Spectrophotometric properties of galaxies at intermediate redshifts (z ∼ 0.2-1.0) -: I.: Sample description, photometric properties and spectral measurements

We present the spectrophotometric properties of a sample of 141 emission-line galaxies at redshifts in the range 0.2 < z < 1.0 with a peak around z is an element of [0.2, 0.4]. The analysis is based on medium resolution (R-s = 500- 600), optical spectra obtained at VLT and Keck. The targets are mostly Canada-France Redshift Survey emission-line galaxies, with the addition of field galaxies randomly selected behind lensing clusters. We complement this sample with galaxy spectra from the Gemini Deep Deep Survey public data release. We have computed absolute magnitudes of the galaxies and measured the line fluxes and equivalent widths of the main emission/ absorption lines. The last two have been measured after careful subtraction of the fitted stellar continuum using the platefit software originally developed for the SDSS and adapted to our data. We present a careful comparison of this software with the results of manual measurements. The pipeline has also been tested on lower resolution spectra, typical of the VIMOS/VLT Deep Survey (R-s = 250), by resampling our medium resolution spectra. We show that we can successfully deblend the most important strong emission lines. These data are primarily used to perform a spectral classification of the galaxies in order to distinguish star-forming galaxies from AGNs. Among the initial sample of 141 emission-line galaxies, we find 7 Seyfert 2 (narrow-line AGN), 115 star-forming galaxies and 16 candidate star-forming galaxies. Scientific analysis of these data, in terms of chemical abundances, stellar populations, etc., will be presented in subsequent papers of this serie.