2 Spectral observations and data reduction

All results presented below were obtained by observations with the Russian 6m telescope, mainly in the snap-shot mode during 16 runs between December 1995 and June 1998. The spectrograph SP-124 attached to the Nasmyth-1 focus of the telescope and equipped with a new Photometrix CCD-detector PM1024 (with $24~\times~24~\mu$m pixel size) was used in most of the runs. We used the gratings either with 300 grooves/mm or with 600 grooves/mm (see journal of observations in Table 1). Due to an experimental set-up of the CCD detector, it was not properly optimized, and significant vignetting prevented us in several runs in 1995 and early 1996 to get spectra in the UV ($\lambda \leq 4000$ Å). Various spectral set-ups were used with dispersions from 2.4 to 5.5 Å/pixel. The long slit with a length of 40$\hbox{$^{\prime\prime}$}$ was used. The slit width was 2$\hbox{$^{\prime\prime}$}$ in almost all observations. The scale along the slit was 0.4$\hbox{$^{\prime\prime}$}$ or 0.5$\hbox{$^{\prime\prime}$}$/pixel. Normally, short exposures were used (2 - 5 min) in order to detect strong emission lines, to measure redshifts and make some crude classification. Several spectra were taken with the Long Slit spectrograph (LSS in Table 1) (Afanasiev et al. 1995) at the 6m telescope prime focus equipped with a CCD-detector on base of an ISD015A chip ($530\times580$ pixels, with rectangular pixel size $18\times24~\mu$m), produced at SAO RAS. Long slit spectra ($2\hbox{$^{\prime\prime}$}\times180\hbox{$^{\prime\prime}$}$) were obtained with the grating having 650 grooves/mm and a dispersion 3.1 Å/pixel, and a scale along the slit of 0.41$\hbox{$^{\prime\prime}$}$/pixel. Reference spectra of an Ar-Ne-He lamp were recorded before or after each observation to provide a wavelength calibration. The spectrophotometric standard stars from Massey et al. (1988) were observed for flux calibration at least twice a night.

  

Table 1:   Journal of observations at the SAO 6m telescope

All observations and data acquisition with the spectrograph SP-124 have been conducted under software package NICE in MIDAS, described by Kniazev & Shergin (1995).

2.1 Data reduction

The data reduction was done at SAO with the MIDAS software package. The context LONG was adapted to the SAO data formats to perform an automatic reduction. The reduction of the originally two-dimensional CCD data included standard steps such as: bias and dark subtraction, flat-fielding, cosmic-ray removal. After wavelength mapping the subsequent night sky background subtraction was performed. 1-D spectra were extracted by adding 6-10 (depending on seeing and galaxy size) consecutive CCD rows centered on the object intensity peak along the slit. Then the corrections for atmospheric extinction and flux calibration were applied. For the flux calibration we used the mean response curve obtained from the observations of standard stars, mainly Feige 34 and HZ 44.

In the final spectra showing emission lines, redshifts and line fluxes are measured applying Gaussian fitting. To determine redshifts for individual galaxies averages are taken over prominent individual emission lines (mostly H$\beta$, H$\alpha$, [OIII]$\lambda$4959, 5007 Å). The line [OII]$\lambda$3727 Å is not used for redshift determination since for most of the objects its observed wavelength is determined with significantly larger uncertainties due to the extrapolation of the linear scale below the first line of HeI$\lambda$3889 Å in the reference spectrum.

The emission line fluxes are computed by summing up the pixel intensities inside the line region by using standard MIDAS program tools. For all spectra, the individual emission line fluxes of the H$\alpha$,[NII]$\lambda\lambda$6548, 6583 Å and [SII]$\lambda\lambda$6716, 6731 Å line blends are obtained by summing up of pixel intensities over the total blend and then modelling the individual line fluxes using Gaussian fitting.