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]λ9533 Å, was used to study the kinematics of the ionized gas in the nuclear region with a 0. 1 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) × 10 8 M for an assumed disk inclination of i = 25 deg or M BH = (6.5 ± 0.7) × 10 7 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, h 3 and h 4 , 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 ( 0.6 pc). Thus Centaurus A ranks among the best cases for supermassive Black Holes in galactic nuclei.

Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei

“…The modeling assumes that the gas is rotating in a thin disc configuration (neglecting hydrodynamical effects) under the effect of the gravitational potential of the stellar mass and of a pointlike dark mass M BH that is the BH. The value of M BH and other unknown parameters of the model are obtained by fitting the rotation curves (see Marconi et al (2006) and references therein for details). Because of the many unknown parameters of the model, to better constrain the fit, usually many spectra of the galactic nucleus are obtained each with a different orientation of the spectroscopic slit and a simultaneous fit of all of them is performed.…”

“…The reason is very simple and is related to the fact that the full PVD depends on the unknown intrinsic flux distribution of the emission line. Marconi et al (2006) showed that line profiles do depend on the assumed flux distribution while, on the contrary, such dependence is much weaker on the first moment of the line profile (i.e. the mean velocity).…”

“…The model takes also into account the effect of the instrumental Point Spread Function that is modeled with a Gaussian function with a given FWHM, and the effect of the others instrumental setup parameters: the spectral resolution, the slit width and position angle, the detector's pixel size. With this model we can simulate a longslit spectrum of a gas emission line in a particular galaxy nucleus (see Marconi et al 2006, and references therein for a detailed description).…”

Uncovering the kiloparsec-scale stellar ring of NGC 5128

“…To estimate the fraction, a radial model for the stellar emission is needed. We adopted the radial luminosity profile given by Marconi et al (2006) who fitted the intensity profile using an oblate spheroidal function. Using this model, we integrated the emission of the radial emission model along each line-of-sight towards the dust features to the point closest to the observer where that line-of-sight connects with the warped disk structure.…”

Integral-Field Spectroscopy of the Centaurus A Nucleus

“…• (ground-and spacebased observations of gas kinematics, Marconi et al 2006). The last study also constrained the size of the central massive object c 2006 RAS to r ∼ 0.6 pc suggesting that it is indeed a black hole.…”