Systematic Uncertainties in Black Hole Masses Determined From Single-Epoch Spectra

Abstract

We explore the nature of systematic errors that can arise in measurement of black hole masses from singleepoch (SE) spectra of active galactic nuclei (AGNs) by utilizing the many epochs available for NGC 5548 and PG1229+204 from reverberation mapping (RM) databases. In particular, we examine systematics due to AGN variability, contamination due to constant spectral components (i.e., narrow lines and host galaxy flux), data quality (i.e., signal-to-noise ratio (S/N)), and blending of spectral features. We investigate the effect that each of these systematics has on the precision and accuracy of SE masses calculated from two commonly used line width measures by comparing these results to recent RM studies. We calculate masses by characterizing the broad Hβ emission line by both the full width at half maximum and the line dispersion, and demonstrate the importance of removing narrow emission-line components and host starlight. We find that the reliability of line width measurements rapidly decreases for S/N lower than ∼ 10-20 (per pixel), and that fitting the line profiles instead of direct measurement of the data does not mitigate this problem but can, in fact, introduce systematic errors. We also conclude that a full spectral decomposition to deblend the AGN and galaxy spectral features is unnecessary, except to judge the contribution of the host galaxy to the luminosity and to deblend any emission lines that may inhibit accurate line width measurements. Finally, we present an error budget which summarizes the minimum observable uncertainties as well as the amount of additional scatter and/or systematic offset that can be expected from the individual sources of error investigated. In particular, we find that the minimum observable uncertainty in SE mass estimates due to variability is 0.1 dex for high S/N ( 20 pixel −1 ) spectra.

THE RADIUS-LUMINOSITY RELATIONSHIP FOR ACTIVE GALACTIC NUCLEI: THE EFFECT OF HOST-GALAXY STARLIGHT ON LUMINOSITY MEASUREMENTS. II. THE FULL SAMPLE OF REVERBERATION-MAPPED AGNs

“…A second consistency check is provided by spectral decomposition, i.e., by fitting multiple components of a spectrum rather than an image. Our experience, however, is that spectral decomposition is certainly no less ambiguous than image decomposition, but when carefully done with very good data shows reasonable consistency (e.g., the modeling of the optical spectrum of NGC 5548 by Denney et al 2009 yields a hostgalaxy flux in agreement with the value we find here). A third check, which applies at least in the case of the higher luminosity AGNs, is that the total luminosity of the host galaxy, based on the model fit, must not exceed the luminosity of known bright normal galaxies.…”

Spectroscopic Indication of a Centi-Parsec Supermassive Black Hole Binary in the Galactic Center of Ngc 5548

“…Fe II emission is relatively weak in NGC5548 (e.g., Wamsteker et al 1990;Vestergaard & Peterson 2005;Denney et al 2009;Mehdipour et al 2015). Vestergaard & Peterson (2005) selected 73 spectra of NGC5548 spanning 13 yr of the AGN Watch project and quantitatively showed that, on average, the flux ratio of optical Fe II over the range 4250-5710 Å (rest-frame) to broad Hβ line is 0.66.…”

“…Denney et al (2009) studied exhaustively the contamination of He IIλ4686 on Hβ for NGC5548, again using the spectra from the AGN Watch project. Their results showed that although deblending He II slightly increases the flux of the far blue wing of Hβ, its influence on the Hβ line width (in term of full width at half maximum) is safely negligible (see their Figures 16 and 19).…”

DETECTION OF QUASAR FEEDBACK FROM THE THERMAL SUNYAEV–ZEL’DOVICH EFFECT INPLANCK

“…The spectroscopically-derived estimates of M BH we use for each quasar in our sample are single-epoch M BH estimates based on the broad emission lines, and have possible systematic uncertainties that are currently poorlyconstrained (Denney et al 2009;Rafiee & Hall 2011;Denney et al 2013). …”