probably because they have the least intrinsic scatter in the M - _* relationship. These comparisons with predictions from host galaxy properties imply that black hole mass determinations from the BLR are surprisingly accurate - as accurate as the best stellar-dynamical estimates. This accuracy of black hole mass estimates made using the BLR provides strong support for all type-1 AGNs being very similar as far as the structure and kinematics of the BLR goes, and for orientation effects being minimal. The accuracy of AGN black hole mass estimates is thus consistent with there being a substantial turbulent BLR velocity component and type-1 AGNs being seen close to pole-on.

Figure 14. The M - Lhost relationship for 100 AGNs with 0.13 < z < 0.34. See Gaskell & Kormendy (2009) for details. M has been estimated by the Dibai method. The diagonal line is the OLS-bisector fit, M Lhost0.84.