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[*] Dynamics of the nuclear gas and dust disk in the E4 radio galaxy NGC 7052
van den Bosch F., van der Marel R.P.
MNRAS, 274, 884-898, 1995
© 1995. The Royal Astronomical Society. All Rights Reserved.

[*] Citations to this paper in the ADS

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 (V_rot = 250 km/s at 1.5 arcsec). The emission-line widths increase towards the nucleus (nuclear FWHM 500-600 km/s).

The images are well fitted by an axisymmetric model with the gas and dust in a disc of 1.5 arcsec radius (340 pc) in the equatorial plane of the stellar body, viewed at inclination i=70 degrees. The flux distribution of the ionized gas is very centrally concentrated, and is unresolved inside the seeing radius. The central cusp-steepness alpha of the stellar luminosity density (j = cst x r^alpha for r ---> 0) is not well constrained by the observed surface brightness distribution, because of the dust absorption.

We model the gas kinematics with the gas on circular orbits in the equatorial plane, with a local velocity dispersion due to turbulence (or otherwise non-gravitational motion). 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 fitted by a model with alpha = -1.3, either with or without a black hole. Turbulent velocities > 300 km/s must be present at radii < 0.5 arcsec to fit the observed nuclear line widths. Seeing convolution of a Keplerian rotation curve around a 10^9 solar mass black hole can fit the observed widths without turbulence, but such a model predicts nuclear emission line shapes with pronounced peaks at v = +/- 300 km/s, which are not observed. 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.

Although this upper limit is not very stringent, it is already 5 times smaller than the black hole mass inferred for M87 from HST data. We show that HST observations of NGC 7052 should improve significantly the constraints on the mass of any possible black hole. Kinematic observations of nuclear gas discs are likely to become a widely used tool in the search for massive black holes in galactic nuclei. The modelling and analysis techniques presented here will be useful for the interpretation of such data.


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Last modified December 8, 1998.
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