After two ALMA observing cycles, only a handful of [C II] 158 µm emission line searches in z > 6 galaxies have reported a positive detection, questioning the applicability of the local [C II]-SFR relation to high-z systems. To investigate this issue we use the Vallini et al. (2013, V13) model, based on high-resolution, radiative transfer cosmological simulations to predict the [C II] emission from the interstellar medium of a z ≈ 7 (halo mass M h = 1.17 × 10 11 M ) galaxy. We improve the V13 model by including (a) a physically-motivated metallicity (Z) distribution of the gas, (b) the contribution of Photo-Dissociation Regions (PDRs), (c) the effects of Cosmic Microwave Background on the [C II] line luminosity. We study the relative contribution of diffuse neutral gas to the total [C II] emission (F diff /F tot ) for different SFR and Z values. We find that the [C II] emission arises predominantly from PDRs: regardless of the galaxy properties, F diff /F tot ≤ 10% since, at these early epochs, the CMB temperature approaches the spin temperature of the [C II] transition in the cold neutral medium (T CMB ∼ T CNM s ∼ 20 K). Our model predicts a high-z [C II]-SFR relation consistent with observations of local dwarf galaxies (0.02 < Z/Z < 0.5). The [C II] deficit suggested by actual data (L CII < 2.0 × 10 7 L in BDF3299 at z ≈ 7.1) if confirmed by deeper ALMA observations, can be ascribed to negative stellar feedback disrupting molecular clouds around star formation sites. The deviation from the local [C II]-SFR would then imply a modified Kennicutt-Schmidt relation in z > 6 galaxies. Alternatively/in addition, the deficit might be explained by low gas metallicities (Z < 0.1 Z ).

Angular fluctuations of the Near InfraRed Background (NIRB) intensity are observed up to scales < ∼ 1 • . 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 H > ∼ 10 25 cm −2 ) DCBHs formed in metal-free halos with virial temperature 10 4 K at z > ∼ 12, can explain the observed level ≈ 10 −3 (nW m −2 sr −1 ) 2 of the 3.6 and 4.5 µm 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 θ > 100 ′′ DCBHs produce a CXB (0.5-2 keV)-NIRB (4.5µm) cross-correlation signal of ≃ 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 ≈ 10 25 cm −2 ) adopted and in agreement with the recent measurements by Cappelluti et al. (2012a). 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.

We study the Initial Mass Function (IMF) and hosting halo properties of Intermediate Mass Black Holes (IMBH, 10 4−6 M ⊙ ) formed inside metal-free, UV illuminated atomic cooling haloes (virial temperature T vir 10 4 K) either via the direct collapse of the gas or via an intermediate Super Massive Star (SMS) stage. These IMBHs have been recently advocated as the seeds of the supermassive black holes observed at z ≈ 6. We achieve this goal in three steps: (a) we derive the gas accretion rate for a proto-SMS to undergo General Relativity instability and produce a direct collapse black hole (DCBH) or to enter the ZAMS and later collapse into a IMBH; (b) we use merger-tree simulations to select atomic cooling halos in which either a DCBH or SMS can form and grow, accounting for metal enrichment and major mergers that halt the growth of the proto-SMS by gas fragmentation. We derive the properties of the hosting haloes and the mass distribution of black holes at this stage, and dub it the "Birth Mass Function"; (c) we follow the further growth of the DCBH by accreting the leftover gas in the parent halo and compute the final IMBH mass. We consider two extreme cases in which minihalos (T vir < 10 4 K) can (fertile) or cannot (sterile) form stars and pollute their gas leading to a different IMBH IMF. In the (fiducial) fertile case the IMF is bimodal extending over a broad range of masses, M ≈ (0.5 − 20) × 10 5 M ⊙ , and the DCBH accretion phase lasts from 10 to 100 Myr. If minihalos are sterile, the IMF spans the narrower mass range M ≈ (1 − 2.8) × 10 6 M ⊙ , and the DCBH accretion phase is more extended (70 − 120 Myr). We conclude that a good seeding prescription is to populate halos (a) of mass 7.5 < log(M h /M ⊙ ) < 8, (b) in the redshift range 8 < z < 17, (c) with IMBH in the mass range 4.75 < (log M • /M ⊙ ) < 6.25.

It has been proposed that the first, intermediate-mass (≈ 10 5−6 M ⊙ ) black holes might form through direct collapse of unpolluted gas in atomic-cooling halos exposed to a strong Lyman-Werner (LW) or near-infrared (NIR) radiation. As these systems are expected to be Compton-thick, photons above 13.6 eV are largely absorbed and reprocessed into lower energy bands. It follows that direct collapse black holes (DCBHs) are very bright in the LW/NIR bands, typically outshining small high-redshift galaxies by more than 10 times. Once the first DCBHs form, they then trigger a runaway process of further DCBH formation, producing a sudden rise in their cosmic mass density. The universe enters the "DCBH era" at z ≈ 20 when a large fraction of atomic-cooling halos are experiencing DCBH formation. By combining the clustering properties of the radiation sources with Monte Carlo simulations we show that in this scenario the DCBH mass density rises from ∼ 5 M ⊙ Mpc −3 at z ∼ 30 to the peak value ∼ 5×10 5 M ⊙ Mpc −3 at z ∼ 14 in our fiducial model. However, the abundance of active (accreting) DCBHs drops after z ∼ 14, as gas in the potential formation sites (unpolluted halos with virial temperature slightly above 10 4 K) is photoevaporated. This effect almost completely suppresses DCBH formation after z ∼ 13. The DCBH formation era lasts only ≈ 150 Myr, but it might crucially provide the seeds of the supermassive black holes (SMBHs) powering z ∼ 6 quasars. LWBackground LW intensity from normal galaxies J BH LW Background LW intensity from DCBHs J crit LW Critical LW intensity for DCBH formation f gal ⋆ Star formation efficiency of normal galaxies f pop3 ⋆ Star formation efficiency of Pop III stars M pop3 crit,0 Critical halo mass for galaxy/DCBH formation under photoevaporation feedback ρ pop3 Mass density of Pop III stars ρ BH Mass density of active DCBHs ρ cum BH Cumulative DCBH mass density p g Probability of genetic enrichment p J Probability that a halo sees a super-critical LW intensity p W Probability that a halo being enriched by metals carried by galactic winds p J−WProbability that a halo sees a super-critical LW intensity but without being polluted by galactic winds P (> z, M |M 0 , z 0 ) For a halo with mass M 0 at z 0 , the probability that its most massive progenitor with mass M formed before z dP dz (z, M |M 0 , z 0 ) Redshift derivative of above probability