Star formation histories of nearby starburst galaxies

Optical spectroscopic observations of a sample of 12 M82-type starburst galaxies were carried out to investigate the star formation activity previous to the current burst in the central kpc region. In spite of showing strong Ha emission lines, sample nuclei show Hdelta line in absorption. Selected line and continuum bands of the observed spectra were fitted with synthesized spectra, that were formed by combining spectra of 10 different star formation episodes spread over the lifetime of the galaxies. Intermediate age (50-500 Myr) populations were found to be common in the starburst nuclei, that along with the underlying disk stars are responsible for Balmer absorption lines. In the majority of the galaxies, the mass of this intermediate age population, inside a region of 1 kpc size, exceeds that of the young burst by factors more than 2. We suggest a method to quantify the dilution of HP emission equivalent width due to the presence of intermediate age populations.