next previous
Up: Identification of active galaxies galaxies


3 Optical spectroscopy

The spectroscopic observations aimed at confirming the nature of the selected candidates were carried out at the F/15 Cassegrain focus of the 1.93 m telescope at the Observatoire de Haute-Provence (CNRS, France) during the nights of April 8-15, 1997 and April 20-21, 27-29, 1998. The long-slit spectrograph CARELEC [(Le-maitre etal. 1990)] was operated with a 150 lines mm-1 grating leading to a dispersion of 26 nm mm-1 in the wavelength range $\lambda\lambda 380.0-725.0$ nm. The detector was a $512 \times 512$ Tektronix CCD with 27 $\mu$m pixels and a rms read-out noise of 9.2 electrons. Throughout the observing runs the atmospheric seeing usually ranged from $1\hbox{$.\!\!^{\prime\prime}$}5$ to $2\hbox{$.\!\!^{\prime\prime}$}0$. Most of the observations were made with a $2\hbox{$.\!\!^{\prime\prime}$}5$ slit width, which corresponds to a projected slit width on the detector of 62 $\mu$m or 2.3 pixels. With this set-up, the overall instrumental resolution FWHM was 1.4 nm as measured on the He arc lines used for the wavelength calibration. An exposure time of 3600 s was enough to yield a signal-to-noise ratio larger than 10 in the spectra of the faintest candidates ($B\approx 18$).

The sub-dwarf O star HZ 44 [(Oke 1990)] was used as a spectro-photometric standard during both observing runs. Depending on the atmospheric stability of the night, the absolute flux calibration is uncertain at the 20 to 50% level. The data were reduced and analyzed independently by two of us, with the LONG-SLIT package implemented within MIDAS (CL), and with IRAF (DVG). After flat-field correction and wavelength calibration, the object signal was extracted using the Horne algorithm ([Horne 1986]). It ensures an optimal signal-to-noise ratio for weak spectra while it possibly slightly underestimates the emission-line strength in extended sources.


next previous
Up: Identification of active galaxies galaxies

Copyright The European Southern Observatory (ESO)