OPTICAL AND NEAR-INFRARED SPECTRAL ENERGY-DISTRIBUTIONS OF BLUE COMPACT GALAXIES FROM EVOLUTIONARY SYNTHESIS

The broad band optical and near infrared spectral energy distributions (UBVRIJHKL) of blue compact galaxies (BCGs) have been calculated with evolutionary synthesis models. A starburst lasting 5 . 10(6) yr has been simulated in a 15 Gyr old galaxy with an underlying component of red stars. Stellar evolutionary tracks for two different metallicities (Z = 0.02 and 0.001) and metallicity dependent photometric calibrations have been adopted to account for the metallicity range observed in BCGs. Emission of the ionised gas in H II regions (gaseous continuum plus lines) has been superimposed upon the spectrum of the stellar component, also consistent with the respective metallicities. The evolution of the spectral energy distributions (SED) obtained from UBVRIJHKL fluxes is studied during and after the starbursts and compared with observed SEDs of BCGs. During the starburst phase our models give good agreement with the SEDs of BCGs obtained from CCD surface photometry and aperture photometry. The gaseous emission component is crucial for explaining the SEDs of starbursts in BCGs. The stellar emission alone gives much less agreement with the observed SEDs, especially in the near infrared. Furthermore, the metallicity significantly affects the shape of the SED. Best agreement with the observed SEDs is obtained for models matching the observed metallicities of the galaxies. This emphasises the need for metallicity consistent photometric models in studies of galaxies with different metallicities. Most of the optical emission in BCGs is produced by the line emission of H II regions and by young stars formed during the starburst, whereas the near infrared emission in the majority of BCGs is dominated by old giant stars in the underlying component. Only in stronger starbursts (the ratio b between the stellar mass formed during the starburst and the mass of stars ever formed in the underlying galaxy being b greater than or equal to 0.01) the gas continuum provides a significant contribution to the near infrared emission. It dominates in the strongest starbursts (b similar to 0.1). The burst strengths deduced for BCGs are in the range b = 0.001 to 0.05. These values are by about an order of magnitude lower than those found for ultraluminous IRAS galaxies. The BCGs studied here show a trend of stronger starbursts for smaller galaxy masses.