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Published December 10, 2016 | Published
Journal Article Open

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


We observe Arp 220, the nearest ultra-luminous infrared galaxy, over 4 GHz in the K and Ka bands, providing constraints for the kinematics and morphology, and identifying molecular species on scales resolving both nuclei (0".6 or 230 pc). We detect multiple molecular species, including hydroxyl (OH ^2Π_(3/2)J = 9/2 F= 4-4; 5-5) in both cores, and tentatively detect H_2O(6_(15)-5_(23)) 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)–(5, 5), as well as the (9, 9) inversion line; together, they 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 local thermal equilibrium (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.

Additional Information

© 2016. The American Astronomical Society. Received 2015 October 29; revised 2016 September 9; accepted 2016 September 13; published 2016 December 6. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00175.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. This research made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com. The authors thank Brent Groves and Yancy Shirley for insightful discussions concerning ammonia properties and LVG analysis. We also thank Elizabeth Mills for insight concerning ammonia masers and column densities. Finally, we thank the anonymous referee for exceptionally insightful and relevant comments leading to substantial improvement of this manuscript.

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