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2 Observations

Eight galaxies were selected (see Sect. 1) for observations on the three dark nights allocated at the ESO 3.6 m telescope on La Silla, from August 30 to September 2 1995. Observing conditions were photometric all nights. The seeing (measured at the telescope) was slightly above one arcsecond. The exposure times ranged between 24 minutes (1 minute per channel) and 160 minutes (almost 7 minutes per channel), plus calibration exposures. The used instrument was CIGALE, attached to the Cassegrain focus of the telescope. CIGALE is basically composed of a focal reducer (bringing the original f/8 focal ratio of the Cassegrain focus to f/3), a scanning Fabry-Perot interferometer, an interference filter (to isolate the emission line and suppress the sky brightness) and an IPCS detector (2-D photon-counting system). The basic principles of this instrument were described in Amram et al. (1991). The pixel size, projected on the sky, is 0.91$\hbox{$^{\prime\prime}$}$ with a resulting field of view of $\sim\!\!4\hbox{$^\prime$}$. The IPCS, with a time resolution of 1/50 seconds and zero readout noise makes it possible to scan the interferometer rapidly, avoiding problems with varying sky transparency, airmass and seeing during long exposures; and thus has several advantages over a CCD for this application. Narrow band interference filters were used to isolate the region around the redshifted H$\alpha$ line. Different interference filters were used for observing galaxies with different radial velocities, see Table 2. Only in two cases (ES0 350-IG38 and Tololo 0341-407) the [N II]${\lambda 6548~{\mbox \AA}}$ line might be partly transmitted. This line is however observed to be very weak as compared to H$\alpha$ in the two galaxies (Bergvall & Östlin 1999; Terlevich et al. 1991) so there is no risk that this affect our results.

In order to save observing time and to increase the the spectral resolution, with respect to the small velocity range of the observed galaxies, we decided to use a high order Fabry-Perot interferometer (hereafter simply referred as to FP2 for Fabry-Perot interferometer number 2, see Table 1) giving at H$\alpha_0$ a mean finesse of 10, a free spectral range (FSR) of 117 km s-1, and a scanning step of 4.8 km s-1. The FSR is the separation between two consecutive interference orders and is therefore the maximum wavelength (velocity) range that can be observed before wavelength overlap occurs. The finesse is effectively the ratio between the FSR and the width of the Airy function. The Airy function (or apparatus function) is the instrumental line profile. Thus by scanning the FSR in a number of steps that equals at least twice the finesse, adequate spectral sampling is obtained. The spectral resolution, R, given in Table 1 is calculated from the width of the apparatus function, but for high signal to noise (S/N) the effective R will be higher.

One galaxy was also observed using a lower order Fabry-Perot interferometer (hereafter simply referred to as FP1, see Table 1), which gave us the possibility to check the consistency of our results. Calibrations were obtained by scanning a narrow neon line before and after the observations of each galaxy. Table 1 lists some characteristics of the set-up used for the observations. Table 2 gives the target names and some parameters for the observed galaxies.


  
Table 1: Journal of observations

\begin{tabular}
{lll}
\hline
Observations & Telescope & ESO 3.6~m\\ & Location &...
 ...ctral resolution & $R \ge 26000$\space ~ (12 km~s$^{-1}$) \\ \hline\end{tabular}


  
Table 2: List of observed galaxies. R.A. and Decl. give the right ascension (h m s) and declination ($\hbox{$^\circ$}~ \hbox{$^\prime$}~ \hbox{$^{\prime\prime}$}$) of the targets for epoch 1950. mV is the apparent magnitude in V. $v_{\rm hel,obs}$ is the velocity used for selecting narrow band filter for the observations. "Filter" is the central wavelengths of the used interference filters as measured in laboratory and corrected to the observing conditions: a temperature of $\approx 5^\circ$C and a focal ratio of f/8. FWHM is the full width half maximum of the used narrow band filter. The transmission curve for the filters have a quite squared shape, thus the full width at (practically) zero transmission is not much larger than the FWHM. Unless noted otherwise, all observations were made with FP2

\begin{tabular}
{llllllllll}
\hline
Target Name & Other Name & R.A. & Decl. & $m...
 ...45 29& 15.7 &
4500& ~1 Sept. 1995 & 0.40 &6653/20.5& 7, 8 \\ \hline\end{tabular}
\begin{tabular}
{l}
Notes/references:\\ 1. Position and velocity reference 
\cit...
 ...n and velocity reference 
\cite[Bergvall et~al. (1998)]{ber:jo}.\\ \end{tabular}


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