Common Correlations between 60, 100, and 140$\, {\mu \mathrm {m}}$ 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.