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 b een simulated in a 15 Gyr old galaxy with an underlying component of r ed stars. Stellar evolutionary tracks for two different metallicities (Z = 0.02 and 0.001) and metallicity dependent photometric calibration s have been adopted to account for the metallicity range observed in B CGs. Emission of the ionised gas in H II regions (gaseous continuum pl us lines) has been superimposed upon the spectrum of the stellar compo nent, also consistent with the respective metallicities. The evolution of the spectral energy distributions (SED) obtained from UBVRIJHKL fl uxes is studied during and after the starbursts and compared with obse rved SEDs of BCGs.During the starburst phase our models give good agre ement with the SEDs of BCGs obtained from CCD surface photometry and a perture photometry. The gaseous emission component is crucial for expl aining the SEDs of starbursts in BCGs. The stellar emission alone give s much less agreement with the observed SEDs, especially in the near i nfrared. Furthermore, the metallicity significantly affects the shape of the SED. Best agreement with the observed SEDs is obtained for mode ls matching the observed metallicities of the galaxies. This emphasise s the need for metallicity consistent photometric models in studies of galaxies with different metallicities. Most of the optical emission i n BCGs is produced by the line emission of H II regions and by young s tars formed during the starburst, whereas the near infrared emission i n the majority of BCGs is dominated by old giant stars in the underlyi ng component. Only in stronger starbursts (the ratio b between the ste llar mass formed during the starburst and the mass of stars ever forme d 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 tr end of stronger starbursts for smaller galaxy masses.