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 of galaxies from the spectral energy distribution (SED) at wavelengths of λ <= 100 μm. It is difficult to derive the total far-infrared luminosity from only the IRAS data, though it is one of the most important properties of galaxies. Observations by Infrared Telescope in Space (IRTS) indicate that the SED of the diffuse emission from the Galactic plane in this wavelength region can be derived from the 60 μm to 100 μm color. This empirical SED relation was improved in order to obtain a better fit to the Galactic plane data for I_ν(60 μm) / I_ν(100 μm) > 0.6, and applied to 96 IRAS galaxies for which ISOPHOT and KAO data are available at λ > 100 μm. As a result, the empirical relation well describes the far-infrared (FIR) SED for a majority of galaxies. Additionally, the total FIR flux for λ >= 40 μm was derived from the flux densities at 60 and 100 μm by using this model. For the 96 IRAS galaxies, the uncertainty in the total far-infrared flux of the present method is 26%. The present method is more accurate than the previous one widely used to derive the total infrared flux from the IRAS 60 and 100 μm data.