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Title: A far-infrared spectral sequence of galaxies: trends and models

We present a framework for the interpretation of the far-infrared spectra of galaxies in which we have expanded the model parameters compared with previous work by varying the ionization parameter U, column density N(H), and gas density at the cloud face n(H{sup +}) for a central starburst or active galactic nucleus (AGN). We compare these models carried out with the Cloudy spectral synthesis code to trends in line-to-total far-infrared luminosity ratios, far-infrared fine-structure line ratios, IRAS colors, and OH and H{sub 2}O column densities found in the well-studied sample of 10 nearby galaxies from the IRAS Bright Galaxy Sample with infrared luminosities greater than 10{sup 10} L {sub ☉} and IRAS 60 μm fluxes equal to or greater than that of the nearby ultraluminous infrared galaxy (ULIRG) Arp 220. We find that the spectral sequence extending from normal starburst-type emission-line-dominated spectra to ULIRG-type absorption-dominated spectra with significant absorption from excited levels can be best explained by simultaneously increasing the hydrogen column density, from as low as 10{sup 21} cm{sup –2} to as high as 10{sup 24.8} cm{sup –2} or greater, and the ionization parameter, from as low as 10{sup –4} to as high as 1. The starburst models best reproducemore » most of the sequence, while AGN models are somewhat better able to produce the high OH and H{sub 2}O column densities in Arp 220. Our results suggest that the molecular interstellar medium in ULIRG-like, molecular-absorption-dominated systems is located close to and at least partially obscures the source of power throughout much of the far-infrared, which must be taken into account in order to properly interpret diagnostics of both their sources of power and of feedback.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [6]
  1. Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Avenue SW, Washington DC 20375 (United States)
  2. MCGP Department, University of Cincinnati, Clermont College, Batavia, OH 45103 (United States)
  3. Departamento de Física y Matemáticas, Universidad de Alcalá, Campus Universitario, E-28871 Alcalá de Henares, Madrid (Spain)
  4. Formerly at Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Avenue SW, Washington DC 20375 (United States)
  5. School of Physics, Astronomy, and Computational Sciences, George Mason University, MS 3F3, 4400 University Drive, Fairfax, VA 22030 (United States)
  6. Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels (Belgium)
Publication Date:
OSTI Identifier:
22370247
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 795; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; COLOR; COMPARATIVE EVALUATIONS; DENSITY; EMISSION; EXCITED STATES; FAR INFRARED RADIATION; FEEDBACK; FINE STRUCTURE; GALAXIES; GALAXY NUCLEI; HYDROGEN; INFRARED SPECTRA; LUMINOSITY; SYNTHESIS; WATER