1 Introduction

The fact that early-type galaxies are not necessarily oblate major-axis rotators has been well known since the end of the 1970s. The number of objects showing significant minor-axis rotation attained ten in 1988 (Wagner et al. 1988), and some of these objects later showed evidence for kinematically decoupled core structures (NGC 4365, NGC 4406: Bender 1988; NGC 4589: Moellenhoff & Bender 1989; NGC 5982: Wagner 1990). The most cited formation scenario for such structures is the hierarchical model, which involves single or multiple merging steps (see e.g.  Cole et al. 1994 and references therein). Observations of kinematically decoupled cores can therefore be used to constrain the merger tree of galaxies. However the total fraction of early-type galaxies containing such a core is poorly known (de Zeeuw & Franx 1991). Such cores are generally revealed using long-slit measurements with 1 $^{\prime\prime}$ FWHMor worse spatial resolution: this introduces an a priori assumption on the geometry of the central structures, and prevents the detection of cores with small apparent sizes. Finally, the rough characteristics of these cores (fraction in mass, relative age and metallicity) remain very uncertain, as very few studies exist and are often limited to morphological data (see Carollo et al. 1997 for 15 KDCs).

Recent studies (e.g. Verolme et al. 2002) show that state-of-the-art models combined with two-dimensional integral-field spectroscopic data are required to constrain precisely the global physical parameters of early-type galaxies, such as the inclination angle or the mass-to-light ratio, as well as the central characteristics for objects exhibiting features such as kinematically decoupled cores (e.g. the core mass of IC 1459, Cappellari et al.  2002). Moreover, integral-field spectroscopy also provides access to line-strength maps: the detailed kinematic information can thus be coupled to the line indices in order to more easily disentangle the core from the host galaxy (Davies et al. 2001).

The SAURON survey (Bacon et al. 2001a; de Zeeuw et al.  2002) was carried out in order to study the integral field kinematics of a sample of 72 early-type galaxies over a wide field of view (>33 $^{\prime\prime}$ $\times$ 41 $^{\prime\prime}$). In parallel, to obtain complementary high spatial resolution data, we designed a program to observe their central parts ($\sim$5 $^{\prime\prime}$ $\times$ 5 $^{\prime\prime}$) with the integral field spectrograph OASIS (Bacon et al. 2000). The aim is to probe the central spatial morphology and dynamics of a subset of galaxies in the SAURON sample, in order to link them with the global properties of the host galaxies provided by SAURON and other wide field studies. The additional use of PUEO (Rigaut et al. 1998), the Canada France Hawaii Telescope AO bonnette, with OASIS allows us to reach resolutions of about $0\hbox{$.\!\!^{\prime\prime}$ }25$ ($\sim$12 pc at 10 Mpc) in the red part of the visible when bright guiding sources are available.

NGC 4621, an S0 galaxy (Lauer et al. 1995), with M_V=9.6, located in the Virgo cluster (D= 18.3 Mpc, Tonry et al. 2001), is the first target of the subsample that was observed with OASIS/PUEO. Its steep power-law central luminosity profile ( $\gamma = 2.03$, Gebhardt et al. 1996) is ideally adapted to serve as a guiding source for the AO. NGC 4621 is close to edge-on, and its innermost $6\hbox{$^{\prime\prime}$ }\$isodensities reveal a nuclear disk (Sil'Chenko et al. 1997).

In this paper, we present the first results of our OASIS observations of NGC 4621, with the discovery of the smallest counter-rotating stellar core observed to date. The scale of the core is 60 pc, while the previously detected cores have an average radius of around 1 kpc (see the sample of Carollo et al. 1997). Detailed information on the different data sets used in this work, including HST/WFPC2 and STIS observations, are provided in Sect. 2. The corresponding measured photometry and stellar kinematics are presented in Sect. 3. We computed a two-integral distribution function (DF) model of the galaxy, which is presented in Sect. 4. The discussion is carried out in Sect. 5.