Infrared background signatures of the first black holes
  Angular fluctuations of the Near InfraRed Background (NIRB) intensity are
observed up to scales $\simlt 1^{\ensuremath{^{\circ}}}$. 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 (DCBH), which are too faint to be detected individually in current
surveys, could explain the observed fluctuations. We find that a population of
highly obscured ($N_{\rm H}\simgt 10^{25} \rm cm^{-2}$) DCBHs formed in
metal-free halos with virial temperature $10^4$ K at $z\simgt 12$, can explain
the observed level $\approx 10^{-3}$ (nW m$^{-2}$ sr$^{-1})^2$ of the 3.6 and
4.5 $\mu$m fluctuations on scales $>100''$. The signal on smaller scales is
instead produced by undetected galaxies at low and intermediate redshifts.
Albeit Compton-thick, at scales $\theta> 100''$ DCBHs produce a CXB (0.5-2
keV)-NIRB ($4.5 \rm \mu m$) cross-correlation signal of $\simeq 10^{-11}$ erg
s$^{-1}$ cm$^{-2}$ nW m$^{-2}$ sr$^{-1}$ slightly dependent on the specific
value of the absorbing gas column ($N_{\rm H} \approx 10^{25} \rm cm^{-2}$)
adopted and in agreement with the recent measurements by
\cite{2012arXiv1210.5302C}. At smaller scales the cross-correlation is
dominated by the emission of high-mass X-ray binaries (HMXB) 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.
