A Chandra View of the Anomalous Half-Merger NGC520

Abstract

(abridged) High spatial and spectral resolution Chandra X-ray observations of the anomalous merging galaxy NGC520, a similarly-evolved system to the well-known Antennae galaxies, are presented here. Of great interest is the fact that NGC520, on account of it being due to the result of an encounter between one gas-rich disk and one gas-poor disk, appears in X-rays to be only 'half a merger'; Whereas a ULX lies at the primary (SE), 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 disk 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, one sees that, relative to its SFR, 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 ~2 less than that expected based on NGC520's evolutionary merger stage. 15 X-ray sources are detected within the system, including a small number of bright ULXs that flatten the source XLF 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, though 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 features seen in evolutionarily earlier systems (eg the Mice), but smaller and less massive than as seen in later systems (eg the Antennae), or classic starbursts.