2 Observations and data reduction

2.1 Narrow and broad-band imaging

Two narrow-band images of NGC 6221 were extracted from the ESO NTT archive. They were taken on the night of May 6, 1994 with the NTT Telescope at La Silla. The No. 25 Tektronix TK1024M CCD with $1024\times1024$ pixels was used as detector in combination with SUSI. Each $\rm 24\,\mu m \times 24\, \mu m$ image pixel corresponds to 0.13$''\times0.13''$. The CCD gain and the readout noise were determined to be 3.4 e^- per ADU and 5.9 e^-, respectively.

The emission-band image was obtained with the interference ESO filter No. 692 isolating the spectral region containing the redshifted H$\alpha$ and [$\rm N \;{\scriptsize II}$] ($\lambda$ 6583.4 Å) emission lines. The continuum-band image was taken with the interference ESO filter No. 696 isolating an emission-free spectral region. The exposure time was 20 minutes for each filter. The seeing was about 1.7''

Standard reduction of the two images was performed using the original available data. The two images were aligned using field stars as reference (with an accuracy better than 0.2 pixel). In each image the sky level was estimated in areas of the frames unaffected by the galaxy light and then subtracted. The continuum-band image was subtracted from the emission-band image after an appropriate intensity scaling, allowing for transmission differences between the filters. No attempt was made to calibrate in flux the resulting pure-emission image of NGC 6221.

Two broad-band images of NGC 6221 in the I and the J-band respectively were extracted from the DENIS data archive (Epchtein et al. 1997). Their reduction was routine.

The reduction of the broad and narrow-band images and the surface photometry of the NTT continuum and DENIS I and J-band frames was performed by means of standard procedures within the ESO-MIDAS software package.

2.2 Long-slit spectroscopy

The spectroscopic observations of NGC 6221 were carried out at the ESO 1.52 m Spectroscopic Telescope at La Silla using the Cassegrain Boller & Chivens Spectrograph in the nights of April 30 and May 2, 1992. The No. 26 1200 $\rm grooves\;mm^{-1}$ grating blazed at 5730 Å, was used in the first order in combination with a 2.5$''\times2.1'$ slit. The No. 24 FA2048L CCD with 2048$\times$2048 pixels was adopted as detector. It yielded a wavelength coverage of $\sim2000$ Å between about 4900 and about 6900 Å with a reciprocal dispersion of 65.1 $\rm \AA\;mm^{-1}$. The instrumental resolution was determined measuring the FWHM of the emission lines of the comparison spectra. We checked that such measured FWHM's do not depend on wavelength and we found a mean value of FWHM = 2.35 Å (i.e. $\sigma = 1.0$ Å which corresponds in the range of the observed emission lines to $\sim\! 50$ $\rm km\;s^{-1}$). Each $\rm 15\,\mu m \times 15\, \mu m$spectrum pixel corresponds to 0.98 Å $\times 0.81''$.

The long-slit spectra of NGC 6221 were obtained at different position angles in order to map the velocity field. The observing log is given in Table 2. The seeing value was about 1.2''. Each object spectrum was bracketed by two helium-argon calibration spectra. The stars HR 3431 (K4 III), HR 5601 (K0.5 III), HR 6318 (K4 III) and HR 7597 (K0 III) were observed as velocity templates. The standard spectral reduction was performed by using the ESO-MIDAS package. All the spectra were bias subtracted, flat-field corrected by quartz lamp exposures and wavelength calibrated by fitting the position of the comparison lines with cubic polynomials. Pixels affected by cosmic ray events were identified and then corrected. The contribution of night sky was determined from the edge regions (not contaminated by galaxy light) of each spectrum and then subtracted.

 

Table 2: Observing log for NGC 6221

The stellar velocities and velocity dispersions were measured from the absorption lines in the wavelength range between about 4900 Å and 6200 Å using the Fourier Correlation Quotient technique (Bender 1990) as applied by Bender et al. (1994). The measured radial velocities were corrected to the heliocentric frame of reference. The heliocentric correction was $\Delta v=+15.6$ $\rm km\;s^{-1}$ and +14.9 $\rm km\;s^{-1}$ for April 30 and May 2 respectively.

The stellar kinematics measured along the different position angles is given in Table 3. The table provides the position angle (Col. 1), the radial distance from the galaxy centre in arcsec (Col. 2), the observed heliocentric velocity with its respective error (Cols. 3 and 4), and the velocity dispersion with errors (Cols. 5 and 6) in $\rm km\;s^{-1}$, and the Gauss-Hermite coefficients with errors h₃ (Cols. 7 and 8) and h₄ (Cols. 9 and 10).

The gas velocities and velocity dispersions were measured by means of the MIDAS package ALICE from the H$\alpha$ and [$\rm N \;{\scriptsize II}$] ($\lambda$ 6583.4 Å) emission lines. The position, the FWHM and the uncalibrated flux of each emission line were individually determined by interactively fitting a Gaussian plus a second order polynomial to the emission and its surrounding continuum. The wavelength of the Gaussian centre was converted to the velocity cz, and then the heliocentric correction was applied. The velocity errors were derived as in Bertola et al. (1996). The Gaussian FWHM was corrected for the instrumental FWHM and then converted to the velocity dispersion $c\sigma/\lambda_0$. The velocities and the velocity dispersions derived from the two emission lines agree within 5%. At each radius the weighted mean of the velocities and the average of the velocity dispersions were taken as the velocity and velocity dispersion of the NGC 6221 ionized gas respectively.

The ionized gas kinematics measured along the different position angles is given in the five Tables 4-8. Each table provides the radial distance from the galaxy centre in arcsec (Col. 1), the H$\alpha$ observed heliocentric velocity with its respective error (Cols. 2 and 3) and velocity dispersion (Col. 4) in $\rm km\;s^{-1}$, the [$\rm N \;{\scriptsize II}$] observed heliocentric velocity with error (Cols. 5 and 6) and velocity dispersion (Col. 7) in $\rm km\;s^{-1}$, the [$\rm N \;{\scriptsize II}$]/H$\alpha$ flux ratio (Col. 8), and the averaged ionized gas heliocentric velocity with error (Cols. 9 and 10) and velocity dispersion with error (Cols. 11 and 12) in $\rm km\;s^{-1}$.