Cross-correlating cosmic IR and X-ray background fluctuations: evidence
  of significant black hole populations among the CIB sources
  In order to understand the nature of the sources producing the recently
uncovered cosmic infrared background (CIB) fluctuations, we study
cross-correlations between the fluctuations in the source-subtracted CIB from
Spitzer/IRAC data and the unresolved cosmic X-ray background from deep Chandra
observations. Our study uses data from the EGS/AEGIS field, where both data
sets cover an 8'x45' region of the sky. Our measurement is the cross-power
spectrum between the IR and X-ray data. The cross-power signal between the IRAC
maps at 3.6 um and 4.5 um and the Chandra [0.5-2] keV data has been detected,
at angular scales 20'', with an overall significance of 3.8 sigma and 5.6
sigma, respectively. At the same time we find no evidence of significant
cross-correlations at the harder Chandra bands. The cross-correlation signal is
produced by individual IR sources with 3.6 um and 4.5 um magnitudes m_AB 25-26
and [0.5-2] keV X-ray fluxes 7e-17 cgs.We determine that at least 15%-25% of
the large scale power of the CIB fluctuations is correlated with the spatial
power spectrum of the X-ray fluctuations. If this correlation is attributed to
emission from accretion processes at both IR and X-ray wavelengths, this
implies a much higher fraction of accreting black holes than among the known
populations. We discuss the various possible origins for the cross-power signal
and show that neither local foregrounds nor the known remaining normal galaxies
and active galactic nuclei can reproduce the measurements. These observational
results are an important new constraint on theoretical modeling of the near-IR
CIB fluctuations.
