The X‐ray evolution of merging galaxies

We present here the first study of the X‐ray properties of an evolutionary sample of merging galaxies. Both ROSAT PSPC and HRI data are presented for a sample of eight interacting galaxy systems, each believed to involve a similar encounter between two spiral discs of approximately equal size. The mergers span a large range in age, from completely detached to fully merged systems. A great deal of interesting X‐ray structure is seen, and the X‐ray properties of each individual system are discussed in detail. Along the merging sequence, several trends are evident: in the case of several of the infrared bright systems, the diffuse emission is very extended, and appears to arise from material ejected from the galaxies. The onset of this process seems to occur very soon after the galaxies first encounter one another, and these ejections soon evolve into distorted flows. More massive extensions (perhaps involving up to 10 10  M⊙ of hot gas) are seen at the ‘ultraluminous’ peak of the interaction, as the galactic nuclei coalesce. The amplitude of the evolution of the X‐ray emission through a merger is markedly different from that of the infrared and radio emission, however. 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‐infrared. This, we believe, may well be linked with the large extensions of hot gas observed. The late, relaxed remnants appear relatively devoid of gas, and possess an X‐ray halo very different from that of typical ellipticals, a problem for the ‘merger hypothesis’, whereby the merger of two disc galaxies results in an elliptical galaxy. However, these 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.