The Sloan Digital Sky Survey Reverberation Mapping Project: How Broad Emission Line Widths Change When Luminosity Changes

Abstract

Quasar broad emission lines are largely powered by photoionization from the accretion continuum. Increased central luminosity will enhance line emissivity in more distant clouds, leading to increased average distance of the broad-line-emitting clouds and decreased averaged line width, known as the broad-line region (BLR) "breathing". However, different lines breathe differently, and some highionization lines, such as C IV, can even show "anti-breathing" where the line broadens when luminosity increases. Using multi-year photometric and spectroscopic monitoring data from the Sloan Digital Sky Survey Reverberation Mapping project, we quantify the breathing effect (∆ log W = α∆ log L) of broad Hα, Hβ, Mg ii, C IV, and C III] for statistical quasar samples over z ≈ 0.1 − 2.5. We found that Hβ displays the most consistent normal breathing expected from the virial relation (α ∼ −0.25), Mg ii and Hα on average show no breathing (α ∼ 0), and C IV (and similarly C III] and Si IV) mostly shows anti-breathing (α > 0). The anti-breathing of C IV can be well understood by the presence of a nonvarying core component in addition to a reverberating broad-base component, consistent with earlier findings. The deviation from canonical breathing introduces extra scatter (a luminosity-dependent bias) in single-epoch virial BH mass estimates due to intrinsic quasar variability, which underlies the long argued caveats of C IV single-epoch masses. Using the line dispersion instead of FWHM leads to less, albeit still substantial, deviations from canonical breathing in most cases. Our results strengthen the need for reverberation mapping to provide reliable quasar BH masses, and quantify the level of variability-induced bias in single-epoch BH masses based on various lines.

OzDES Reverberation Mapping Program: The first Mg II lags from five years of monitoring

“…This assumes that the velocity broadening of the iron features does not change between epochs. In principle the broad emission lines can have a "breathing mode" where the line width changes as the continuum varies (e.g., Korista & Goad 2004;Guo et al 2020;Wang et al 2020). Assuming 𝑅 ∝ 𝐿 0.5 according to previous H𝛽 RM studies (e.g., Bentz et al 2009Bentz et al , 2013, the virial theorem predicts 𝑤 ∝ 𝐿 −0.25 .…”

“…If we vary the broadening width 𝑤 by 2.5% when calculating the Mg II line flux, the changes in the line flux are generally much smaller than the line flux uncertainties. While broad lines can have different breathing behaviours, the breathing is generally weaker than the 𝑤 ∝ 𝐿 −0.25 correlation (e.g., Guo et al 2020;Wang et al 2020), so the discussion above provides a conservative estimate. Therefore, the variability of the broadening width does not have a significant impact on our Mg II line flux measurements.…”

Extreme Variability Quasars in Their Various States. I: The sample selection and composite SDSS spectra

“…Besides, the single-epoch virial black hole mass estimates may also biased by variability (the breathing of broad line region, i.e., the change of line width with lu-minosity in individual AGNs). It was found while Hβ line display normal breathing expected from the virial relation (Gibson et al 2008;Denney et al 2008;Park et al 2011;Barth et al 2015;Runco et al 2016), Mg II shows much weaker breathing (e.g., Shen 2013;Yang et al 2019;Guo et al 2020b;Homan et al 2020), and C IV exhibits even anti-breathing (e.g., Wilhite et al 2005a;Shen et al 2008;Sun et al 2018;Wang et al 2020). Simply assuming no breathing of Mg II and C IV (i.e., the line width does not vary with luminosity) 6 , we could further apply a correction to the single-epoch SMBH mass through replacing L λ with L λ,"cor" during the calculation of mass in Equation 5:…”

“…Contrarily, the ILR may appear non-reverberating or reverberating at much longer time. The observed "antibreathing" of C IV line (i.e., the line broadens when luminosity increases) could also be attributed to the combination the two components (e.g., Denney 2012;Wang et al 2020).…”

Synchronized Co-evolution between Supermassive Black Holes and Galaxies Over the Last Seven Billion Years as Revealed by the Hyper Suprime-Cam

“…As a result, our virial M BH is an unbiased estimator of the true M BH irrespective of AGN luminosity. Therefore, we are ignoring the possible luminositydependent virial M BH biases inShen & Kelly (2010), which is a suitable assumption for our relatively low-z and low-luminosity (log L 5100 = 44.4) sample based on Hβ(Shen & Kelly 2012;Shen 2013;Wang et al 2020).…”