Dissecting the Extended X-Ray Emission in the Merging Pair NGC 6240: Photoionization and Winds

Abstract

We present a detailed spectral and imaging analysis of the central 15″ radius (~7.5 kpc) region of the merger galaxy NGC 6240 that makes use of all the available Chandra-ACIS data (0.3-3 keV effective exposure of ~190 ks). This region shows extended X-ray structures with lower-energy counterparts imaged in CO, [O III], and Hα line emission. We find both photoionized phases of possible nuclear excitation and thermal shock-excited emission in the different large-scale components: the northwest "loop" detected in Hα, the region surrounding the two nuclei, the large outflow region to the northeast 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 kpc, with a free-flowing wind at larger radii; if the confinement is magnetic, we estimate B-field values of ~100 μG, similar to those measured in the halo of M82. The thermal gas of the extended halo at kT ~ 1 keV absorbs soft X-rays from the active galactic nucleus, but not the extreme ultraviolet radiation leading to a rapid increase in F _{[O III]}/F _X beyond ~3 kpc. The α-element to Fe abundance ratios of the thermal components in the different regions of the extended X-ray emission are generally compatible with Type II supernova yields, confirming the importance of active star formation in NGC 6240.