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Session 75 - Dust & PAHs in Galaxies.
Display session, Friday, January 09
Exhibit Hall,

[75.05] Far-infrared Spectroscopy of Normal Galaxies with ISO-LWS: Physical conditions in the ISM

S. Malhotra, S. D. Lord, G. Helou, C. Beichman (IPAC/Caltech), H. L. Dinerstein (U. Texas), D. J. Hollenbach (NASA/Ames), D. A. Hunter (Lowell Obs.), K. Y. Lo (U. Illinois), N. Y. Lu (IPAC/Caltech), R. H. Rubin (NASA/Ames), N. Silbermann (IPAC/Caltech), G. Stacey (Cornell U.), H. A. Thronson Jr. (NASA HQ/U. Wyoming), M. W. Werner (JPL/Caltech)

We report observations of atomic fine structure lines in 45 normal galaxies with the Long Wavelength Spectrograph (LWS) on the Infrared Space Observatory (ISO). These observations are a part of US-ISO Key project to characterize the ISM and star-formation in normal star-forming galaxies spanning a range of morphologies (E-Irr), infrared luminosities, far-infrared colors and far-infrared to blue luminosity. Up to 7 lines are observed for each galaxy. The [CII]/FIR ratio decreases with increasing ratio of 60/100 micron flux and far-infrared to blue luminosity. Also, the ratio of [CII]/[OI] line fluxes decrease with increasing 60/100 micron flux ratio. This indicates that G_0/n cm^3 increases with density n cm^-3, where G_0 is the UV radiation intensity in the units of local interstellar field, and n is the density of the ISM. This implies that the UV radiation field from massive stars increases with density in the ISM with a steeper than linear dependence. The ratio of the line fluxes to total far-infrared luminosity, measures the ratio of the cooling of warm gas and dust and the efficiency of the grain photoelectric heating process. This ratio (L_[CII] + L_[OI]) /L_FIR decreases with 60/100 micron flux and far-infrared to blue luminosity. This may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons.

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