Astron. Astrophys. 325, 954-960 (1997)

3. The spectral properties of the sample

The ESP data provides a large data-base of galaxy spectra which, when fully analyzed, will constitute a reference for the next generation of larger and deeper redshift surveys that will probe galaxy evolution (). These data are well suited for the study of the intrinsic properties of galaxies in terms of their stellar populations, and their relation with the environment and redshift (cosmic time). This will be presented in a future paper.

Emission lines are present in a large fraction () of the galaxies in our sample. We mainly detect [OII] , , [OIII] and . Our preliminary analysis focused on the [OII] doublet, since this line is the most useful star formation tracer (Kennicutt 1992) in the wavelength range covered by our spectra. Galaxies showing the [OII] line correspond, at increasing equivalent width, to three main categories: spiral galaxies, where the line originates mostly from HII regions in the disks, galaxies undergoing a significant burst of star formation, or AGNs.

Great care is required when dealing with statistical analysis of emission lines properties of galaxies, because the detectability of lines in a spectrum strongly depends on the signal-to-noise ratio of the adjacent continuum. The ratio in the blue part of our spectra ranges from 2 to 10, with an average value of the order of 4. With such a value the [OII] line can be detected only if its equivalent width is larger than about 5 Å. For the spectra of poorer quality, however, the minimum detectable [OII] equivalent width is of the order of 20 Å (see, for example, Fig. 7 in Vettolani et al. 1994). For this reason, in order to properly analyze the distribution of the equivalent widths below 20 Å and to study the possible correlations of the equivalent width with other intrinsic properties, one should also take into account the information carried by the upper limits. In this paper, in order to avoid dealing with upper limits, we present a preliminary analysis of the galaxies with an [OII] equivalent width greater than 20 Å.

Up to now the measurement of the equivalent width of the [OII] line, or of an upper limit for the galaxies in which the line is not detected, has been done for a subsample of 2550 galaxies, corresponding to 76% of the total sample. From this subsample we find that about 13% of the galaxies show an [OII] equivalent width greater than 20 Å and can therefore be classified as actively star-forming galaxies. This fraction, however, is a strong function of absolute magnitude, ranging from a few percent for the brightest galaxies up to about 40% for the galaxies fainter than   (see Fig. 3). This result is qualitatively similar to what has been found in the AUTOFIB Redshift Survey (see Fig. 8 in Ellis et al. 1996). Obviously, this correlation has to be taken into account in the determination of the evolution of the star formation rate as estimated by comparing bright and local samples of galaxies with fainter and more distant ones.

Fig. 3. Percentage of galaxies showing [OII] with an emitted equivalent width greater than 20 Å as a function of absolute magnitude.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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