Tracing the Ionization Structure of the Shocked Filaments of NGC 6240

TL;DR

This study maps the ionization and shock structures in NGC 6240, revealing how outflows and shocks influence molecular and ionized gas over several kiloparsecs, using high-resolution multi-wavelength observations.

Contribution

It provides detailed spatially-resolved analysis of shock fronts, ionization states, and molecular gas behavior in a merging galaxy with active outflows, combining data from HST, Keck, and ALMA.

Findings

Shock fronts are traced by high [O III]/Hβ ratios.

Molecular gas is only near the outflow base, not outside the shock front.

Shock-excited H₂ extends over 4 kpc, indicating widespread shock influence.

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.