Astron. Astrophys. 351, 477-486 (1999)

3. Measurements

Our main goal is to study the stellar population of BCGs. To do this, we have used the synthesis method described in Schmitt, Bica & Pastoriza (1996). The method minimizes the differences between the observed and synthetic equivalent widths for a set of spectral features. To measure the equivalent widths accurately, it is extremely important first to have a good fit to the continuum. Hence the analysis of each of the sample spectra proceeds in two steps (1) determining a pseudo-continuum at selected pivot-points, and (2) measuring the equivalent widths (EWs) for a set of selected spectral lines.

The continuum and EW measurements followed the method outlined in Bica & Alloin (1986), Bica (1988) and Bica et al. (1994), Cid Fernandes et al. (1998), and subsequently used in several studies of both normal and emission line galaxies (e.g., Jablonka et al. 1990, Storchi-Bergmann et al. 1995, McQuade et al. 1995, Bonatto et al. 1998). This method first determines a pseudo-continuum at a few pivot-wavelengths, and then integrates the flux difference with respect to this continuum in the defined wavelength windows (Table 3) to determine the EWs. The pivot wavelengths used in this work are based on the same as those used by the above authors; these were chosen to avoid regions of strong emission or absorption features (Table 2). Four point flux values (3784, 3814, 3866, 3918 Å) were used for the Balmer discontinuity. The use of a compatible set of pivot points and wavelength windows is important, since it allows a detailed quantitative analysis of the stellar population by the synthesis techniques using the spectral library of star clusters (Bica & Alloin 1986).

Table 2. Continuum points relative to (5870 Å), corrected for the foreground reddening

Table 3. Measured equivalent widths in Å for blue compact galaxies.

The determination of the continuum was done interactively, taking into account the flux level, noise and small uncertainties in wavelength calibration, as well as the presence of emission lines. The 5870 Å point, in particular, is sometimes buried underneath the HeI 5876 Å emission line. In such cases, adjacent wavelength regions guided the placement of the continuum. The final measured fluxes, normalized to the flux at 5870 Å, are given in Table 2. After fitting the continuum points to a continuum spectrum, we measured the equivalent widths of seven characteristic absorption lines. When a noise spike was present in the wavelength window, it was necessary to make `cosmetic' corrections. The results are given in Table 3. The first two rows give the names of the spectral lines and corresponding wavelength range. The EWs are in units of Å, and negative sign denotes emission lines.

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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