The counterrotating core and the black hole mass discrepancy of IC1459

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  • 7 February 2002
Abstract
The E3 giant elliptical galaxy IC1459 is the prototypical galaxy with a fast counterrotating stellar core. We have obtained one HST/STIS long-slit spectrum along the major axis of IC1459, and CTIO spectra along five position angles. We present fully general self-consistent three-integral axisymmetric models of the stellar kinematics, obtained with Schwarzschild's orbit superposition method. We study the internal dynamics of IC1459, and constrain the formation process of the observed counterrotating core. In particular our study indicates that the stars in this component counterrotate in a disk along orbits that are close to circular. We are able to estimate the mass involved to be ~0.5% of the total galaxy mass or ~3*10^9 Msun. This value is close to the mass of the Black Hole (BH) M_BH=(2.6+/-1.1)*10^9 Msun that we measure in the nucleus of this object. These results are consistent with the idea that the stellar disk formed from an externally acquired gaseous component, which settled in a thin disk and subsequently turned into stars. The BH mass estimate we obtain from the analysis of the stellar kinematics of this galaxy is inconsistent with previous measurements obtained from the FOS gas kinematics M_BH=2--6*10^8 Msun (Verdoes Kleijn et al. 2000) and with our own STIS gas kinematical modeling. While there exist plausible explanations, it is disconcerting that the BH masses differ so considerably (~0.8 dex). It will be important to perform similar comparisons for other galaxies, to determine 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 galaxy global parameters (e.g., velocity dispersion), and in particular its scatter (believed to be only ~0.3 dex). [Abridged]

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