Photoionization Modeling of Infrared Fine-Structure Lines in Luminous Galaxies with Central Dust-Bounded Nebulae
Abstract
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 II]158 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 cm(exp -3)) and high (10(exp 5) cm(exp -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 micron/[N II] 122 micron line ratio is found to drop with ionization parameter.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 2001
- Accession Number
- ADA464702
Entities
People
- C. C. Dudley
- Howard A. Smith
- Jacqueline Fischer
- Mark G. Wolfire
- Michael L. Luhman
- Robert Allen
- Shobita Satyapal
Organizations
- United States Naval Research Laboratory