The lick AGN monitoring project: The M_BH- σ_* relation for reverberation-mapped active galaxies

To investigate the black hole mass versus stellar velocity dispersion (M_BH-σ_*) relation of active galaxies, we measured the velocity dispersions of a sample of local Seyfert 1 galaxies, for which we have recently determined black hole masses using reverberation mapping. For most objects, stellar velocity dispersions were measured from high signal-to-noise ratio optical spectra centered on the Ca II triplet region (∼ 8500 Å), obtained at the Keck, Palomar, and Lick Observatories. For two objects, in which the Ca II triplet region was contaminated by nuclear emission, the measurement was based on high-quality H-band spectra obtained with the OH-Suppressing Infrared Imaging Spectrograph at the Keck-II telescope. Combining our new measurements with data from the literature, we assemble a sample of 24 active galaxies with stellar velocity dispersions and reverberation-based black hole mass measurements in the range of black hole mass 10⁶ < M_BH/M_⊙ < 10⁹. We use this sample to obtain reverberation-mapping constraints on the slope and intrinsic scatter of the M_BH-σ_* relation of active galaxies. Assuming a constant virial coefficient f for the reverberation-mapping black hole masses, we find a slope β = 3.55 ± 0.60 and the intrinsic scatter σ_int = 0.43 ± 0.08 dex in the relation log(M_BH/M_⊙) = α + β log(σ_*/200kms^-1), which are consistent with those found for quiescent galaxies. We derive an updated value of the virial coefficient f by finding the value which places the reverberation masses in best agreement with the M_BH-σ_* relation of quiescent galaxies; using the quiescent M_BH-σ_* relation determined by Gültekin etal., we find log f = 0.72 ^+0.09 _-0.10 with an intrinsic scatter of 0.44 ± 0.07 dex. No strong correlations between f and parameters connected to the physics of accretion (such as the Eddington ratio or line-shape measurements) are found. The uncertainty of the virial coefficient remains one of the main sources of the uncertainty in black hole mass determinations using reverberation mapping, and therefore also in single-epoch spectroscopic estimates of black hole masses in active galaxies.