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Session 89 - Star-forming Galaxies.
Oral session, Friday, January 09
International Ballroom East,
As the closest and brightest of the class of ultraluminous infrared galaxies, Arp 220 is often considered to be the prototype of its class. As is typical for galaxies in this class, Arp 220 is in the final stages of the merging of two galaxies. Whether or not Arp 220 contains a luminous active galactic nucleus that powers most of its infrared luminosity, is a question hotly debated and important for our understanding of the evolution of galaxies and of the origin of active galactic nuclei.
We discuss the implications of a low resolution (R = \lambda /\Delta \lambda \sim 200) far-infrared (43-197 \mum) spectrum of the ultraluminous galaxy Arp 220 obtained from deep, full range scans using the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO). The spectrum is vastly different from the spectra of less luminous IR-bright galaxies: the fine-structure lines that are typical of IR-bright, starburst galaxies are weak or absent and the spectrum is dominated by absorption lines of OH, H_2O, CH, NH_3 and [O I]. Based on our fit to the 43-197 \mum continuum, we derive that \tau_d \sim 1 at 150 \mum and our simple radiative transfer solution indicates a cool central source. The upper limits on the FIR fine-structure lines indicate a softer radiation field in Arp 220 than in starburst galaxies such as M82 or in AGN. We present the results of simple models of the starburst and molecular regions that are probed by our far-infrared spectroscopy of this prototypical ultraluminous galaxy.
