Common Correlations between 60, 100 and 140 um Intensities in the Galactic Plane and Magellanic Clouds

We investigate the far-infrared SED of the Galaxy and the Magellanic Clouds by using the COBE (COsmic Background Explorer) / DIRBE (Diffuse InfraRed Background Experiment) ZSMA (Zodi - Subtracted Mission Average) maps at wavelengths of 60 um, 100 um and 140 um. We analyze three regions: the Galactic plane region with the Galactic latitude |b|<5 degree, the Large Magellanic Cloud (LMC) region, and the Small Magellanic Cloud (SMC) region. Because the dust optical depth is much smaller in the far-infrared than in the visible, we may observe cumulative far-infrared radiation from regions with various interstellar radiation field (IRSF) in a line of sight. As consequence of considering such an effect, we find a common far-infrared color correlation between the 140 - 100 um and 60 - 100 um intensity ratios in all the three galaxies. Although this color correlation cannot be explained by any existing model, it fits very well the far-infrared color of nearby star forming galaxies.