The large scale structure of the soft X-ray background III. Cosmological implications

The auto-correlation function of the X-ray background in the range of angul ar scales 0 degrees.3-20 degrees is determined using the ROSAT All-Sky Surv ey. In the power-law approximation, our best fit estimate is w(theta) = (0 degrees.0058/theta)(1.1). At the small separation end, this is substantiall y above previous estimates based on ROSAT pointings. The present upward rev ision of the correlation amplitude results from variations in the backgroun d intensity at angular scales larger than the field of view of the X-ray te lescope. Various assumptions for the local clustering amplitude, clustering evolutio n rate and luminosity/density evolution of the sources producing the backgr ound are explored, and the predicted auto-correlation functions compared wi th the measured fluctuations. We show that the large amplitude of the backg round angular auto-correlation function is incompatible with oro the weak c lustering amplitude observed for normal galaxies, while it is marginally co mpatible with some measurements of the AGN clustering amplitude at moderate redshifts.