Is MgIIλ2800 a reliable virial broadening estimator for quasars?

Abstract

Context. Broad Mgiiλ2800 and Hβ lines have emerged as the most reliable virial estimators of the black hole mass in quasars. Which is more reliable? Part of the challenge centers on comparing Mgiiλ2800 and Hβ line profiles in order to improve the ±1 dex M BH uncertainties inherent in single-epoch FWHM measures from noisy spectra. Aims. We compare Mgiiλ2800 and Hβ profile measures in the same sources and especially FWHM measures that provide the virial broadening estimator. Methods. We identify 680 bright Sloan Digital Sky Survey Data Release 7 quasars with spectra showing both Mgiiλ2800 and Hβ lines at redshift 0.4 ≤ z ≤ 0.75. The S/N of these spectra are high enough to allow binning in the "four-dimensional (4D) eigenvector 1" optical plane and construction of high S/N composite spectra. Results. We confirm that Mgiiλ2800 shows a profile that is ≈20% narrower as suggested in some previous studies. FWHM measures for Population B sources (i.e., with FWHM of Hβ larger than 4000 km s −1 ) are uncertain because they show complex profiles with at least two broad-line components involving a nearly unshifted broad and redshifted very-broad component. Only the broad component is likely to be a valid virial estimator. If Hβ and Mgiiλ2800 are not corrected for the very broad component then black hole mass M BH values for Population B sources will be systematically overestimated by up to Δ logM BH ≈ 0.3−0.4 dex. We suggest a simple correction that can be applied to the majority of sources. Mgiiλ2800 is the safer virial estimator for Population B sources because the centroid shifts with respect to rest frame are lower than for Hβ. In the broad and very broad component profile interpretation, this is a consequence of the lower very broad to broad component intensity ratio for Mgiiλ2800. Eigenvector-based studies show that effective discrimination of black hole mass and Eddington ratio at fixed redshift is not achieved via luminosity binning but rather by binning in a "4D eigenvector 1" context that reflects different broad line region geometry/kinematics probably driven by the Eddington ratio.

Blue outliers among intermediate redshift quasars

“…More expedient in the analysis of large ensembles of quasar spectra is to estimate the quasar redshift from lowionization emission lines (typically Hβ or Hα narrow component, the partially resolved [OII]λ3727 doublet, or [NII]λ6548,6583). There are several caveats also with this approach but these lines are customarily sharply peaked and, with good S/N ( > ∼ 20), make possible the definition of a conventional quasar rest frame with a precision ≈ ±50 km s −1 (Marziani et al 2013). Progressing to high redshift is not easy.…”

“…The HE spectra were modeled with a χ 2 minimization procedure using the routine specfit implemented in the IRAF package (Kriss 1994). The details of the analysis techniques have been described elsewhere (Marziani et al 2009(Marziani et al , 2013Negrete et al 2014). Here we recall that the main elements of the multicomponent fits are continuum shape (a power-law), Feii emission template, a Lorentzian function to represent Pop.…”

“…The dynamical relevance of the shift i.e., the shift amplitude normalized by the line half-width half maximum (Marziani et al 2013), is ∆v( Bian et al (2005) and HE samples. BOs are a small fraction of the total sample at the extreme end of the shift distribution in both low-z samples.…”

Black Hole Mass Estimation in Type 1 AGN: Hβ vs. Mg II Lines and the Role of Balmer Continuum

“…Note here that the literature in the field presents comparisons on the use of the optical and the UV lines, like Hβ and Mg II, as virial estimators of the M BH in AGN, pointing out that in some cases Mg II is more reliable as M BH estimator then Hβ (e.g., Marziani et al, 2013, and references therein). However, it has been shown that the Mg II wings are emitted from the weakly gravitationally bounded gas (see Kovačević-Dojčinović and Popović, 2015;Jonić et al, 2016), i.e., the assumption of virialization may be problem in using Mg II line as the BH mass estimator.…”

SALT long-slit spectroscopy of LBQS 2113-4538: variability of the Mg II and Fe II component

“…(Marziani et al 2013). These templates give systematic residuals at 2950 Å due to the absence of Fe I emission in the model (see Marziani et al, for the discussion), but our fits did not extend to such wavelengths.…”