next previous
Up: Star formation history of environments


3 Observations and data reduction

Long slit spectra have been obtained, during three different runs, at the 1.5 m ESO telescope (La Silla). The telescope was equipped with a Boller & Chivens spectrograph and a UV coated CCD Fa2048L $(2048\times 2048)$camera. Parameters of the observations and range of the variations of the seeing for each run are reported in Table 4. Table 5 details: the object identification (1), the observing run (2), the slit position angle (oriented NE) (3), the exposure time (4), the spectrum quality (5) and the galaxy portion enclosed in 5$^{\prime\prime}$ in terms of equivalent diameter $D^{\rm B}_{\rm e}$ (i.e. the diameter enclosing half of the total light) (6). The quality of the spectrum is defined as the average S/N-1) ratio in the wavelength range 4000 Å- 5000 Å. Higher S/N spectra are indicated with 3 (S/N > 20) while lower ones are indicated with 1 ($S/N \leq 10$). Galaxies observations were split into multiple exposures (typically three) in order to perform cosmic rays cleaning by a median average among the frames. The slit has been oriented along the major axis for almost all the objects. Some pairs have been observed along the line connecting the nuclei. He-Ar lamps were taken before and after each exposure to allow an accurate wavelength calibration.

Each frame has been treated separately. Pre-reduction, wavelength calibration and sky subtraction have been performed using the IRAF package. Relative flux calibration has been obtained with the use of a sequence of standard stars. Comparing calibrated stellar spectra common to different nights and different runs, we estimate that the flux uncertainty is about 1.0% for data relative to the run n.1, about 0.4% for those relative to the run n.2 and 3.5% for those relative to the last run.

Before being calibrated in flux, frames have been corrected for the atmospheric extinction (using the ESO La Silla coefficients) and for the Galactic extinction (using the values of E(B-V) from Burstein & Heiles 1982). Multiple spectra, relative to the same object, have then been co-added: their alignment was better than 0.5 pixel. From each flux calibrated spectrum, the portion corresponding to the central 5$^{\prime\prime}$ of the galaxy nucleus has been extracted and co-added producing a one dimensional spectrum. Objects spectra have been convolved with a gaussian curve ($\sigma$ = 3.37 Å) in order to degrade their resolution up 8.2 Å (FWHM), to match the Lick-IDS data resolution (see Sect. 4).


next previous
Up: Star formation history of environments

Copyright The European Southern Observatory (ESO)