We examine what types of galaxies the conversion formula from dust infrared (IR) luminosity into the star formation rate (SFR) derived by R. C. Kennicutt is applicable to. The ratio of the observed IR luminosity, L IR, to the intrinsic bolometric luminosity of the newly (≲ 10 Myr) formed stars, LSF, of a galaxy can be determined by a mean dust opacity in the interstellar medium and the activity of the current star formation. We find that the area given by these parameters, 0.5 ≤ L IR/LSF ≤ 2.0, is very large, and many nearby normal and active starforming galaxies really fall in this area. It results from offsetting two effects of a small dust opacity and a large cirrus contribution of normal galaxies relative to starburst galaxies on the conversion of the stellar emission into the dust IR emission. In conclusion, the SFR determined from the IR luminosity under the assumption of LIR = LSF, like Kennicutt's, is reliable within a factor of 2 for all galaxies except for dust-rich but quiescent galaxies and extremely dust-poor galaxies.
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