The supermassive black hole in Centaurus A: a benchmark for gas kinematical measurements

Abstract

We present new HST Space Telescope Imaging Spectrograph observations of the nearby radio galaxy NGC 5128 (Centaurus A). The bright emission line with longest wavelength accessible from HST, [S III]λ 9533 Å, was used to study the kinematics of the ionized gas in the nuclear region with a 0.1 arcsec spatial resolution. The STIS data were analized in conjunction with the ground-based near-infrared Very Large Telescope ISAAC spectra used by Marconi et al. (2001, ApJ, 549, 915) to infer the presence of a supermassive black hole and measure its mass. The two sets of data have spatial resolutions differing by almost a factor of five but provide independent and consistent measures of the BH mass, which are in agreement with our previous estimate based on the ISAAC data alone. The gas kinematical analysis provides a mass of M_BH(1.1±0.1)× 10⁸ {M}_⊙ for an assumed disk inclination of i=25 deg or M_BH= (6.5±0.7)× 10⁷ {M}_⊙ for i=35 deg, the largest i value allowed by the data. We performed a detailed analysis of the effects on M_BH of the intrinsic surface brightness distribution of the emission line, a crucial ingredient in the gas kinematical analysis. We estimate that the associated systematic errors are no larger than 0.08 in log M_BH, comparable with statistical errors and indicating that the method is robust. However, the intrinsic surface brightness distribution has a large impact on the value of the gas velocity dispersion. A mismatch between the observed and model velocity dispersion is not necessarily an indication of non-circular motions or kinematically hot gas, but is as easily due to an inaccurate computation arising from too course a model grid, or the adoption of an intrinsic brightness distribution which is too smooth. The observed velocity dispersion in our spectra can be matched with a circularly rotating disk and also the observed line profiles and the higher order moments in the Hermite expansion of the line profiles, h₃ and h₄, are consistent with emission from such a disk. To our knowledge, Centaurus A is the first external galaxy for which reliable BH mass measurements from gas and stellar dynamics are available and, as in the case of the Galactic Center, the M_BH gas kinematical estimate is in good agreement with that from stellar dynamics. The BH mass in Centaurus A is in excellent agreement with the correlation with infrared luminosity and mass of the host spheroid but is a factor ∼ 2-4 above the one with the stellar velocity dispersion. But this disagreement is not large if one takes into account the intrinsic scatter of the M_BH-σ_e correlation. Finally, the high HST spatial resolution allows us to constrain the size of any cluster of dark objects alternative to a BH to r_•<0.035 arcsec (≃0.6 pc). Thus Centaurus A ranks among the best cases for supermassive Black Holes in galactic nuclei.