Infrared background signatures of the first black holes

Abstract

Angular fluctuations of the Near InfraRed Background (NIRB) intensity are\nobserved up to scales $\\simlt 1^{\\ensuremath{^{\\circ}}}$. Their interpretation\nis challenging as even after removing the contribution from detected sources,\nthe residual signal is $>10$ times higher than expected from distant galaxies\nbelow the detection limit and first stars. We propose here a novel\ninterpretation in which early, intermediate mass, accreting direct collapse\nblack holes (DCBH), which are too faint to be detected individually in current\nsurveys, could explain the observed fluctuations. We find that a population of\nhighly obscured ($N_{\\rm H}\\simgt 10^{25} \\rm cm^{-2}$) DCBHs formed in\nmetal-free halos with virial temperature $10^4$ K at $z\\simgt 12$, can explain\nthe observed level $\\approx 10^{-3}$ (nW m$^{-2}$ sr$^{-1})^2$ of the 3.6 and\n4.5 $\\mu$m fluctuations on scales $>100''$. The signal on smaller scales is\ninstead produced by undetected galaxies at low and intermediate redshifts.\nAlbeit Compton-thick, at scales $\\theta> 100''$ DCBHs produce a CXB (0.5-2\nkeV)-NIRB ($4.5 \\rm \\mu m$) cross-correlation signal of $\\simeq 10^{-11}$ erg\ns$^{-1}$ cm$^{-2}$ nW m$^{-2}$ sr$^{-1}$ slightly dependent on the specific\nvalue of the absorbing gas column ($N_{\\rm H} \\approx 10^{25} \\rm cm^{-2}$)\nadopted and in agreement with the recent measurements by\n\\cite{2012arXiv1210.5302C}. At smaller scales the cross-correlation is\ndominated by the emission of high-mass X-ray binaries (HMXB) hosted by the same\nlow-$z$, undetected galaxies accounting for small scale NIRB fluctuations.\nThese results outline the great potential of the NIRB as a tool to investigate\nthe nature of the first galaxies and black holes.\n