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infrared sourcesA variety of objects emitting strongly in the infrared (see infrared radiation). Many stars exhibit an infrared excess in that they radiate more strongly in the 1–20 μm waveband than would be expected from their spectral type. In most cases the excess arises from circumstellar shells containing cosmic dust grains that are heated by the visible and UV radiation from the central star. Protostars and their associated dust clouds are also strong infrared emitters. Many T Tauri stars emit infrared radiation, in some cases by thermal bremsstrahlung and/or emission from heated dust. At longer wavelengths (60–200 μm) cool molecular clouds containing dust are found to be infrared sources.
Infrared line emission occurs from a variety of different complex regions, particularly from H II regions and associated molecular-cloud interfaces. Lines from atomic hydrogen are seen in the near-infrared (Brackett and Paschen lines), and in the mid- and far-infrared the lines occur from neutral and ionized states of neon, oxygen, nitrogen, sulfur, and iron. Highly ionized species of, for example, oxygen (O III) and nitrogen (N III) can be identified by the forbidden lines in their infrared spectra. The molecule carbon monoxide has been detected in the far-infrared from some sources, the lines measured arising from the rotational spectrum of the molecule.
Infrared emission is also found in extragalactic sources (see IRAS) and is strongly correlated with galaxy type: elliptical and lenticular galaxies emit little or no infrared while most spiral galaxies are infrared emitters. Active galaxies, such as Seyfert type II, BL Lac objects, and quasars, exhibit infrared emission from their nuclei. It is unclear at the present time whether this is thermal emission by dust in the nuclei or is due to nonthermal synchrotron emission. Galaxies that are interacting or merging are also seen to be strong infrared continuum emitters. This may be due to the triggering of star formation in one or both galaxies.