3 Results

To avoid any confusion due to the complexity in both the photometry and the kinematics of NGC 4621, we will use the following convention: the photometric major axis of the galaxy (as measured by HST) defines the x-axis, the negative values being in the SE quadrant, referred to as the left side of the galaxy. The center (0,0) is defined as the center of the HST isophotes of NGC 4621 within 1 $^{\prime\prime}$, excluding the central 0 $.\!\!^{\prime\prime}$1, the isophotes of which exhibit a significant asymmetry (see next section). The centering and rotation procedures of the HST frames have been performed using an algorithm which minimizes the standard deviation of the recentered and rotated frame subtracted by its flipped counterpart. This leads to accuracies of 0.1 degrees and 0 $.\!\!^{\prime\prime}$005. All figures in this paper share a common orientation, as shown in Fig. 3.

   
3.1 Stellar velocity and velocity dispersion

The OASIS stellar mean velocity and dispersion maps are presented in Fig. 1. The datacubes have been centered and rotated in order to match the HST data, using the center and angle provided by the PSF fitting procedure mentioned in Sect. 2. The velocity field reveals a clear counter-rotating core (CRC). The position of the zero-velocity curve is used to measure the center and the size of the CRC: it is 1 $.\!\!^{\prime\prime}$7 (150 pc) in diameter, and off-centered by $\sim$ $0\hbox{$.\!\!^{\prime\prime}$ }2$ towards the SE. The total velocity amplitude observed within the central 1 $^{\prime\prime}$ along the major axis is 100 km s^-1, while the peak-to-peak velocity amplitude of the CRC's is only 35 km s^-1. The stellar velocity dispersion map peaks at $\sim$ $330 \pm 25$ km s^-1. The dispersion map however exhibits high frequency substructures in the central 0 $.\!\!^{\prime\prime}$5 (Fig. 1), the minor-axis dispersion profile even having a local minimum at the centre. Considering the amplitude of these structures and their spatial scales (comparable to the local FWHM), we should wait for data with better signal to noise ratio to discuss these features.

The BSG94 data show no significant minor-axis rotation, and a maximum velocity of 140 km s^-1 at 30 $^{\prime\prime}$ (see BSG94 or Fig. 7). Besides a relatively weak velocity amplitude in the innermost 4 $^{\prime\prime}$ (<60 km s^-1), these data do not show any hint for the existence of the counter-rotating core. We will use the BSG94 kinematics to constrain the dynamical model outside the OASIS field of view.

The STIS velocities and dispersions are shown in Fig. 2. In these data the size of the CRC is 0 $.\!\!^{\prime\prime}$7 ($\sim$60 pc) and its velocity center is located at $x=-0\hbox{$.\!\!^{\prime\prime}$ }05$ ($\sim$4.5 pc). We have carefully checked the centering of the STIS data with respect to the reference WFPC2 images, and found that this offset is robust. At STIS resolution the major-axis velocities of the CRC reach $\sim$75 km s^-1with respect to the systemic. The maximum velocity dispersion is $320\pm27$ km s^-1, at $x = 0\hbox{$.\!\!^{\prime\prime}$ }05$. These values are consistent with the OASIS data considering the higher spatial resolution of the STIS data.

3.2 HST photometry

Both F555W and F814W images reveal a peculiar structure in the core. Indeed, the luminosity peak is offset from the center of the outer isophotes by 0 $.\!\!^{\prime\prime}$01 towards the east. The photometric peak is located in the upper left quadrant (Fig. 3). This feature does not seem to be an artefact, as it is clearly visible on each individual HST frame, and in both bands, and cannot be attributed to centering uncertainties (0 $.\!\!^{\prime\prime}$005).

The two HST frames were then PSF-crossconvolved and divided to obtain a V - I colour image (Fig. 4). It reveals a central gradient (V-Iincreases towards the center) with a y-axis elongated structure centered at (-0 $.\!\!^{\prime\prime}$02, 0 $.\!\!^{\prime\prime}$01) (see dashed line on Fig. 3). The isochromes have slightly higher ellipticities than the isophotes ( $\varepsilon=0.5$ at 2 $.\!\!^{\prime\prime}$5 for V-I, vs. 0.4 for the V frame).