Infrared Emission from the Galaxy

This lecture summarizes empirical knowledge on the infrared emission from the Galaxy in the plane and at high Galactic latitude. In discussing observations we convey to the reader a basic understanding of the origin of the infrared emission of a normal galaxy such as the Milky Way and of the composition of interstellar dust. We analyze the infrared emission from the different components of the interstellar medium: neutral atomic, ionized and molecular gas, and the contribution of dust grains, small particles and large molecules to the emission from the near-infrared to 1mm. Most observations analyzed here were obtained with the Infrared Astronomy Satellite (IRAS) which surveyed the sky at 4’ resolution at 12, 25, 60 and 100µm. The Cosmic Background Explorer (COBE) has recently extended these observations to both shorter and longer wavelengths. We briefly discuss how these observations, when analyzed, will complement and test current knowledge on interstellar matter. A motivation for studying the Galactic emission is the necessity to subtract it before one may discuss the existence of an infrared extragalactic background. Since the school was mostly focused on cosmology special attention is given to the infrared emission seen in the region of minimum emission including its structure on small scale which complicates the observation of faint extragalactic objects.