The X-ray Evolution of Merging Galaxies

Abstract

Abstract. From a Chandra survey of nine interacting galaxy systems the evolution of X-ray emission during the merger process has been investigated. From comparing LX/LK and LFIR/LB it is found that the X-ray luminosity peaks ∼300 Myr before nuclear coalescence, even though we know that rapid and increasing star formation is still taking place at this time. It is likely that this drop in X-ray luminosity is a consequence of outflows breaking out of the galactic discs of these systems. At a time ∼1 Gyr after coalescence, the merger-remnants in our sample are X-ray dim when compared to typical X-ray luminosities of mature elliptical galaxies. However, we do see evidence that these systems will start to resemble typical elliptical galaxies at a greater dynamical age, given the properties of the 3 Gyr system within our sample, indicating that halo regeneration will take place within low L X merger-remnants.

A Chandra view of the anomalous half-merger NGC 520

“…Furthermore, Brassington et al (2005) observe a relationship between the integrated luminosity of the ULX sources and the far‐infrared luminosity in their merging galaxy sample: log L ULX ∝ log L 0.54±0.04 FIR . The small number of NGC 520 ULXs are all bright, but, even so, NGC 520 lies significantly below the observed L ULX – L FIR relationship.…”

“…There is evidence to suggest that galaxies with a greater SFR contain a greater number of ULXs (Swartz et al 2004), and it is these sources that flatten the XLF. Brassington, Read & Ponman (2005) have presented a relationship between the number of ULXs and the far‐infrared luminosity for a small number of interacting and merging galaxies: log N (ULX) ∝ log L 0.18±0.13 FIR . NGC 520 appears unusual in that it does not fit into this picture at all, having a large L FIR value, very like that of, for example, the Mice or the Antennae, but a relatively small number of ULXs (three to four).…”

“…the Antennae, 45 per cent; Fabbiano et al 2001), and even when compared with very early‐stage interacting systems (e.g. Arp 270, 29 per cent; Brassington et al 2005). Were the progenitor disc that formed the NW nucleus initially gas‐rich, then f diff for NGC 520 would certainly be larger (although not by a factor of 2), and perhaps more in line with other merging systems.…”

Diffuse X-ray Emission from Galaxies

“…It may even be that two double-bubbles are seen in diffuse X-rays within this system, and that therefore something rather unexpected may be occurring in merging galaxies. The evolution of the diffuse X-ray emission in merging galaxies is investigated more fully in Brassington et al (2005).…”