Observations of Arp 220 using Herschel-SPIRE: An Unprecedented View of the Molecular Gas in an Extreme Star Formation Environment

TL;DR

This study uses Herschel SPIRE-FTS observations to analyze the molecular gas and outflows in Arp 220, revealing a complex multi-temperature gas environment, a massive molecular outflow, and evidence for an X-ray luminous AGN.

Contribution

First detailed Herschel SPIRE-FTS spectral analysis of Arp 220 revealing warm and cold molecular gas components and a massive outflow with implications for AGN activity.

Findings

Detection of high-J CO and water ladders with comparable luminosities

Presence of a massive molecular outflow with >10^7 solar masses

Evidence for an X-ray luminous AGN in Arp 220

Abstract

We present Herschel SPIRE-FTS observations of Arp~220, a nearby ULIRG. The FTS continuously covers 190 -- 670 microns, providing 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 ladders with comparable total luminosity; very high-J HCN absorption; OH+, H2O+, and HF in absorption; and CI and NII. Modeling of the continuum yields warm dust, with T = 66 K, and an unusually large optical depth of ~5 at 100 microns. Non-LTE modeling of the CO shows two temperature components: cold molecular gas at T ~ 50 K and warm molecular gas at T ~1350 K. The mass of the warm gas is 10% of the cold gas, but dominates the luminosity of the CO ladder. The temperature of the warm gas is in excellent agreement with H2 rotational lines. At 1350 K, H2 dominates the cooling (~20 L_sun/M_sun) in the ISM compared to CO (~0.4 L_sun/M_sun). We found that only a non-ionizing source such as the mechanical energy from supernovae and stellar winds can excite the warm gas and satisfy the energy budget of ~20 L_sun/M_sun. We detect a massive molecular outflow in Arp 220 from the analysis of strong P-Cygni line profiles observed in OH+, H2O+, and H2O. The outflow has a mass > 10^{7} M_sun and is bound to the nuclei with velocity < 250 km/s. The large column densities observed for these molecular ions strongly favor the existence of an X-ray luminous AGN (10^{44} ergs/s) in Arp 220.