High Resolution Observations of Molecular Lines in Arp 220: Kinematics, Morphology, and Limits on the Applicability of the Ammonia Thermometer

TL;DR

This study uses high-resolution molecular line observations of Arp 220 to analyze its kinematics, morphology, and molecular composition, revealing complex ammonia excitation conditions and confirming outflows in both nuclei.

Contribution

First detailed high-resolution molecular line study of Arp 220 addressing ammonia thermometry and non-LTE effects.

Findings

Detection of multiple molecular species including hydroxyl and tentative water masers.

Complex ammonia excitation indicating non-LTE conditions and importance of non-metastable transitions.

Confirmation of outflows in both nuclei through comparison with ALMA data.

Abstract

We observe Arp 220, the nearest Ultra-Luminous Infrared Galaxy (ULIRG), over 4 GHz in the K and Ka bands. We provide constraints for the kinematics,morphology, and identify molecular species on scales resolving both nuclei (0.6" or 230 pc). We detect multiple molecular species, including hydroxyl in both cores. We tentatively detect H2O at 21.84 GHz in both nuclei, indicating the likely presence of maser emission. The observed frequency range also contains metastable ammonia transitions from (J,K) = (1,1) to (5,5), as well as the (9,9) inversion line, which, together are a well-known thermometer of dense molecular gas. Furthermore, the non-metastable (4,2) and (10,9) and possibly the (3,1) lines are also detected. We apply a standard temperature analysis to Arp 220. However, the analysis is complicated in that standard LTE assumptions do not hold. There are indications that a substantial fraction of ammonia could be in the non-metastable transitions as opposed to only the metastable ones. Thus, the non-metastable transitions could be essential to constraining the temperature. We compare all of these data to ALMA observations of this source, confirming the outflow previously observed by other tracers in both nuclei.