A Chandra view of the anomalous half-merger NGC 520

Abstract

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.

New H$_{\mathsf 2}$O masers in Seyfert and FIR bright galaxies II. The intermediate luminosity range

“…Chandra observations also revealed 15 point sources within the optical boundaries of the galaxy (Read 2005). The third brightest of these, source 12, is an ultraluminous X-ray source (ULX) with L X (0.2-10 keV) ∼ 9 × 10 39 erg s −1 .…”

“…No Fe-K line seems to be present in the X-ray spectrum but an ULX source (source 12, see Sect. 3.2) with a "power law spectrum" (N H = 2.3 × 10 22 cm −2 ) is detected close to the main nucleus (Read 2005). Figure 7 shows a plot in which the positions for the main centers of activity are marked.…”

Diffuse X-Ray-emitting Gas in Major Mergers

“…The Chandra X-ray map of Arp 157 was previously presented by Read (2005), who called it a half-merger. Using Galactic absorption and an absorbed MEKAL model, they obtained a gas temperature of 0.58 ± 0.09 0.11 keV and an absorption-corrected 0.2 − 10 keV luminosity of 8.03 × 10 39 erg s −1 .…”

The X-ray Evolution of Merging Galaxies

“…It is likely that this difference is due to the inclusion of the merging galaxy NGC 520 (a system which has been described as an 'anomalous half-merger' (Read (2005))), in the Read & Ponman (1998) study. We suggest that the reason that L X peaks at a time before nuclear coalescence is a consequence of large-scale diffuse outflows being driven out of the interacting systems by the starbursts taking place within these galaxies.…”