THE X-RAY EVOLUTION OF MERGING GALAXIES

We present here the first study of the X-ray properties of an evolutio nary sample of merging galaxies. Both ROSAT PSPC and HRI data are pres ented for a sample of eight interacting galaxy systems, each believed to involve a similar encounter between two spiral discs of approximate ly equal size. The mergers span a large range in age, from completely detached to fully merged systems. A great deal of interesting X-ray st ructure is seen, and the X-ray properties of each individual system ar e discussed in detail. Along the merging sequence, several trends are evident: in the case of several of the infrared bright systems, the di ffuse emission is very extended, and appears to arise from material ej ected from the galaxies. The onset of this process seems to occur very soon after the galaxies first encounter one another, and these ejecti ons soon evolve into distorted flows. More massive extensions (perhaps involving up to 10(10) M-circle dot of hot gas) are seen at the 'ultr aluminous' peak of the interaction, as the galactic nuclei coalesce. T he amplitude of the evolution of the X-ray emission through a merger i s markedly different from that of the infrared and radio emission, how ever. Although the X-ray luminosity rises and falls along the sequence , the factor by which the X-ray luminosity increases, relative to the optical, appears to be only about a tenth of that seen in the far-infr ared. This, we believe, may well be linked with the large extensions o f hot gas observed. The late, relaxed remnants appear relatively devoi d of gas, and possess an X-ray halo very different from that of typica l ellipticals, a problem for the 'merger hypothesis', whereby the merg er of two disc galaxies results in an elliptical galaxy. However, thes e systems are still relatively young in terms of total merger lifetime , and they may still have a few Gyr of evolution to go through before they resemble typical elliptical galaxies. (C) 1998 RAS.