THE FUNDAMENTAL GAS DEPLETION AND STELLAR-MASS BUILDUP TIMES OF STAR-FORMING GALAXIES

Abstract

Stars do not form continuously distributed over star-forming galaxies. They form in star clusters of different masses. This nature of clustered star formation is taken into account in the theory of the integrated galactic stellar initial mass function (IGIMF) in which the galaxy-wide initial mass function (IMF) on galaxy-wide scales is calculated by adding all IMFs of young star clusters. For massive stars, the IGIMF is steeper than the universal IMF in star clusters and steepens with decreasing star formation rate (SFR) which is called the IGIMF effect. The current SFR and the total Hα luminosity of galaxies therefore scale nonlinearly in the IGIMF theory compared to the classical case in which the galaxy-wide IMF is assumed to be constant and identical to the IMF in star clusters. Here we apply for the first time the revised SFR–LHα relation on a sample of local volume star-forming galaxies with measured Hα luminosities. The fundamental results are: (1) the SFRs of galaxies scale linearly with the total galaxy neutral gas mass, (2) the gas depletion timescales of dwarf irregular and large disk galaxies are about 3 Gyr, implying that dwarf galaxies do not have lower star formation efficiencies than large disk galaxies, and (3) the stellar-mass buildup times of dwarf and large galaxies are only in agreement with downsizing in the IGIMF context, but contradict downsizing within the traditional framework that assumes a constant galaxy-wide IMF.