A New Empirical Method for Estimating the Far-Infrared Flux of Galaxies

We propose a new empirical method to estimate the total far-infrared flux ofgalaxies from the spectral energy distribution (SED) at wavelengths lambdashorter than 100 microns. It is difficult to derive the total far-infraredluminosity from only the IRAS data, though it is one of the most importantproperties of galaxies. Observations by Infrared Telescope in Space (IRTS)indicate that the SED of the diffuse emission from the Galactic plane in thiswavelength region can be derived from the 60 microns to 100 microns color. Thisempirical SED relation was improved in order to obtain a better fit to theGalactic plane data for I(60 microns)/I(100 microns) > 0.6, and applied to 96IRAS galaxies for which ISOPHOT and KAO data are available at lambda > 100microns. As a result, the empirical relation describes well the far-infrared(FIR) SED for a majority of the galaxies. Additionally, the total FIR flux forlambda longer than 40 microns is derived from the flux densities at 60 and 100microns by using this model. For the 96 IRAS galaxies, the uncertainty in thetotal far-infrared flux of the present method is 26%. The present method ismore accurate than the previous one widely used to derive the total infraredflux from the IRAS 60 and 100 microns data.Comment: 24 pages including 5 figures; to appear in PASJ, Vol. 54, No.