O. Lahav et al., A SIGNIFICANT CONTRIBUTION TO THE COSMIC X-RAY-BACKGROUND FROM SOURCES ASSOCIATED WITH NEARBY GALAXIES, Nature, 364(6439), 1993, pp. 693-695
THE origin of the cosmic X-ray background remains a mystery after thir
ty years of study. The three properties of the background radiation co
mmonly used for tackling this problem-its spectrum, isotropy and resol
ved component-are well defined by observations, but do not lead to a s
imple interpretation. A different approach to the problem1,2, in which
fluctuations in the unresolved component are cross-correlated with ga
laxy 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 t
hat such analyses must allow for contributions from X-ray sources whic
h 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 co
rrelated with nearby galaxies. Extrapolating these results up to a red
shift of approximately 5, we find that a smaller, but still significan
t, fraction of the X-ray background (30 +/- 15%) can be accounted for
by these sources. To explain the residual background emissions, evolut
ion of the source properties and/or a new population of sources at hig
h redshift is required.