Dissecting the Extended X-ray Emission in the Merging Pair NGC 6240

Abstract

We present a detailed spectral and imaging analysis of the central $15^{\prime \prime }$ radius ($\sim 7.5\text{ kpc}$) region of the merger galaxy NGC 6240 that makes use of all the available \textit{Chandra}-ACIS data ($0.3-3\text{ keV}$ effective exposure of $\sim 190\text{ ks}$). This region shows extended X-ray structures with lower energy counterparts imaged in CO, [O III] and H$\alpha$ line emission. We find both photo-ionized phases of possible nuclear excitation and thermal shock-excited emission in the different large-scale components: the north-west "loop" detected in H$\alpha$, the region surrounding the two nuclei, the large outflow region to the north-east detected in [O III], and the southern X-ray extensions. The latter could be the ionization cone of the northern nucleus, with the N counterpart being obscured by the galaxy disk. The radial distribution of the X-ray surface brightness suggests a confined hot interstellar medium at $r<2.5\text{ kpc}$, with a free-flowing wind at larger radii; if the confinement is magnetic, we estimate B-field values of $\sim 100\mu \text{G}$ , similar to those measured in the halo of M82. The thermal gas of the extended halo at $kT\sim 1\text{ keV}$ absorbs soft X-rays from the AGN, but not the extreme ultraviolet radiation leading to a rapid increase in $F_{\text{[O III]}}/F_{\text{X}}$ beyond $\sim 3\text{ kpc}$. The $\alpha$ element to Fe abundance ratios of the thermal components in the different regions of the extended X-ray emission are generally compatible with SNe II yields, confirming the importance of the active star formation in NGC 6240.

36 pages, 16 figures, 4 tables, accepted for publication on ApJ

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• Deep Chandra Observations of NGC 5728: Morphology and Spectral Properties of the Extended X-ray Emission
• Jet-ISM interaction in NGC 1167 / B2 0258+35, A LINER with an AGN past
• Mapping the diffuse interstellar bands $\lambda$5780 and $\lambda$6284 in the luminous infrared galaxy merger NGC 6240
• Termination Shocks and the Extended X-ray Emission in MRK 78