Spectroscopy of the near-nuclear regions of Cygnus A: estimating the mass of the supermassive black hole

We use a combination of high spatial resolution optical and near-infrared spectroscopic data to make a detailed study of the kinematics of the narrow-line region (NLR) gas in the near-nuclear regions of the powerful, FRII radio galaxy Cygnus A (z = 0.0560), with the overall goal of placing limits on the mass of any supermassive black hole in the core. Our K -band infrared observations (0.75-arcsec seeing) - taken with NIRSPEC on the Keck II telescope - show a smooth rotation pattern across the nucleus in the Paalpha and H-2 emission lines along a slit position (PA180degrees) close to perpendicular to the radio axis, however, there is no evidence for such rotation along the radio axis (PA105degrees). Higher spatial resolution observations of the [O iii]lambda5007 emission line - taken with STIS on the Hubble Space Telescope (HST ) - confirm the general rotation pattern of the gas in the direction perpendicular to the radio axis, and provide evidence for steep velocity gradients within a radius of 0.1 arcsec of the core - corresponding to the high surface brightness structure visible in high-resolution narrow-band images. The [O iii] line remains broad throughout the core region (FWHM similar to 300-900 km s(-1) ), but the spatial distribution of [O iii] provides no evidence for an unresolved inner narrow-line region (INLR). Assuming that the ionized gas is circularly rotating in a thin disc and that the large linewidths are due to activity-induced turbulence, the circular velocities measured from both the Keck and HST data lead to an estimate of the mass of the supermassive black hole of 2.5 +/- 0.7 x 10(9) M-.. For the host galaxy properties of Cygnus A, this mass is consistent with the global correlations between black hole mass and host galaxy properties deduced for non-active galaxies. Therefore, despite the extreme power of its radio source and the quasar-like luminosity of its active galactic nucleus (AGN), the black hole in Cygnus A is not unusually massive considering the luminosity of its host galaxy. Indeed, the estimated mass of the black hole in Cygnus A is similar to that inferred for the supermassive black hole in the FRI radio galaxy M87, despite the fact that the AGN and radio jets of Cygnus A are two to three orders of magnitude more powerful. Overall, these results are consistent with the idea that the properties and powers of the AGN in radio galaxies are determined as much by the mass accretion rates as by the absolute masses of their supermassive black holes. As well as providing evidence for a supermassive black hole in the core of Cygnus A, our data also demonstrate that nuclear activity has an important effect on the kinematics of the circumnuclear gas on a subkiloparsec scale. Most notably, the velocity offsets measured in the two outer HST /STIS slit positions are consistent with the presence of an activity-induced outflow in the NW cone.