The space density of ultra-luminous QSOs at the end of reionization epoch by the QUBRICS Survey and the AGN contribution to the hydrogen ionizing background

Abstract

Motivated by evidences favoring a rapid and late hydrogen reionization process completing at z ∼ 5.2 − 5.5 and mainly driven by rare and luminous sources, we have reassessed the estimate of the space density of ultra-luminous QSOs at z ∼ 5 in the framework of the QUBRICS survey. A ∼ 90% complete sample of 14 spectroscopically confirmed QSOs at M 1450 ≤ −28.3 and 4.5 ≤ z ≤ 5.0 has been derived in an area of 12, 400 sq. deg., thanks to multi-wavelength selection and GAIA astrometry. The space density of z ∼ 5 QSOs within −29.3 ≤ M 1450 ≤ −28.3 is three times higher than previous determinations. Our results suggest a steep bright-end slope for the QSO luminosity function at z ∼ 5 and a mild redshift evolution of the space density of ultra-bright QSOs (M 1450 ∼ −28.5) at 3 < z < 5.5, in agreement with the redshift evolution of the much fainter AGN population at M 1450 ∼ −23. These findings are consistent with a pure density evolution for the AGN population at z > 3. Adopting our z ∼ 4 QSO luminosity function and applying a mild density evolution in redshift, a photo-ionization rate of Γ HI = 0.46 +0.17 −0.09 × 10 −12 s −1 has been obtained at z = 4.75, assuming an escape fraction of ∼ 70% and a steep faint-end slope of the AGN luminosity function. The derived photo-ionization rate is ∼ 50 − 100% of the ionizing background measured at the end of the reionization epoch, suggesting that AGNs could play an important role in the cosmological reionization process.

Astraeus VI: Hierarchical assembly of AGN and their large-scale effect during the Epoch of Reionization

“…We stress that this is not in conflict with the studies of e.g. Grazian et al (2018Grazian et al ( , 2021 and Boutsia et al (2021): in our model, AGN do take over the UV background at 𝑧 5 (with variations around this value slightly depending on the assumptions for 𝑓 AGN esc ). We note that despite careful modelling, we found no DCBH seed in our set of simulations.…”