Spectroscopic study of blue compact galaxies III. Empirical population synthesis

This is the third paper of a series dedicated to the study of the star formation rates, star formation histories, metallicities and dust contents of a sample of blue compact galaxies (BCGs). We constrain the stellar contents of 73 blue compact galaxies by analyzing their continuum spectra and the equivalent widths of strong stellar absorption features using a technique of empirical population synthesis based on a library of observed star-cluster spectra. Our results indicate that blue compact galaxies are typically age-composite stellar systems; in addition to young stars, intermediate-age and old stars contribute significantly to the 5870 Å continuum emission of most galaxies in our sample. The stellar populations of blue compact galaxies also span a variety of metallicities. The ongoing episodes of star formation started typically less than a billion years ago. Some galaxies may be undergoing their first global episode of star formation, while for most galaxies in our sample, older stars are found to contribute up to half the optical emission. Our results suggest that BCGs are primarily old galaxies with discontinuous star formation histories. These results are consistent with the results from deep imaging observations of the color-magnitude diagrams of a few nearby BCGs using HST and large ground-based telescopes. The good quality of our population synthesis fits of BCG spectra allow us to estimate the contamination of the Hα, Hβ, Hγ and Hδ Balmer emission lines by stellar absorption. The absorption equivalent widths measured in the synthetic spectra range from typically 1.5 Å for Hα, to 2-5 Å for Hβ, Hγ, and Hδ. The implied accurate measurements of emission-line intensities will be used in a later study to constrain the star formation rates and gas-phase chemical element abundances of blue compact galaxies.