The Contribution of z<6 Sources to the Spatial Coherence in the Unresolved Cosmic near-Infrared and X-ray Backgrounds

A spatial clustering signal has been established in Spitzer/IRAC measurements of the unresolved Cosmic near-Infrared Background (CIB) out to large angular scales, ~1 deg. This CIB signal, while significantly exceeding the contribution from the remaining known galaxies, was further found to be coherent at a highly statistically significant level with the unresolved soft Cosmic X-ray Background (CXB). This measurement probes the unresolved CXB to very faint source levels using deep near-IR source subtraction. We study contributions from extragalactic populations at low to intermediate redshifts to the measured positive cross-power signal of the CIB fluctuations with the CXB. We model the X-ray emission from AGN, normal galaxies and hot gas residing in virialized structures, calculating their CXB contribution including their spatial coherence with all infrared emitting counterparts. We use a halo model framework to calculate the auto and cross-power spectra of the unresolved fluctuations based on the latest constraints of the halo occupation distribution and the biasing of AGN, galaxies and diffuse emission. At small angular scales (<1'), the 4.5mic vs 0.5-2 keV coherence can be explained by shot noise from galaxies and AGN. However, at large angular scales (~10 arcmin) we find that the net contribution from the modeled populations is only able to account for ~3% of the measured CIBxCXB cross-power. The discrepancy suggests that the CIBxCXB signal originates from the same unknown source population producing the CIB clustering signal out to ~1 deg.