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

TL;DR

This paper analyzes the extended X-ray emission in NGC 6240, revealing complex ionization and thermal processes, and provides insights into the galaxy's interstellar medium, outflows, and star formation activity.

Contribution

It offers a detailed spectral and imaging analysis of NGC 6240's extended X-ray structures, highlighting the coexistence of photo-ionized and shock-excited regions and estimating magnetic field strengths.

Findings

Presence of both photo-ionized and shock-excited emission regions.

Detection of a confined hot interstellar medium and large-scale outflows.

Element abundance ratios consistent with supernovae II yields.

Abstract

We present a detailed spectral and imaging analysis of the central $15''$ 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.