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

Abstract

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 Ha, [OII]3727, IR, radio (1.4GHz) luminosities and neutral hydrogen gas masses, we estimated star formation rates(SFR) and gas depletion timescales of 72 star-forming BCGs. The SFRs of the BCGs in our sample span nearly four orders of magnitude, from approximately 10^-2 to 10^2M_sun/yr, with a median SFR of about 3M_sun/yr. The typical gas depletion timescale of BCGs is about one billion years. We found that subtracting underlying stellar absorption is very important to calculate both dust extinction and SFR of galaxies. Otherwise, the intrinsic extinction will be overestimated, the SFRs derived from [OII] and Ha 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 Ha, [OII], IR and radio SFR indicators. Finally, we find a good correlation between the measured SFR and the absolute blue magnitude, metallicity, interstellar extinction of BCGs. Our results indicate that faint, low-mass BCGs have lower star formation rates.