Infrared background signatures of the first black holes

Angular fluctuations of the near-infrared background (NIRB) intensity are observed up to scales less than or similar to 1 degrees. Their interpretation is challenging as even after removing the contribution from detected sources, the residual signal is > 10 times higher than expected from distant galaxies below the detection limit and first stars. We propose here a novel interpretation in which early, intermediate-mass, accreting direct collapse black holes (DCBHs), which are too faint to be detected individually in current surveys, could explain the observed fluctuations. We find that a population of highly obscured (N-H greater than or similar to 10(25) cm(-2)) DCBHs formed in metal-free haloes with virial temperature 10(4) K at z greater than or similar to 12 can explain the observed level approximate to 10(-3) (nW m(-2) sr(-1))(2) of the 3.6 and 4.5 mu m fluctuations on scales > 100 arcsec. The signal on smaller scales is instead produced by undetected galaxies at low and intermediate redshifts. Albeit Compton-thick, at scales theta > 100 arcsec DCBHs produce a cosmic X-ray background (0.5-2 keV)-NIRB (4.5 mu m) cross-correlation signal of similar or equal to 10(-11) erg s(-1) cm(-2) nW m(-2) sr(-1) slightly dependent on the specific value of the absorbing gas column (N-H approximate to 10(25) cm(-2)) adopted and in agreement with the recent measurements by Cappelluti et al. At smaller scales the cross-correlation is dominated by the emission of high-mass X-ray binaries hosted by the same low-z, undetected galaxies accounting for small-scale NIRB fluctuations. These results outline the great potential of the NIRB as a tool to investigate the nature of the first galaxies and black holes.