Faint progenitors of luminous $z\sim 6$ quasars: why don't we see them?

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 $z\sim 6$ SMBH with $M_{\mathrm{B}\mathrm{H}}\sim {10}^9{M}_{\odot }$. We find an average Compton thick fraction of $\sim 45\mathrm{\%}$ and large typical column densities ($N_H\gtrsim {10}^{23}\mathrm{c}{\mathrm{m}}^2$). 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 ($f_{\rm act} \sim 1 %$), 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.