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Nearby Galaxies and the Ginga X-ray Background

We present here the results of cross-correlating the X-ray background measured by Ginga in the 2-10 keV band with several catalogues of extragalactic objects. Positive signals with an amplitude of a few per cent have been found for some catalogues implying that some fraction of the X-ray background is produced either by the class of catalogued sources or by other classes spatially related to them. Detailed X-ray background simulations have been used to assess the significance of the results and, for the first time, the full angular shape of the cross-correlation. The inferred X-ray volume emissivity in the local Universe, j_0, has been estimated for two galaxy catalogues (UGC and IRAS) for which the cross-correlation is highly significant. We obtain j_0= (0.74 +/- 0.07) for UGC and j_0= (1.15 +/- 0.10) for IRAS, in units of 1.E39 h erg/s/Mpc3. Extrapolating this result back to z~1-4 leads to the conclusion that <~10-30 per cent of the X-ray background could be produced by a non-evolving population of galaxies. These values are shown to be consistent with upper limits on the Auto Correlation Function derived here.