The X-ray background-foreground galaxy cross-correlation: evidence for weak lensing?

A significant cross-correlation between the unresolved X-ray background (XR B) at soft energies (0.5 to 2 keV) and foreground bright galaxies has now b een reported in several studies. This cross-correlation has been interprete d in terms of a low redshift and a low luminosity active galactic nuclei (A GN) population, clustered similar to optically bright galaxies, as responsi ble for the unresolved component of the XRB. In contrast to such a low reds hift population, we suggest that a correlation between the unresolved XRB a nd bright optical galaxies can exist due to a high redshift population of X -ray emitting AGNs through weak lensing effects of low redshift large scale structure traced by foreground optical galaxies. We further investigate th is possibility and suggest that a substantial fraction of the detected cros s-correlation signal can arise from this scenario. The most likely explanat ion for the observed cross-correlation is that both a population of low red shift sources and a population of high redshift low luminous sources contri bute through clustering and lensing effects, respectively. The exact weak g ravitational lensing contribution to the detected signal can eventually be used to constrain cosmological parameters, foreground galaxy bias and, more importantly, models of high redshift X-ray emitting sources.