Year: 1990
Type: article
Abstract: Infrared and submillimeter 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 IRAS infrared continuum measurements, infrared spectroscopy of the fine-structure lines of O I 63 micron and C II 158 micron, and the 2.6 mm CO (J = 1-0) rotational transition. The diagnostic capabilities of the O I 145 micron and Si II 35 micron lines are also addressed. We 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. We use the theoretical photodissociation region models of Tielens and Hollenbach 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 G_0_, and the temperature of the atomic gas T_a_. In modeling the ensemble of clouds present in galactic nuclei, we adopt a global model of the interstellar medium consisting of molecular cloud cores with atomic envelopes. In addition to n, G_0_, and T_a_, we determine the mass of the molecular and warm atomic gas components as well as estimates of the area and volume filling factors and the number and radii of clouds. As examples, the procedure is applied to the Galactic photodissociation region behind the Orion H II region as well as to ensembles of clouds in the Galactic center, and the nucleus of the starburst galaxy M82. We find that the Orion photodissociation region is composed of warm (T ~ 500 K) high-density (n ~ 10^5^ cm^-3^) gas which is illuminated by a far- ultraviolet radiation field of intensity ~ 2 x 10^4^ times the local Galactic field. Within 5 pc of the Galactic center we find ~ 100 clouds of size r ~ 0.4 pc and density n ~ 10^5^ cm^-3^. A far-ultraviolet radiation field, most likely from a central source with L ~ 2-3 x 10^7^ L_sun_, illuminates the clouds with an intensity ~ 10^5^ times greater than the local Galactic field and heats gas in the surface layers to ~ 700 K. The gas phase Si abundance is <~ 4.7 x 10^-5^ in the atomic layers of these clouds. For the case of M82, we find that the neutral interstellar medium within the nuclear region of diameter less than 650 pc consists of a large number, N_cloud_ ~ 3 x 10^5^, of small, r ~ 0.4 pc, dense, n ~ 5 x 10^4^ cm^-3^, clouds which are irradiated by a far-ultraviolet radiation field ~ 10^4^ times larger than the local Galactic far-ultraviolet field. Thus, except for the scale size, the physical conditions in the center of our galaxy and of M82 are very similar to those in the region surrounding the trapezium stars in Orion.
Source: The Astrophysical Journal
DOI: 10.1086/168966
Authors: Mark G. Wolfire, A. G. G. M. Tielens, David Hollenbach
Institutions: University of Chicago, Ames Research Center
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2085098780