Tracing the Ionization Structure of the Shocked Filaments of NGC 6240

Creators: Medling, Anne M.; Kewley, Lisa J.; Calzetti, Daniela; Privon, George C.; Larson, Kirsten; Rich, Jeffrey A.; Armus, Lee; Allen, Mark G.; Bicknell, Geoffrey V.; Díaz-Santos, Tanio; Heckman, Timothy M.; Leitherer, Claus; Max, Claire E.; Rupke, David S. N.; Treister, Ezequiel; Messias, Hugo; Wagner, Alexander Y.

Abstract

We study the ionization and excitation structure of the interstellar medium in the late-stage gas-rich galaxy merger NGC 6240 using a suite of emission-line maps at ∼25 pc resolution from the Hubble Space Telescope, Keck/NIRC2 with Adaptive Optics, and the Atacama Large Millimeter/submillimeter Array (ALMA). NGC 6240 hosts a superwind driven by intense star formation and/or one or both of two active nuclei; the outflows produce bubbles and filaments seen in shock tracers from warm molecular gas (H₂ 2.12 μm) to optical ionized gas ([O iii], [N ii], [S ii], and [O i]) and hot plasma (Fe XXV). In the most distinct bubble, we see a clear shock front traced by high [O iii]/Hβ and [O iii]/[O i]. Cool molecular gas (CO(2−1)) is only present near the base of the bubble, toward the nuclei launching the outflow. We interpret the lack of molecular gas outside the bubble to mean that the shock front is not responsible for dissociating molecular gas, and conclude that the molecular clouds are partly shielded and either entrained briefly in the outflow, or left undisturbed while the hot wind flows around them. Elsewhere in the galaxy, shock-excited H₂ extends at least ∼4 kpc from the nuclei, tracing molecular gas even warmer than that between the nuclei, where the two galaxies' interstellar media are colliding. A ridgeline of high [O iii]/Hβ emission along the eastern arm aligns with the southern nucleus' stellar disk minor axis; optical integral field spectroscopy from WiFeS suggests this highly ionized gas is centered at systemic velocity and likely photoionized by direct line of sight to the southern active galactic nucleus.

Additional Information

© 2021. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 January 11; revised 2021 September 19; accepted 2021 October 11; published 2021 December 20. We would like to thank many for helpful discussions related to the interpretation of this rich data set, including Phil Appleton, Bruce Draine, Rosalie McGurk, Patrick Ogle, Rick Pogge, and Ralph Sutherland, and Emanuele Nardini and Junfeng Wang for sharing their processed X-ray maps. We also thank the referee for careful readings and, especially, the scientific editor for patience while pandemic situations delayed our responses. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community; we are privileged to be guests on your sacred mountain. We wish to pay respect to the Gamilaraay/Kamilaroi language group Elders—past, present, and future—of the traditional lands on which the Siding Spring Observatory stands and to the Atacameño community of the Chajnantor Plateau, whose traditional home now also includes the ALMA observatory. Support for A.M.M. was provided in part by NASA through Hubble Fellowship grant #HST-HF2-51377 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555, and in part from the National Science Foundation under grant No. 2009416. A.M.M. and L.J.K. acknowledge the support of the Australian Research Council (ARC) through the Discovery project DP130103925. Parts of this research were supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. C.E.M. acknowledges support by the National Science Foundation under award number AST-0908796. K.L.L. is supported by NASA through grants HST- GO-13690.002-A and HST-GO-15241.002-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. T.D.-S. acknowledges support from ALMA-CONICYT project 31130005 and FONDECYT regular project 1151239. E.T. acknowledges support from CATA-Basal AFB-170002, FONDECYT Regular grant 1190818, ANID Anillo ACT172033, and Millennium Nucleus NCN19_058 (TITANs). This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 12552, 13690, and 10592. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We enthusiastically thank the staff of the W. M. Keck Observatory and its AO team for their dedication and hard work. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00370.S and ADS/JAO.ALMA#2015.1.00003.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with the National Research Council Canada, the Academia Sinica Institute of Astronomy and Astrophysics (Taiwan), and the Korea Astronomy and Space Science Institute (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated University, Inc./NRAO, and the National Astronomical Observatory of Japan. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Facilities: HST (ACS and - WFC3) - , ATT (WiFeS) - , Keck:II (Laser Guide Star Adaptive Optics - , OSIRIS - , and NIRC2) - , ALMA - , and CXO (ACIS). - Software: LZIFU (v1.1; Ho et al. 2016) and PyWiFeS (v0.6.0; Childress et al. 2014).

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