A sequence of optical images of merging spiral galaxies arranged in an approximate order from top left to bottom right: approaching, interacting and finally merging into a single elliptical galaxy. We have studied the way in which hot gas is produced by supernova explosions triggered during this process - see discussion below.
Galaxies were once thought of as `island universes', evolving slowly in complete isolation. This is now known not to be the case. Galaxies can interact in a variety of ways with satellite and neighbouring galaxies, and collisions and mergers of galaxies are now believed to be key evolutionary mechanisms. There are probably very few galaxies in the universe that were not shaped by interactions or even outright mergers. The position of a galaxy in Hubble's morphological sequence may depend mainly on the number and severity of merger events in its past history. Pure disc systems, formed from relatively isolated protogalactic gas clouds, appear at one end of the sequence, giant ellipticals, possibly produced through mergers of similar spirals, appear at the other. In between, mergers between galaxies of differing mass produce galaxies with a wide range of bulge to disc ratios.
The image sequence above shows the sample that we used in the first ever study of the X-ray properties of an evolutionary sample of merging galaxies. During the merger of two disc galaxies, the conversion of orbital to internal energy during a close tidal encounter causes the two progenitor systems to sink together and coalesce violently into a centrally condensed system, disrupting any pre-existing discs, and largely randomizing the stellar motions. Gas flows into the centre of the galaxies during this process, where it fuels massive bursts of star formation, leading to multiple supernova explosions which heat gas to very high temperatures.
In studying the evolution of the X-ray properties of merging galaxies at Birmingham, we used data from the ROSAT PSPC and HRI instruments for the above sample of eight interacting galaxy systems, each believed to involve a similar encounter between two spiral discs of approximately equal size, ranging in age from completely detached to fully merged systems. With the recent launch of Chandra and XMM, we are now extending our research, studying these and other merging galaxy systems. Very recent Chandra images of a few merging galaxy systems can be found here.
Topics being addressed by our work include: