Stellar populations in blue compact galaxies

Blue compact galaxies (BCGs) are compact galaxies that are dominated by intense star formation. Comparing the observational properties with the predictions of the stellar population synthesis model, we have analyzed the nuclear stellar population and emission line spectra in a sample of 10 BCGs. The results indicate that the continuum flux fractions at 5870 Å due to old stellar components and young stellar components are both important. The contribution from intermediate age components is different in different galaxies. Our results suggest that BCGs are old galaxies, in which star formation occurs in short intense bursts separated by long quiescent phases. We have also derived the internal reddening for the stellar population by the population synthesis method, and the internal reddening for the emitting gas clouds by the Balmer line ratio. The former is significantly smaller than the latter for BCGs. A model of clumpy foreground dust, with different covering factors for the gas and stars, can explain the difference. Combining the internal reddening value and the stellar population, we have decreased the effecting from the internal reddening and underlying stellar absorption, and accurately measured most emission lines for each BCGs. Using these emission lines, we have attempted to identify the ionizing mechanism of BCGs. The ionizing mechanism for these emission line regions of BCGs is typical of photoionization by stars, characteristics of a low extinction HII regions.