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

TL;DR

This study models the X-ray emission of high-redshift black hole progenitors, revealing they are often obscured but still detectable, with low active fractions limiting observations, suggesting large-area surveys are more effective.

Contribution

Introduces a model accounting for super-Eddington accretion and obscuration in high-z SMBH progenitors, explaining their observational scarcity.

Findings

Faint high-z progenitors are often obscured but detectable in X-ray surveys.

Active fraction of these black holes is very low (~1%), limiting their observability.

Large area, shallower surveys are more effective for detecting high-z SMBH progenitors.

Abstract

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 $M_{\rm BH} \sim 10^9 \, M_\odot$. We find an average Compton thick fraction of $\sim 45\%$ and large typical column densities ($N_H \gtrsim 10^{23} \rm \, cm^2$). 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 ($f_{\rm act} \sim 1 \%$), 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.