Faint progenitors of luminous z ∼ 6 quasars: Why do not we see them?
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 black holes (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 MBH ∼ 109 M⊙. We find an average Compton-thick fraction of ∼45 per cent and large typical column densities (NH ≳ 1023 cm2). 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 of 2. In our simulated sample, observations of faint quasars are mainly limited by their very low active fraction (fact ∼ 1 per cent), which is the result of short, supercritical 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 Chandra Deep Field South.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- April 2017
- DOI:
- 10.1093/mnras/stw3243
- arXiv:
- arXiv:1612.04188
- Bibcode:
- 2017MNRAS.466.2131P
- Keywords:
-
- black hole physics;
- galaxies: active;
- galaxies: evolution;
- galaxies: high-redshift;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 12 pages, 9 figures, accepted for publication in MNRAS