Dynamics of the nuclear gas and dust disc in the E4 radio galaxy NGC 7052

Abstract

We present high spatial resolution ground-based broad-band imaging, H-alpha +[NII] narrow-band imaging and long-slit spectroscopy for the E4 radio galaxy NGC 7052, which has a nuclear dust disc. We detect ionized gas with a LINER spectrum, residing also in a nuclear disc. The gas rotates rapidly and the emission line widths increase towards the nucleus. The images are well fit by an axisymmetric model with the gas and dust in a disc of 1.5" radius (340 pc) in the equatorial plane of the stellar body, viewed at an inclination of 70 degrees. We assume the gas to be on circular orbits in the equatorial plane, with in addition a local turbulent velocity dispersion. The circular velocity is calculated from the combined gravitational potential of the stars and a possible nuclear black hole. The observed gas rotation curve is well fit, either with or without a black hole. Turbulent velocities >300 km/s must be present at radii <0.5" to fit the observed nuclear line widths. Models with a black hole and no turbulence can also fit the line widths, but these models cannot fit the observed nuclear line shapes. Models with both a black hole and gas turbulence can fit the data well, but the black hole is not required by the data, and if present, its mass must be < 5 x 10^8 solar masses. This upper limit is 5 times smaller than the black hole mass inferred for M87 from HST data. Our modelling and analysis techniques will be useful also for the study of the kinematics of nuclear gas discs in other galaxies. Such data will yield important information on the presence of massive black holes.