Blue compact galaxies (BCGs) are characterized by their very blue color, compact appearance, high gas content, strong nebular emission lines, and low chemical abundances (Kunth & Östlin 2000; Östlin et al. 2001). These properties are typical of unevolved systems, thus suggesting that BCGs should have suffered very few bursts of star formation during their lives and that some of them are probably experiencing their first burst. In a recent review, Kunth & Östlin (2000) argued that, despite a few remaining young galaxy candidates (like I Zw 18, SBS 0335-052), in most BCGs an old underlying stellar population does exist, revealing at least another burst of star formation (SF) prior to the present one (Papaderos et al. 1996; Kong & Cheng 2002). In addition, these properties make BCGs represent an extreme environment for star formation that differs from that in the Milky Way and in other quiescent nearby galaxies (Izotov & Thuan 1999; Izotov et al. 2001). Detailed studies of BCGs are important not only for understanding their intrinsic properties, but also for understanding of the chemical evolution of galaxies, for constraining models of stellar nucleosynthesis, for understanding star formation processes and galaxy evolution in different environments.
To measure the current star formation rates, stellar components, metallicities, and star formation histories and evolution of BCGs, we have prepared an atlas of optical spectra of the central regions of 97 blue compact galaxies in the first paper of this series (Kong & Cheng 2002, Paper I). Because we want to combine the optical spectra we obtained with H I data to constrain simultaneously the stellar and gas contents of BCGs, we selected most of our sample from H I surveys by Gordon & Gottesman (1981). The spectra were obtained at the 2.16 m telescope at the XingLong Station of the Beijing Astronomical Observatory (BAO) in China. A 300 line mm-1 grating was used to achieve coverage in the wavelength region from 3580 to 7400 Å with about 10 Å resolution.
In the present paper, we provide measurements of emission line equivalent widths and fluxes, equivalent widths of absorption lines, 4000 Å Balmer break index, as well as fluxes at several points of the continuum for our BCGs sample. We explore the trends in emission line fluxes and equivalent widths in the integrated spectra of SFGs in the sample. The absorption features and the continuum colors will be used to study the stellar population components and star formation history of blue compact galaxies. The emission line equivalent widths and fluxes will be used to determine the physical parameters of blue compact galaxies.
The paper is organized as follows. In Sect. 2, we classify spectra according to their emission lines. Emission line equivalent widths and fluxes measurements are presented in Sect. 3. Section 4 describes the continuum determination and the measurements of stellar absorption equivalent widths. In Sect. 5 we present an analysis of the emission line equivalent widths, line ratios and blue magnitudes of BCGs. Section 6 summarizes our conclusions.
Copyright ESO 2002