Observational searches for faint active nuclei at z > 6 have been extremely elusive, with a few candidates whose high-z nature is still to be confirmed. Interpreting this lack of detections is crucial to improve our understanding of high-z supermassive black holes (SMBHs) formation and growth. In this work, we present a model for the emission of accreting BHs in the X-ray band, taking into account super-Eddington accretion, which can be very common in gas-rich systems at high-z. We compute the spectral energy distribution for a sample of active galaxies simulated in a cosmological context, which represent the progenitors of a z ∼ 6 SMBH with M BH ∼ 10 9 M . We find an average Compton thick fraction of ∼ 45% and large typical column densities (N H 10 23 cm 2 ). However, faint progenitors are still luminous enough to be detected in the X-ray band of current surveys. Even accounting for a maximum obscuration effect, the number of detectable BHs is reduced at most by a factor 2. In our simulated sample, observations of faint quasars are mainly limited by their very low active fraction ( f act ∼ 1%), which is the result of short, super-critical growth episodes. We suggest that to detect high-z SMBHs progenitors, large area surveys with shallower sensitivities, such as Cosmos Legacy and XMM-LSS+XXL, are to be preferred with respect to deep surveys probing smaller fields, such as CDF-S. 1 Hereafter we adopt a Lambda Cold Dark Matter (ΛCDM) cosmology with parameters Ω M = 0.314, Ω Λ = 0.686, and h = 0.674 (Planck Collaboration et al. 2014).

We present the 78-ks Chandra observations of the z = 6.4 quasar SDSS J1148+5251. The source is clearly detected in the energy range 0.3-7 keV with 42 counts (with a significance 9σ). The X-ray spectrum is best-fitted by a power-law with photon index Γ = 1.9 absorbed by a gas column density of N H = 2.0We measure an intrinsic luminosity at 2-10 keV and 10-40 keV equal to ∼ 1.5 × 10 45 erg s −1 , comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value (α OX = −1.76 ± 0.14) of J1148 is consistent with the one found in quasars with similar rest-frame 2500Å luminosity (L 2500 ∼ 10 32 erg s −1Å−1 ). Then we use Chandra data to test a physically motivated model that computes the intrinsic X-ray flux emitted by a quasar starting from the properties of the powering black hole and assuming that X-ray emission is attenuated by intervening, metal-rich (Z Z ⊙ ) molecular clouds distributed on ∼kpc scales in the host galaxy. Our analysis favors a black hole mass M BH ∼ 3 × 10 9 M ⊙ and a molecular hydrogen mass M H2 ∼ 2 × 10 10 M ⊙ , in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.

Several evidences indicate that Lyman Break Galaxies (LBG) in the Epoch of Reionization (redshift z > 6) might host massive black holes (MBH). We address this question by using a merger-tree model combined with tight constraints from the 7 Ms Chandra survey, and the known high-z super-MBH population. We find that a typical LBG with MUV = −22 residing in a Mh ≈ 1012M⊙ halo at z = 6 host a MBH with mass M• ≈ 2 × 108M⊙. Depending on the fraction, fseed, of early halos planted with a direct collapse black hole seed (Mseed = 105M⊙), the model suggests two possible scenarios: (a) if fseed = 1, MBH in LBGs mostly grow by merging, and must accrete at a low (λE ≃ 10−3) Eddington ratio not to exceed the experimental X-ray luminosity upper bound $L_X^* = 10^{42.5} {\rm erg\, s}^{-1}$; (b) if fseed = 0.05 accretion dominates (λE ≃ 0.22), and MBH emission in LBGs must be heavily obscured. In both scenarios the UV luminosity function is largely dominated by stellar emission up to very bright mag, $M_{\rm UV} {\,\, \gtrsim \,\,}-23$, with BH emission playing a subdominant role. Scenario (a) poses extremely challenging, and possibly unphysical, requirements on DCBH formation. Scenario (b) entails testable implications on the physical properties of LBGs involving the FIR luminosity, emission lines, and presence of outflows.

The nature of the seeds of the observed high-z super-massive black holes (SMBH) is unknown. Although different options have been proposed, involving e.g. intermediate mass direct collapse black holes, BH remnants of massive stars remain the most natural explanation. To identify the most favorable conditions (if any) for their rapid growth, we study the accretion rate of a M • = 100M BH formed in a typical z = 10 galaxy under different conditions (e.g. galaxy structure, BH initial position and velocity). We model the galaxy baryonic content and follow the BH orbit and accretion history for 300 Myr (the time span in 10 > z > 7), assuming the radiation-regulated accretion model by Park & Ricotti (2013). We find that, within the limits of our model, BH seeds cannot grow by more than 30%, suggesting that accretion on light-seed models are inadequate to explain high-z SMBH. We also compute the X-ray emission from such accreting stellar BH population in the [0.5 − 8] keV band and find it comparable to the one produced by high-mass X-ray binaries. This study suggests that early BHs, by X-ray pre-heating of the intergalactic medium at cosmic dawn, might leave a specific signature on the HI 21 cm line power spectrum potentially detectable with SKA.

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