Implications of the ISO LWS Spectrum of the Prototypical Ultraluminous Galaxy: ARP 220
NAVAL RESEARCH LAB WASHINGTON DC REMOTE SENSING DIV
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We present a low resolution R lamba Delta lamba 200 far-infrared 43-197 micrometer 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 galaxies are weak or absent and the spectrum is dominated by absorption lines of OH, H2O, CH, NH3 and 0 I. From the 43-197 micrometer continuum fit we derive that the optical depth of the thermal dust emission, Tau sub dust, is about unity at 150 micrometer. From our continuum fit we derive a Lyman continuum production rate of Q9 x 10exp 54 secexp -1 and our simple spherical 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. The low Lsub CII 158 Lsub FIR ratio in Arp 220 cannot be easily explained by dust obscuration or saturation effects. These results may be explained by a starburst model with a low upper mass cutoff or by unusually high dust absorption of UV photons within the ionized regions of a starburst or AGN.
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