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Present and future X-ray deep surveys
Ian McHardy

I review the results of recent deep X-ray surveys with particular emphasis on their galaxy content. The existence of galaxies as the identifications of sources in deep X-ray surveys is well established. However what is not so well established is the origin of the X-ray emission in these galaxies. Some galaxies show only absorption lines in their optical spectra and deep optical imaging usually shows that they are ellipticals living in poor groups, so bremsstrahlung is the most likely emission mechanism. However the majority of X-ray galaxies are spirals displaying narrow optical emission lines (NELGs). For the latter galaxies I review the evidence concerning both starburst activity and true AGN activity from a massive black hole as the origin of the X-ray emission. I conclude that most X-ray galaxies display a mixture of the two activities, but that the relative proportion varies from galaxy to galaxy with the higher X-ray luminosity galaxies containing the greatest AGN fraction. I also consider the link between the NELGs and other faint galaxy populations, such as the general field galaxy population and, in particular, the sub-mJy radio source population. I consider the implications for the origin of the remainder of the unresolved X-ray background and highlight the advances in our understanding that are likely to arise as a result of the next generation of X-ray surveys.

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