Faint progenitors of luminousz ∼ 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 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).

Observational evidence for intermediate-mass black holes

“…The short phases of super-Eddington growth, followed by longer periods of quiescence, expected to occur in IMBHs (e.g., Volonteri & Rees 2005;Madau et al 2014;Pezzulli et al 2016) could also decrease the probability of detecting accreting BHs (Pezzulli et al 2017). Modeling the X-ray emission of accreting BHs at z ∼6 and taking into account super-Eddington accretion, Pezzulli et al (2017) found that faint AGN progenitors at z ∼6 should be luminous enough to be detected in current X-ray surveys even when accounting for maximum obscuration.…”

The infrared-luminous progenitors of high-zquasars

“…To assess the possibility of detecting BH activity in the nuclei of high-z HyLIRGs, we compared the predicted X-ray luminosity in the (observed-frame) soft ([0.5 -2] keV) and hard ([2 -10] keV) X-ray bands with the flux limits of recent surveys. Following Pezzulli et al (2017), we modeled the X-ray luminosity of accreting BHs considering the primary emission from the hot corona and the reflection component due to the surrounding neutral medium. The first one is parametrized as a power law, Lν ∝ ν −Γ+1 e hν/Ec , with an exponential cutoff energy of Ec = 300 keV (Sazonov et al 2004;Yue et al 2013) and photon spectral index Γ = 0.32 log λ Edd + 2.27, that we assume to depend on the Eddington ratio λ Edd = L bol /L Edd (Brightman et al 2013).…”

“…5.c, the resulting net attenuation of fluxes by obscuration in the observed-frame soft X-ray band is about a factor of 2, while fluxes in the observed-frame hard X-ray band are almost unaffected by attenuation. This can be explained in terms of the photoelectric cross section decreasing for increasing energy (see a thorough discussion in Pezzulli et al 2017). Finally, we compared the predicted, obscuration-attenuated F BH X with the flux limits obtained by Chandra observations of the recent 7 Ms Chandra Deep Field South Survey (7 Ms CDF-S; Luo et al 2017), i.e., FCDF−S = 6.4 × 10 −18 erg s −1 cm −2 in the soft band and FCDF−S = 2.7 × 10 −17 erg s −1 cm −2 in the hard band.…”

Unseen Progenitors of Luminous High-z Quasars in the Rh = ct Universe

“…Our flux calculations and subsequent estimates on the number of observable quasars should therefore be taken as upper limits. However, the results of Pezzulli et al (2017) indicate that even if absorption is important, the overall effect out to z∼10 is not expected to be severe enough to impact our results.…”

“…Since the greatest sensitivity to the emission produced in our model occurs in soft X-rays (see, e.g., Pezzulli et al 2017), we consider only the soft X-ray band between ò l =0.5 keV and ò u =2 keV. For a BH at redshift z, the luminosity emitted in this band is given by the expression…”