2 Observational data sets

2.1 OASIS data

2.1.1 Observations

Ca-triplet OASIS exposures of NGC 4621 were obtained in January 2000 using the medium spectral configuration (MR3: 8346-9152 Å) at the f/20 ( PUEO) focus of the CFHT, and a spatial sampling of 0 $.\!\!^{\prime\prime}$16. Eight out of ten 30 min exposures were fully reduced using the dedicated XOasis package, and merged into a single frame (two frames were discarded due to bad seeing conditions and associated loss of guiding). The resulting 3D dataset probes about $4\hbox{$^{\prime\prime}$ }\times4\hbox{$^{\prime\prime}$ }$ (300 $\times$ 300 pc²), with a resolution of $\sigma=70$ km s^-1. We modeled the PSF of the merged datacube using the sum of two concentric Gaussians: their parameters were retrieved (via a least-square procedure) by comparison with the WFPC2/HST F814W frame as in Bacon et al. (2001b). The resulting FWHM of the merged datacube is 0 $.\!\!^{\prime\prime}$51.

The total S/N (summed over the bandwidth) drops very quickly towards the outer parts, as it is 65 at the center, 20 at 1 $^{\prime\prime}$ and 10 at 2 $^{\prime\prime}$ on the major-axis. We thus convolved the datacube with a Gaussian of variable width in order to obtain a higher S/N over the field (see Fig. 1).

2.1.2 Kinematic measurements

We used a tuned version of the Fourier Correlation Quotient (FCQ; Bender 1990) method to derive the line of sight velocity distributions (LOSVD) at each measured spatial position. We used a single template star (G8III: HD073665), but tested the stability of the kinematical mesurements with different templates, and different continuum subtraction parameters, only resulting in minor differences. The LOSVDs were then fitted using a single Gaussian function, yielding the mean velocity and dispersion maps. We could not obtain reasonable maps of higher order moments, due to the rather low signal to noise ratio (even after the convolution performed on the datacube). The error bars on the velocity and dispersion measurements were computed via a Monte-Carlo algorithm using the measured S/N (see Copin 2000, Ph.D. Thesis).

2.2 Ancillary data

2.2.1 Photometry

In order to obtain an accurate three-dimensional mass model of the galaxy we need both wide-field and high-resolution photometry to probe the visible mass up to large radii and to correctly sample the central structure (the cusp). We used a 20$^\prime$ V band image of the galaxy taken at the OHP 2 m Telescope, kindly provided by R. Michard (Idiart et al. 2002). The central regions were examined with the help of WFPC2/HST data retrieved from the archive (F555W and F814W filters, Faber et al. 1997, #5512). For both bands, three unsaturated frames were merged and cosmic ray-corrected (330 s and 230 s exposure time for F555W and F814W filters respectively). We performed photometric calibration using the VEGAMAG standard, and used the PSFs computed using tinytim. We adjusted the levels of the OHP image to the F555W frame taking into account the different spectral domains and the PSF.

2.2.2 Long-slit kinematics

We used long slit data from Bender et al. (1994, hereafter BSG94; data kindly provided by R. Bender) in order to complement the OASIS data up to large radii: major and minor axis velocity and dispersion measurements are available inside 40 $^{\prime\prime}$ (3.5 kpc), with a seeing of $\sim$1 $.\!\!^{\prime\prime}$8 FWHM (other kinematical - and line-strength - measurements were published by Sil'chenko 1997, but with a lower spatial resolution).

We finally reduced unpublished HST/STIS major-axis data (Green, ID #8018; G750M grating, Ca-Triplet) in order to examine the central kinematics of NGC 4621 at high spatial resolution. Two exposures with a total of 72 min were available. The calibrated data were retrieved via the STScI data archival system (calstis pipeline). Appropriate flat field exposures were retrieved to correct for the fringing ( mkfringeflat/defringe IRAF routines, Goudfrooij & Christensen 1998), critical at these wavelengths, as well as five K0-III kinematical template star exposures (HR7615, Green, #7566). Further rejection of cosmic rays was then performed on the NGC 4621 individual exposures, before recentring and merging. The stellar kinematics were extracted using the same FCQ and fitting methods as for the OASIS data. As for the OASIS datacubes, the S/N of the STIS data was found sufficient only to derive the mean stellar velocity and dispersion within the central arcsecond.

Figure 1: OASIS stellar kinematic data of NGC 4621. Top left: mean velocity field. The zero-velocity curve has been emphasized with a white thick line. Top right: velocity dispersion. White contours are reconstructed isophotes. The data have been smoothed to improve the signal to noise: the black circles represent the beam sizes (FWHM). Below the cuts along the major and the minor axes are shown along with the best fitting two-integral model (dashed lines, see Sect. 4).

Figure 2: STIS mean velocity (left) and dispersion (right) profiles along the major axis. The dashed line corresponds to the best fitting two-integral model (see Sect. 4). The velocity dispersion is clearly underestimated by the present model, which hints for the need of an additional central dark mass, a more general treatment (three-integral model) or both.