OBSERVATIONS OF Arp 220 USING<i>HERSCHEL</i>-SPIRE: AN UNPRECEDENTED VIEW OF THE MOLECULAR GAS IN AN EXTREME STAR FORMATION ENVIRONMENT

Abstract

We present Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (Herschel SPIRE-FTS) observations of Arp 220, a nearby ultra-luminous infrared galaxy. The FTS provides continuous spectral coverage from 190 to 670 m, a wavelength region that is either very difficult to observe or completely inaccessible from the ground. The spectrum provides a good measurement of the continuum and detection of several molecular and atomic species. We detect luminous CO (J = 4-3 to 13-12) and water rotational transitions with comparable total luminosity 2 10 8 L ; very high-J transitions of HCN (J = 12-11 to 17-16) in absorption; strong absorption features of rare species such as OH + , H 2 O + , and HF; and atomic lines of [C i] and [N ii]. The modeling of the continuum shows that the dust is warm, with T = 66 K, and has an unusually large optical depth, with dust 5 at 100 m. The total far-infrared luminosity of Arp 220 is L FIR 2 10 12 L . Non-LTE modeling of the extinction corrected CO rotational transitions shows that the spectral line energy distribution of CO is fit well by two temperature components: cold molecular gas at T 50 K and warm molecular gas at T 1350 +280