Common correlations between 60, 100, and 140 μm intensities in the galactic plane and magellanic clouds

We investigated 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 μm, 100 μm, and 140 μm. We analyzed three regions: the galactic plane region with the galactic latitude |b| < 5°, the Large Magellanic Cloud region, and the Small Magellanic Cloud region. Because the dust optical depth is much smaller in the far-infrared than in the visible, we could observe cumulative far-infrared radiation from regions with various interstellar radiation field in the line of sight. As a consequence of considering such an effect, we found a common far-infrared color correlation between the 140-100 μm and 100-60 μm intensity ratios in all 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.