The Lick Agn Monitoring Project: Reverberation Mapping of Optical Hydrogen and Helium Recombination Lines

Abstract

We have recently completed a 64-night spectroscopic monitoring campaign at the Lick Observatory 3-m Shane telescope with the aim of measuring the masses of the black holes in 12 nearby (z < 0.05) Seyfert 1 galaxies with expected masses in the range ∼ 10 6 -10 7 M ⊙ and also the well-studied nearby active galactic nucleus (AGN) NGC 5548. Nine of the objects in the sample (including NGC 5548) showed optical variability of sufficient strength during the monitoring campaign to allow for a time lag to be measured between the continuum fluctuations and the response to these fluctuations in the broad Hβ emission, which we have previously reported. We present here the light curves for the Hα, Hγ, He II λ4686, and He I λ5876 emission lines and the time lags for the emission-line responses relative to changes in the continuum flux. Combining each emissionline time lag with the measured width of the line in the variable part of the spectrum, we determine a virial mass of the central supermassive black hole from several independent emission lines. We find that the masses are generally consistent within the uncertainties. The time-lag response as a function of velocity across the Balmer line profiles is examined for six of the AGNs. We find similar responses across all three Balmer lines for Arp 151, which shows a strongly asymmetric profile, and for SBS 1116+583A and NGC 6814, which show a symmetric response about zero velocity. For the other three AGNs, the data quality is somewhat lower and the velocity-resolved time-lag response is less clear. Finally we compare several trends seen in the dataset against the predictions from photoionization calculations as presented by Korista & Goad. We confirm several of their predictions, including an increase in responsivity and a decrease in the mean time lag as the excitation and ionization level for the species increases. Specifically, we find the time lags of the optical recombination lines to have weighted mean ratios of τ (Hα) : τ (Hβ) : τ (Hγ) : τ (He I) : τ (He II) = 1.54 : 1.00 : 0.61 : 0.36 : 0.25. Further confirmation of photoionization predictions for broad-line gas behavior will require additional monitoring programs for these AGNs while they are in different luminosity states.

Space Telescope and Optical Reverberation Mapping Project. V. Optical Spectroscopic Campaign and Emission-line Analysis for NGC 5548

“…Gas with very low v r could have a wide range of lags, and a spherical or flat disk distribution of BLR clouds in Keplerian motion could lead to a double-peaked velocityresolved lag profile if the ionizing source is emitting anisotropically Goad & Wanders 1996;Horne et al 2004). Previous studies of the UV and optical lines in NGC 5548 have inferred either Keplerian orbits Wanders et al 1995;Denney et al 2009;Bentz et al 2010b) or infalling motion (Crenshaw & Blackwell 1990;Done & Krolik 1996;Welsh et al 2007;Pancoast et al 2014;Gaskell & Goosmann 2016) for the BLR gas. From our data, the shape of the Hβ velocity-resolved lag profile suggests a BLR dominated by Keplerian motion.…”

The Structure of the Broad-line Region in Active Galactic Nuclei. II. Dynamical Modeling of Data From the AGN10 Reverberation Mapping Campaign

“…To obtain more detailed information about the BLR, a few recent studies have set out to recover the transfer function, or velocity-delay map, that shows exactly how the variations in the continuum emission are mapped into variations in the broad line emission as a function of the line-of-sight velocity of the gas (Bentz et al 2010, Grier et al 2013b). These studies used maximum entropy methods (MEM) implemented in a code called MEMECHO (Horne et al 1991;Horne 1994), to recover the transfer functions.…”

AGN Reverberation Mapping

“…Within the BLR, different emission lines are observed to respond to continuum variations with different time delays, such that species with higher ionization potentials, like C IV λ 1549, respond with a shorter time delay than those with lower ionization potentials, like Hβ (e.g., Peterson & Wandel 2000;Kollatschny et al 2001;Bentz et al 2010b). This behavior points to ionization stratification within the BLR -more highly ionized line emission is radiated from a smaller radius within the BLR, while emission from more neutral gas occurs at larger radii, further from the central ionizing source.…”