Conversion Law of Infrared Luminosity to Star-Formation Rate for Galaxies

We construct a new algorithm for estimating the star formation rate (SFR) of galaxies from their infrared (IR) luminosity by developing the theory of the IR emission from a dusty H II region. The derived formula is ${\rm SFR}/(\MO {\rm yr^{-1}})=\{3.3\times 10^{-10}(1-\eta)/ (0.4-0.2f+0.6 \epsilon)\}(L{\rm ^{obs}_{IR}}/\LO)$, where $f$ is the fraction of ionizing photons absorbed by hydrogen, $\epsilon$ is the efficiency of dust absorption for nonionizing photons from OB stars, and $\eta$ is the cirrus fraction of observed IR luminosity. The previous conversion formulae of SFR from the IR luminosity is applicable to only the case where the observed IR luminosity is nearly equal to the bolometric luminosity (starburst galaxies etc.), except for some empirical formulae. On the other hand, our theoretical SFR is applicable to galaxies even with a moderate star formation activity. That is, our simple and convenient formula is significantly useful for estimating the SFR of various morphologies and types of galaxies --- from early elliptical to late spiral and irregular galaxies, or from active starburst to quiescent galaxies --- as far as they have neither an abnormal dust-to-gas ratio nor an evident active galactic nucleus.