Spectroscopic study of blue compact galaxies IV. Star formation rates and gas depletion timescales

This is the fourth paper in a series studying star formation rates, stellar components, metallicities, and star formation histories of a blue compact galaxy (BCG) sample. Using Hα, [O ii]λ3727, infrared (IR), radio (1.4GHz) luminosities and neutral hydrogen (H i) gas masses, we estimated star formation rates and gas depletion timescales of 72 star-forming BCGs. The star formation rates of the BCGs in our sample span nearly four orders of magnitude, from approximately 10 to 10 M⊙ yr , with a median star formation rate of about 3M⊙ yr . The typical gas depletion timescale of BCGs is about one billion years. Star formation could be sustained at the current level only on a timescale significantly lower than the age of the universe before their neutral gas reservoir is completely depleted. To assess the possible systematic differences among different star formation rate indicators, we compared the star formation rates derived from Hα, [O ii]λ3727, IR, and radio luminosities, and investigated the effects from underlying stellar absorption and dust extinction. We found that subtracting underlying stellar absorption is very important to calculate both dust extinction and star formation rate of galaxies. Otherwise, the intrinsic extinction will be overestimated, the star formation rates derived from [O ii]λ3727 and Hα will be underestimated (if the underlying stellar absorption and the internal extinction were not corrected from the observed luminosity) or overestimated (if an overestimated internal extinction were used for extinction correction). After both the underlying stellar absorption and the dust extinction were corrected, a remarkably good correlation emerges among Hα, [O ii]λ3727, IR and radio star formation rate indicators. Finally, we find a good correlation between the measured star formation rate and the absolute blue magnitude, metallicity, interstellar extinction of BCGs. Our results indicate that faint, low-mass BCGs have lower star formation rates.