Characteristics of diffuse galactic mid- and far-infrared emission from interstellar dust derived by IRTS observation

The spectral characteristics of the diffuse galactic mid- and far infrared emission at lambda = 10 - 60 mum are investigated using the IRTS and IRAS data. We estimate the grain temperature, total far-infrared intensity (FIR) and radiation density from the emission of the large grains at lambda > 100 mum, assuming that the spectrum has a single temperature Planck function with a lambda(-2) emissivity law. We found that the spectral energy distribution of the diffuse galactic emission at wavelengths from 10 mum to 1 mm changed as a function of the dust temperature. In addition, we found clear correlation for the intensity of emissions at 12, 25, and 60 mum. The 12 mum intensity is proportional to FIR, and the ratios of the 25 and 60 mum intensities to FIR are proportional to the strength of the interstellar radiation field. These correlations are consistent with those found in the data of seven H II regions with various physical conditions. This suggests that the emitters of the emission at 12-60 mum are well mixed with large grains radiating at wavelength greater than 100 mum and implies that the ratio of the mass of these emitters to the mass of the large grains is constant in interstellar space. We conclude that the 12 mum emission is largely due to unidentified infrared features and that the 25 and 60 mum emission arises from very small grains transiently heated by multiple-photon (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.