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For refcode 2001ApJ...561..766M:
Retrieve 60 NED objects in this reference.
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Copyright by American Astronomical Society. Reproduced by permission
2001ApJ...561..766M Far-Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium S. Malhotra, M. J. Kaufman, D. Hollenbach, G. Helou, R. H. Rubin, J. Brauher, D. Dale, N. Y. Lu, S. Lord, G. Stacey, A. Contursi, D. A. Hunter, and H. Dinerstein Received 2000 February 23; accepted 2001 June 29 ABSTRACT The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L_[C II]_/L_FIR_ with increasing F_{nu}_(60 microns)/F_{nu}_(100 microns) and L_FIR_/L_B_, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L_[C II]_/L_FIR_ < 2 x 10^-4^ (3 {sigma} upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L_[O I]_/L_[C II]_ shows a tight correlation with F_{nu}_(60 microns)/F_{nu}_(100 microns), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 microns) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L_[O I]_/L_[C II]_ and (L_[C II]_ + L_[O I]_)/L_FIR_ and (2) L_[O I]_/L_FIR_ and F_{nu}_(60 microns)/F_{nu}_(100 microns) yields far-UV flux G_0_ and gas density n. The G_0_ and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G_0_ and n correlate with each other, and G_0_ increases with n as G_0_ is proportional to n^{alpha}^, where {alpha} ~ 1.4 . We interpret this correlation as arising from Stromgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 x 10^4^-1.5 x 10^7^ K cm^-3^) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G_0_ and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G_0_ and n have larger and/or denser star-forming clouds. Subject headings: galaxies: ISM - H II regions - ISM: atoms - ISM: general - ISM: lines and bands - radiation mechanisms: thermal
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