Photoionization Modeling of Infrared Fine-Structure Lines in Luminous Galaxies with Central Dust-Bounded Nebulae
NAVAL RESEARCH LAB WASHINGTON DC REMOTE SENSING DIV
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Far-infrared spectroscopy of a small sample of IR-bright galaxies taken with the Infrared Space Observatory Long Wavelength Spectrometer has revealed a dramatic progression extending from strong fine-structure line emission from photoionized and photodissociated gas in the starburst galaxy Arp 299 to faint C II158 micrometers line emission and absorption in lines of OH, H2O, CH, and O I in the ultraluminous galaxy Arp 220. The progression to weaker emission lines is accompanied by lower excitation and does not appear to be due to far-infrared extinction or density effects. Although aging of short duration starbursts with a relatively small range in age might explain much of the sequence, the spectra of Arp 220 and other ultraluminous galaxies may be more reasonably explained by high ionization parameter, dust-bounded nebulae. To test this hypothesis, we ran photoionization models of mid- and far-infrared lines for power law index -1.5 and starburst cases with moderate 500 cmexp -3 and high 10exp 5 cmexp -3 densities. Indeed, we find that for dust-bounded nebulae and photonization by a power law source, the relative emission line strengths and the high-to-low ionization infrared line ratios decrease with ionization parameter. For starburst ionization sources and dust-bounded nebulae, the N III 57 micronN II 122 micron line ratio is found to drop with ionization parameter.
- Atomic and Molecular Physics and Spectroscopy