Tracing the Ionization Structure of the Shocked Filaments of NGC 6240

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 $\sim$25 pc resolution from the Hubble Space Telescope, Keck NIRC2 with Adaptive Optics, and ALMA. NGC 6240 hosts a superwind driven by intense star formation and/or one or both of two active nuclei; the outflows produce bubbles and filaments seen in shock tracers from warm molecular gas (H${}_2$ 2.12$\mu$m) to optical ionized gas ([O III], [N II], [S II], [O I]) and hot plasma (Fe XXV). In the most distinct bubble, we see a clear shock front traced by high [O III]/H$\beta$ and [O III]/[O I]. Cool molecular gas (CO(2-1)) is only present near the base of the bubble, towards 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 H${}_2$ extends at least $\sim$4 kpc from the nuclei, tracing molecular gas even warmer than that between the nuclei, where the two galaxies' interstellar media are colliding. A ridgeline of high [O III]/H$\beta$ emission along the eastern arm aligns with the south 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 south AGN.

27 pages, 18 figures; accepted for publication in ApJ

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Cited by in corpus (6)

• MICONIC: Dual active galactic nuclei, star formation, and ionised gas outflows in NGC 6240 seen with MIRI/JWST
• The Role of AGN in Luminous Infrared Galaxies from the Multiwavelength Perspective
• JWST/NIRSpec and MIRI observations of an expanding, jet-driven bubble of warm H${}_2$ in the radio galaxy 3C 326 N
• The JWST/NIRSpec view of the nuclear region in the prototypical merging galaxy NGC 6240
• Mapping the diffuse interstellar bands $\lambda$5780 and $\lambda$6284 in the luminous infrared galaxy merger NGC 6240
• Properties of Interstellar Medium in the S0 Galaxy NGC 1222: Evidence for Shocked-enhanced Line Emission