STScI Preprint #1277

Evidence for a 3 × 10⁸ M Black Hole in NGC 7052 from HST Observations of the Nuclear Gas Disk

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

We present a Hubble Space Telescope (HST) study of the nuclear region of the E4 radio galaxy NGC 7052, which has a nuclear disk of dust and gas. The Second Wide Field and Planetary Camera (WFPC2) was used to obtain B, V and I broad-band images and an H + [NII] narrow-band image. The images yield the stellar surface brightness profile, the optical depth of the dust, and the flux distribution of the ionized gas. The Faint Object Spectrograph (FOS) was used to obtain H + [NII] spectra at six different positions along the major axis, using a 0.26" diameter circular aperture. The emission lines yield the rotation curve of the ionized gas and the radial profile of its velocity dispersion. The observed rotation velocity at r = 0.2" from the nucleus is V = 155 ± 17 km/s. The Gaussian dispersion of the emission lines increases from = 70 km/s at r=1", to = 400 km/s on the nucleus.

To interpret the gas kinematics we construct axisymmetric models in which the gas and dust reside in a disk in the equatorial plane of the stellar body, and are viewed at an inclination of 70^o. It is assumed that the gas moves on circular orbits, with an intrinsic velocity dispersion due to turbulence (or otherwise non-gravitational motion). The latter is required to fit the observed increase in the line widths towards the nucleus, and must reach a value in excess of 500 km/s in the central 0.1". The circular velocity is calculated from the combined gravitational potential of the stars and a possible nuclear black hole. Models without a black hole predict a rotation curve that is shallower than observed (V_pred = 92 km/s at r = 0.2"), and are ruled out at > 99% confidence. Models with a black hole of mass M = 3.3^+2.3_-1.3 × 10⁸ M provide an acceptable fit. The best-fitting model with a black hole adequately reproduces the observed emission line shapes on the nucleus, which have a narrower peak and broader wings than a Gaussian.

NGC 7052 can be added to the list of active galaxies for which HST spectra of a nuclear gas disk provide evidence for the presence of a central black hole. The black hole masses inferred for M87, M84, NGC 6251, NGC 4261 and NGC 7052 span a range of a factor 10, with NGC 7052 falling on the low end. By contrast, the luminosities of these galaxies are identical to within ~ 25%. Any relation between black hole mass and luminosity, as suggested by independent arguments, must therefore have a scatter of at least a factor 10.

Status:

Appeared in: The Astronomical Journal, 116:2220-2236, 1998

Affiliations:

¹)Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
²)STScI Fellow
³)Previously at the Institute for Advanced Study, Olden Lane, Princeton, NJ 08540
⁴)Hubble Fellow
⁵)Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580
⁶)Previously at the Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands