Comparing Single-Epoch Virial Black Hole Mass Estimators for Luminous Quasars

Abstract

Single-epoch virial black hole (BH) mass estimators utilizing broad emission lines have been routinely applied to high-redshift quasars to estimate their BH masses. Depending on the redshift, different line estimators (Hα, Hβ, Mg ii λ2798, C iv λ1549) are often used with optical/near-infrared spectroscopy. Here, we use a homogeneous sample of 60 intermediate-redshift (z ∼ 1.5-2.2) Sloan Digital Sky Survey quasars with optical and near-infrared spectra covering C iv through Hα to investigate the consistency between different single-epoch virial BH mass estimators. We critically compare rest-frame UV line estimators (C iv λ1549, C iii] λ1908, and Mg ii λ2798) with optical estimators (Hβ and Hα) in terms of correlations between line widths and between continuum/line luminosities, for the high-luminosity regime (L 5100 > 10 45.4 erg s −1 ) probed by our sample. The continuum luminosities of L 1350 and L 3000 , and the broad-line luminosities are well correlated with L 5100 , reflecting the homogeneity of quasar spectra in the rest-frame UV-optical, among which L 1350 and the line luminosities for C iv and C iii] have the largest scatter in the correlation with L 5100 . We found that the Mg ii FWHM correlates well with the FWHMs of the Balmer lines and that the Mg ii line estimator can be calibrated to yield consistent virial mass estimates with those based on the Hβ/Hα estimators, thus extending earlier results on less luminous objects. The C iv FWHM is poorly correlated with the Balmer line FWHMs, and the scatter between the C iv and Hβ FWHMs consists of an irreducible part (∼0.12 dex), and a part that correlates with the blueshift of the C iv centroid relative to that of Hβ, similar to earlier studies comparing C iv with Mg ii. The C iii] FWHM is found to correlate with the C iv FWHM, and hence is also poorly correlated with the Hβ FWHM. While the C iv and C iii] lines can be calibrated to yield consistent virial mass estimates as Hβ on average, the scatter is substantially larger than Mg ii, and the usage of C iv/C iii] FWHM in the mass estimators does not improve the agreement with the Hβ estimator. We discuss controversial claims in the literature on the correlation between C iv and Hβ FWHMs, and suggest that the reported correlation is either a result based on small samples or only valid for low-luminosity objects.

Reaching the peak of the quasar spectral energy distribution – I. Observations and models

“…However, several studies of high-redshift AGN (e.g. Netzer et al 2007;Shen & Liu 2012;Trakhtenbrot & Netzer 2012) disagree with this result, contending that the line profile of C IV is not suitable for mass estimates, but agreeing that Mg II shows reasonable correlation. Possible explanations put forward for this discrepancy in C IV are the presence of outflows that influence line profiles in high-redshift, high-Eddington-ratio AGN.…”

“…Simultaneously, we identified four objects in the Shen & Liu (2012) sample with XMM X-ray data of varying quality, and IR spectral data from the ARC TripleSpec (TSPEC) instrument. One additional object with an XMM observation was selected from a publicly available, archival GNIRS 2004B data set (project GS-2004B-Q16, PI Todd Boroson) that had not previously been reduced.…”

Reaching the peak of the quasar spectral energy distribution – II. Exploring the accretion disc, dusty torus and host galaxy

“…Wandel, Peterson & Malkan 1999;Vestergaard 2002;Kaspi et al 2005;Matsuoka et al 2013). The choice of emission line and continuum measurement has been shown to be important, as some line profiles are shaped by effects such as outflows that make them susceptible to bias (Shen & Liu 2012). It has also been suggested thatṁ can be estimated from spectral continuum measurements via the so-called bolometric correction (BC) coefficients (e.g.…”

The Extreme Ultraviolet Variability of Quasars

“…Similarly, we estimate M bh using the line strength and not the continuum with the expectation that we are minimizing the affects of magnification from the lens. We use the more reliable low ionization line Mg II and the formulas from Shen & Liu (2012) Alternatively, the formula of Trakhtenbrot & Netzer (2012) yields a different estimate Taking the average of Equations (6) and (7) we arrive at a mass estimate,…”