The spatial distribution of cosmic X-ray sources from the isotropy of the soft X-ray background

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Abstract
Recent studies of the autocorrelation function of the soft X-ray background on arcmin scales, carried out with imaging instruments (basically the Rosat PSPC), are used to constrain the clustering properties of the sources of the residual background, namely what is left after sources brighter than |$\sim 10^{-14} \enspace \text {erg} \enspace \text {cm}^{-2} \enspace \text s^{-1}$| in the 0.5–2 keV band have been removed. It is concluded that sources clustering like optically selected QSOs cannot provide more than about 60 per cent of the residual intensity. If, on the other hand, it is assumed that galaxies produce the bulk of the residual background, then on very general grounds it is concluded that the evolution of their spatial correlation function has to be extremely rapid (not less than the rate of stable clustering) at the relevant redshifts |$0.5 \lesssim z \lesssim 3$|⁠. Future studies of the autocorrelation function on these scales leading to smaller upper limits will show whether clusters are stable or still growing, either linearly or non-linearly, at these redshifts.

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