A SIGNIFICANT CONTRIBUTION TO THE COSMIC X-RAY-BACKGROUND FROM SOURCES ASSOCIATED WITH NEARBY GALAXIES

THE origin of the cosmic X-ray background remains a mystery after thirty years of study. The three properties of the background radiation commonly used for tackling this problem-its spectrum, isotropy and resolved component-are well defined by observations, but do not lead to a simple interpretation. A different approach to the problem1,2, in which fluctuations in the unresolved component are cross-correlated with galaxy catalogues, has led to the suggestion2 that as much as 60% of the background emission can be explained by a population of X-ray sources similar to present-day optically bright galaxies. Here we point out that such analyses must allow for contributions from X-ray sources which cluster with the galaxies, but do not necessarily have a counterpart in galaxy catalogues. For realistic assumptions about clustering, we obtain a revised limit on the local X-ray emissivity due to sources correlated with nearby galaxies. Extrapolating these results up to a redshift of approximately 5, we find that a smaller, but still significant, fraction of the X-ray background (30 +/- 15%) can be accounted for by these sources. To explain the residual background emissions, evolution of the source properties and/or a new population of sources at high redshift is required.