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E. González-Alfonso (U. de Alcalá de Henares), H.A. Smith (Harvard-Smithsonian CfA), J. Fischer (NRL), J. Cernicharo (CSIC)
ISO/LWS observations of the ultraluminous infrared galaxy Arp 220 show absorption in molecular lines of OH, H2O, CH, NH, and NH3, as well as in the [O I] 63 \mum line and emission in the [C II] 158 \mum line. We have modeled the continuum and the emission/absorption of all observed features by means of a non-local radiative transfer code. The continuum from 25 to 1300 \mum is modeled as a warm (106 K) nuclear region that is optically thick in the far-infrared, attenuated by an extended region (size 2'') that is heated mainly through absorption of nuclear infrared radiation. The molecular absorption in the nuclear region is characterized by high excitation due to the high infrared radiation density. The OH column densities are high toward the nucleus and the extended region (~2-6 \times 1017 cm-2). The H2O column density is also high toward the nucleus (2-10 \times 1017 cm-2). The column densities in a halo that accounts for the absorption by the lowest lying levels are similar to those in the diffuse clouds in the Sgr B2 molecular cloud complex near the Galactic Center. Most notable are the high column densities found for NH and NH3 toward the nucleus ~ 1.5 - 3 \times 1016 cm-2, whereas the NH2 column density is \leq 2 \times 1015 cm-2. A combination of PDRs in the extended region and hot cores with enhanced H2O photodissociation and a possible shock contribution in the nuclei may explain the relative column densities of OH and H2O, whereas the nitrogen chemistry may be strongly affected by cosmic ray ionization. The [C II] 158 \mum line is well reproduced by our models and its ``deficit'' relative to the CII/FIR ratio in normal and starburst galaxies is suggested to be mainly a consequence of the dominant non-PDR component of far-infrared radiation, although our models alone cannot rule out extinction effects in the nuclei.
E. G-A thanks Spanish SEEU for funding support under project PR2003-0057, and the Harvard-Smithsonian Center for Astrophysics for its hospitality. This work has been partially supported by NASA Grant NAG5-10659, the NASA LTSA program and the Office of Naval Research.
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Bulletin of the American Astronomical Society, 36 #2
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