The Counterrotating Core and the Black Hole Mass of IC 1459^*

The E3 giant elliptical galaxy IC 1459 is the prototypical galaxy with a fast counterrotating stellar core. We obtained one Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) long-slit spectrum along the major axis of this galaxy and Cerro Tololo Inter-American Observatory (CTIO) spectra along five position angles. The signal-to-noise ratio (S/N) of the ground-based data is such that also the higher order Gauss-Hermite moments (h₃-h₆) can be extracted reliably. We present self-consistent three-integral axisymmetric models of the stellar kinematics, obtained with Schwarzschild's numerical orbit superposition method. The available data allow us to study the dynamics of the kinematically decoupled core (KDC) in IC 1459, and we find that it consists of stars that are well separated from the rest of the galaxy in phase space. In particular, our study indicates that the stars in the KDC counterrotate in a disk on orbits that are close to circular. We estimate that the KDC mass is ≈0.5% of the total galaxy mass or ≈3 × 10⁹ M_☉. We estimate the central black hole (BH) mass M_BH of IC 1459 independently from both its stellar and its gaseous kinematics. Although both tracers rule out models without a central BH, neither yields a particularly accurate determination of the BH mass. The main problem for the stellar dynamical modeling is the fact that the modest S/N of the STIS spectrum and the presence of strong gas emission lines preclude measuring the full line-of-sight velocity distribution (LOSVD) at HST resolution. The main problem for the gasdynamical modeling is that there is evidence that the gas motions are disturbed, possibly as a result of nongravitational forces acting on the gas. These complications probably explain why we find rather discrepant BH masses with the different methods. The stellar kinematics suggest that M_BH = (2.6 ± 1.1) × 10⁹ M_☉ (3 σ error). The gas kinematics suggests that M_BH ≈ 3.5 × 10⁸ M_☉ if the gas is assumed to rotate at the circular velocity in a thin disk. If the observed velocity dispersion of the gas is assumed to be gravitational, then M_BH could be as high as ~1.0 × 10⁹ M_☉. These different estimates bracket the value M_BH = (1.1 ± 0.3) × 10⁹ M_☉ predicted by the M_BH-σ relation. It will be an important goal for future studies to attempt comparisons of BH mass determinations from stellar and gaseous kinematics for other galaxies. This will assess the reliability of BH mass determinations with either technique. This is essential if one wants to interpret the correlation between the BH mass and other global galaxy parameters (e.g., velocity dispersion) and in particular the scatter in these correlations (believed to be only ~0.3 dex).