2005MNRAS.359..455R

High spatial and spectral resolution Chandra X-ray observations of the anomalous merging galaxy NGC 520, a similarly evolved system to the well-known Antennae galaxies, are presented here. Of great interest is the fact that NGC 520, on account of it being supposedly due (as seen in various multiwavelength studies) to the result of an encounter between one gas-rich disc and one gas-poor disc, appears in X-rays to be only `half a merger'; whereas an ultraluminous X-ray source (ULX) lies at the primary (south-eastern), more-massive nucleus, no sources are seen at the secondary nucleus. Whereas what appears to be a starburst-driven galactic wind is seen outflowing perpendicular to the molecular disc surrounding the primary nucleus, no such diffuse structure is seen anywhere near the secondary nucleus. Comparing the X-ray properties with those of other merging galaxies, including famous gas-rich-gas-rich mergers such as the Mice and the Antennae, one sees that, relative to its star formation rate, the number of ULXs seen within the system is rather small. Similarly, the total X-ray luminosity and the fraction of this emission that appears diffuse are both a factor of ∼2 less than that expected based on NGC 520's evolutionary merger stage.

Although only half of NGC 520 appears in X-rays as other mergers do, there is still a wealth of structure and detail: 15 X-ray sources are detected within the system, many of them showing long-term variability, including a small number of bright ULXs that flatten the source X-ray luminosity function to a level similar to that of the Antennae and other mergers. Lastly, to see what appears to be a starburst-driven diffuse galactic wind, with a spectrum entirely consistent with that of other known galactic winds, although unusually, emanating from only one of the nuclei, is a surprise, given that one might have expected such structures to have distorted very quickly in such a rapidly evolving environment. The wind is larger and more massive than structures seen in evolutionarily earlier systems (e.g. the Mice), but smaller and less massive than as seen in later systems (e.g. the Antennae) or classic starbursts. Perhaps these structures can survive for longer than was previously thought.