We present a formalism describing the physical content of cross-correlation functions between a diffuse background and a population of discrete sources. The formalism is used to interpret cross-correlation signals between the unresolved X-ray background and a galaxy population resolved to high redshift in another spectral band. Specifically, we apply it to the so-called faint blue galaxy population and constrain their X-ray emissivity and clustering properties. A model is presented which satisfies the recently measured constraints on all three correlation functions (galaxy/galaxy, background/background and galaxy/background). This model predicts that faint galaxies in the magnitude range B=18-23 (covering redshifts <e1>z<~</e1>0.5) make up ~22 per cent of the X-ray background in the 0.5-2 keV band. At the mean redshift of the galaxy sample, z~0.3, the comoving volume emissivity is rho_X~(6-9)x10^38 h erg s^-1 Mpc^-3. When extrapolated to fainter magnitudes, the faint blue galaxy population can account for most of the residual background at soft X-ray energy. We show how the measurement of the angular and zero-lag cross-correlation functions between increasingly faint galaxies and the X-ray background can in principle allow us to map the X-ray emissivity as a function of redshift.
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 1996
- GALAXIES: EVOLUTION;
- COSMOLOGY: MISCELLANEOUS;
- X-RAYS: GENERAL;
- uuencoded compressed postscript, without figures. The preprint is available with figures at http://www.ast.cam.ac.uk/preprint/PrePrint.html