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Astron. Astrophys. 351, 47-58 (1999)

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3. Observations and data reduction

The spectroscopic observations were carried out on the nights 1997 July 3-5 and 1998 July 25-27 using the EMMI multi-purpose instrument at the NTT (New Technology Telescope) in La Silla Observatory (ESO-Chile). The observations are described in detail in VMBF99.

The data reduction was done using standard methods in IRAF. The spectra were bias subtracted and divided by a normalized flat-field frame (dome flat-field). Illumination corrections along the slit were found to be negligible. The spectra were calibrated in wavelength using comparison spectra of HeAr. Cosmic rays were removed automatically (we obtained at least three similar frames for each object). Sky lines were subtracted and the spectra corrected for atmospheric extinction with the aid of mean extinction coefficients for La Silla. For each night we built a mean response curve from the standard stars observed that night with a wide slit (5 arc sec). Each object frame was flux calibrated with the corresponding response curve. The spectra were also corrected for galactic reddening. The reddening values were based on Burstein & Heiles (1984) maps, using empirical selective function of Cardelli et al. (1989). The redshifted CIII][FORMULA]1909 for SMM J02399-0136 is contaminated by the atmospheric absorption band at [FORMULA]7250 Å. We created the spectrum of the atmospheric band from the 1-D spectrum of a standard star taken with the same slit width as the one used for the radio galaxy (1.5 arc sec). We fitted the continuum and divided the original spectrum by the fit. By removing the absorption features intrinsic to the star, we obtained the spectrum of the atmospheric band. We divided the spectrum of SMM J02399-0136 by it trying different factors. Our conclusion is that the remaining effect of the atmospheric absorption is negligible after correction.

Both IRAF and STARLINK (DIPSO) routines were used to measure the emission line fluxes, FWHMs and wavelengths. 1-D spectra were extracted for all objects from apertures described below. We then fitted Gaussian profiles to the lines. The measured flux and FWHM of the spectral line are the values measured from the fit of the Gaussian. The instrumental profile was subtracted from the observed FWHM in quadrature.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999