5 Discussion and conclusion

In this paper, we report the discovery of a $\sim$60 pc diameter counter-rotating core using new OASIS integral field spectroscopy, a result confirmed with archival STIS long-slit data. A structure at this scale has never been observed before, except in M 31, which exhibits a 20 pc co-rotating decoupled core (Bacon et al. 2001b and references therein). Typical sizes of kinematically decoupled cores are at the order of 1 kpc (See e.g. the sample of Carollo et al. 1997). The computed mass fraction of 0.12% ( $2.1 \times 10^{8}\ M_{\odot}$) is on the low side, as compared for instance to the CRC of IC 1459, which is 0.5% for a core size of 3 kpc (Cappellari et al. 2002), but for which the estimation is of higher accuracy. As a comparison, the black hole mass from the $\sigma-M_\bullet$ relation for IC 1459 is $1.1 \times 10^{9}\ M_{\odot}$ (Merritt & Ferrarese 2001b) and $2.13 \times 10^{8}\ M_{\odot}$ for NGC 4621 (Merritt & Ferrarese 2001a). The mass fraction estimate provided here in the case of NGC 4621 can only be indicative: the DF separation is inaccurate and difficult, and to get a precise result we would need an analysis in integral space.

Both individual OASIS and STIS data sets suggest that the CRC of NGC 4621 is slightly off-centered with respect to the center of the outer isophotes. This offset seems consistent with the weak asymmetry detected in the WFPC2/HST V and I frames and the V-I colour map (Fig. 3). The central structure elongated along the minor axis detected in the V-I colour map represents an increase of V-I=0.03 which could either be due to an intrinsic stellar population gradient, or to dust. In the latter case, this would correspond to A_V=0.06 (assuming R_V=3.1 and R_I=1.86 from models of the Galaxy, Rieke & Lebofsky 1985). Note that we do not detect any high frequency structure in the individual WFPC2 V and I frames.

The two-integral models reasonably reproduce the observed velocity profiles in the outer parts, as well as in the region of the CRC (OASIS and STIS). These axisymmetric models do obviously not take into account the observed off-centering. The predicted central value of the velocity dispersion is significantly too low by 50 km s^-1. We were also unable to simultaneously fit higher order Gauss-Hermite moments: e.g. the predicted h₃ is systematically too high (see Fig. 7). This demonstrates that we need more general dynamical models in order to properly fit the observed kinematics. We are therefore in the process of constructing three-integral Schwarzschild models, with the possibility to include a central dark mass. This may solve the discrepancies mentioned above.

If one assumes a merger-scenario for the CRC's origin, the off-centering, if confirmed, could be either due to the fact that the merging process is still on-going, or may be the result of a stable mode (see e.g. M 31 in Bacon et al.  2001b). The short dynamical timescale at the radius of the CRC (1 Myr) seems to favour the latter hypothesis. Further discussions regarding this issue must however wait for additional spectroscopy. A detailed study at high spatial resolution of the stellar populations in the central arcsecond would certainly help in this context. Existing high resolution STIS spectra at bluer wavelengths can be used for this purpose.

Stellar kinematics at HST resolution is available for only a handful of early-type galaxies. M 31 is one of the rare examples where it is possible to have sufficient spatial resolution to measure kinematical features at parsec scales, such as its 20 pc nucleus (Bacon et al. 2001b and references therein). The OASIS data presented in this paper demonstrate that it is possible to study substructures with a characteristic size of 60 pc in galaxies at the distance of the Virgo cluster. The presence of the CRC in NGC 4621 raises new questions about the dynamical status of the centers of early-type galaxies. What is the fraction of early-type galaxies that have such substructures? Two-dimensional spectroscopy at high spatial resolution is clearly needed to simultaneously study the dynamical and chemical contents of these cores, and to eventually understand their origin.

Acknowledgements

We would like to thank Raymond Michard and Ralf Bender for the data they provided, Tim de Zeeuw for a careful reading of the manuscript and his suggestions, and the referee Dr M. Carollo for her thorough and constructive report.