Physical conditions in photodissociation regions: Application to galactic nuclei

Astronomy and Astrophysics – Astronomy

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Galactic Nuclei, Infrared Astronomy, Infrared Radiation, Interstellar Matter, Molecular Clouds, Photodissociation, Continuums, Emission Spectra, Far Ultraviolet Radiation, Fine Structure, Gas Density, Gas Temperature, Illuminating, Infrared Astronomy Satellite, Infrared Spectroscopy, Mathematical Models, Monatomic Gases

Scientific paper

Infrared and sub-millimeter observations are used in a simple procedure to determine average physical properties of the neutral interstellar medium in Galactic photodissociation regions as well as in ensembles of clouds which exist in the nuclei of luminous infrared galaxies. The relevant observations include the Infrared Astronomy Satellite (IRAS) infrared continuum measurements, infrared spectroscopy of the fine-structure lines of SiII 35 microns, OI 63 microns, and CII 158 microns, and the 2.6 mm CO (J=1-0) rotational transition. The diagnostic capabilities of the OI 145 microns line is also addressed. Researchers attribute these emission lines as well as the continuum to the atomic/molecular photodissociation region on the surfaces of molecular clouds which are illuminated by strong ultraviolet fields. They use the theoretical photodissociation region models of Tielens and Hollenbach (1985, Ap. J., 291, 722) to construct simple diagrams which utilize line ratios and line to continuum ratios to determine the average gas density n, the average incident far-ultraviolet flux Go, and the temperature of the atomic gas T.

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