3 Data reduction

For the reduction of the spectra, we followed closely the method used in a series of papers on early-type galaxies (Simien & Prugniel 1997a-c, 1998: hereafter collectively referred to as SP), and described in full detail in the first of these papers. We thus limit ourselves, here, to a short abstract, with emphasis on a couple of differences with respect to early-type galaxies.

Standard pre-processing was applied to the raw data, up to the rebinning in wavelength. The galaxy centers (r=0) were first determined by a Gaussian fitting to a limited range ($\simeq 12''$) around the intensity peak. Compared to ellipticals, the centering was sometimes more difficult, because dust can cause strong asymmetries. In this case, and when the solution was obvious, we adopted the origin of the rotation symmetry, and we accordingly applied a correction to the r zero point. In the outer regions, cosmic-ray hits were removed with a median filter, and adjacent lines were combined with a variable weighting function (a Gaussian continuously wider faintward, up to 4 arcsec FWHM). A Fourier-Fitting technique determined the central velocity dispersion $\sigma_0$ and, when possible, the radial profile $\sigma(r)$ of the dispersion, together with the projected rotation curve V(r) along the major axis. A two-pass mode (described in SP) allowed to remove cosmic rays on the inner lines, where the spatial resolution must be preserved.

The frequent presence of emission lines in the spectra of spiral galaxies makes it necessary to "cut'' them at the level of the continuum; when the lines are strong, this noticeably reduces the wavelength range effectively used and, in some cases, this can contribute to lower the quality of the results.

The choice of late-type giants as template stars is particularly adapted to spectra of ellipticals and bulges; in deep spectra reaching the disk, we have not seen any evidence for a spectral mismatch: this indicates that the main disk contribution comes from the old disk.