Faint Active Galactic Nuclei and the Ionizing Background

Schirber, Michael; Bullock, James S.

doi:10.1086/345662

Cited by 77 publications

(96 citation statements)

References 146 publications

(178 reference statements)

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“…where α UV = −1.57 (Schirber & Bullock 2003). The value α ν = −1.57 adopted for 475 < λ < 912 Å is consistent with the recent evaluation with the COS spectrograph at HST by Shull et al (2012) and Stevans et al (2014) α ν = −1.41.…”

Section: Agn Hydrogen Ionizing Emissivity and Photoionization Ratesupporting

confidence: 88%

“…The shape of the AGN SED from λ = 1450 Å to λ = 912 Å has been represented by a double power law with slopes adopted from Schirber & Bullock (2003), Telfer et al (2002), andVanden Berk et al (2001). In the integral we have considered all the AGNs that are a factor of 100 brighter or fainter than the break luminosity (corresponding to a faint limit of M 1450 ∼ −18).…”

Section: Agn Hydrogen Ionizing Emissivity and Photoionization Ratementioning

confidence: 99%

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Faint AGNs atz>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe

Giallongo

Grazian

Fiore

et al. 2015

A&A

323

602

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Context. Establishing the number of faint active galactic nuclei (AGNs) at z = 4−6 is crucial to understanding their cosmological importance as main contributors to the reionization of the Universe. Aims. In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at z > 4 in the CANDELS GOODS-South field, which is one of the deepest fields with extensive multiwavelength coverage from Chandra, HST, Spitzer, and various ground-based telescopes. Methods. We have adopted a relatively novel criterion. As a first step, high redshift galaxies are selected in the NIR H band down to very faint levels (H ≤ 27) using reliable photometric redshifts. At z > 4 this corresponds to a selection criterion based on the galaxy rest-frame UV flux. AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of F X ∼ 1.5 × 10 −17 erg cm −2 s −1 (0.5−2 keV), corresponding to a probability of spurious detections of 2 × 10 −4 in the deep X-ray 4 Ms Chandra image. Results. We have found 22 AGN candidates at z > 4 and we have derived the first estimate of the UV luminosity function in the redshift interval 4 < z < 6.5 and absolute magnitude interval −22.5 M 1450 −18.5 typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two to four magnitudes fainter at z ∼ 4−6 than that derived from previous UV surveys. We estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats, the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless, almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies. Both effects could, in principle, significantly reduce the estimated average volume densities and/or ionizing emissivities, especially at the highest redshifts. Conclusions. At z = 4−6.5 we argue that, under reasonable evaluations of possible biases, the probed AGN population can produce photoionization rates consistent with that required to keep the intergalactic medium observed in the Lyman-α forest of high redshift QSO spectra highly ionized, providing an important contribution to the cosmic reionization.

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Section: Agn Hydrogen Ionizing Emissivity and Photoionization Ratesupporting

confidence: 88%

Section: Agn Hydrogen Ionizing Emissivity and Photoionization Ratementioning

confidence: 99%

Faint AGNs atz>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe

Giallongo

Grazian

Fiore

et al. 2015

A&A

323

602

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“…(25) (iii) We convert the B -band magnitude from the Vega system to the AB system following Glikman et al (2010) MAB,B − MB = −0.09 (26) (iv) We extrapolate the B -band magnitude of which the effective wavelength is 4344Å (Blanton & Roweis 2007) to the 1450Å magnitude, assuming the quasar continuum between 1450 and 4344Å has a power-law slope of α q,optical = 0.44 relative to wavelength (Schirber & Bullock 2003). Thus…”

Section: Quasar Luminositymentioning

confidence: 99%

Dark-ages reionization and galaxy formation simulation – X. The small contribution of quasars to reionization

Qin

Mutch

Poole

et al. 2017

Monthly Notices of the Royal Astronomical Society

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Motivated by recent measurements of the number density of faint AGN at high redshift, we investigate the contribution of quasars to reionization by tracking the growth of central supermassive black holes in an update of the Meraxes semi-analytic model. The model is calibrated against the observed stellar mass function at z ∼ 0.6 − 7, the black hole mass function at z 0.5, the global ionizing emissivity at z ∼ 2 − 5 and the Thomson scattering optical depth. The model reproduces a Magorrian relation in agreement with observations at z < 0.5 and predicts a decreasing black hole mass towards higher redshifts at fixed total stellar mass. With the implementation of an opening angle of 80 deg for quasar radiation, corresponding to an observable fraction of ∼23.4 per cent due to obscuration by dust, the model is able to reproduce the observed quasar luminosity function at z ∼ 0.6 − 6. The stellar light from galaxies hosting faint AGN contributes a significant or dominant fraction of the UV flux. At high redshift, the model is consistent with the bright end quasar luminosity function and suggests that the recent faint z ∼ 4 AGN sample compiled by Giallongo et al. (2015) includes a significant fraction of stellar light. Direct application of this luminosity function to the calculation of AGN ionizing emissivity consequently overestimates the number of ionizing photons produced by quasars by a factor of 3 at z ∼ 6. We conclude that quasars are unlikely to make a significant contribution to reionization.

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“…Following Schirber & Bullock (2003) and Telfer et al (2002), we assume the spectral energy distribution for AGN takes a power law form:…”

Section: Source Model: Agnmentioning

confidence: 99%

Constraining the contribution of active galactic nuclei to reionization

Hassan

Davé

Mitra³

et al. 2017

Monthly Notices of the Royal Astronomical Society

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Recent results have suggested that active galactic nuclei (AGN) could provide enough photons to reionise the Universe. We assess the viability of this scenario using a seminumerical framework for modeling reionisation, to which we add a quasar contribution by constructing a Quasar Halo Occupation Distribution (QHOD) based on Giallongo et al. observations. Assuming a constant QHOD, we find that an AGN-only model cannot simultaneously match observations of the optical depth τ e , neutral fraction, and ionising emissivity. Such a model predicts τ e too low by ∼ 2σ relative to Planck constraints, and reionises the Universe at z < ∼ 5. Arbitrarily increasing the AGN emissivity to match these results yields a strong mismatch with the observed ionising emissivity at z ∼ 5. If we instead assume a redshift-independent AGN luminosity function yielding an emissivity evolution like that assumed in Madau & Haardt model, then we can match τ e albeit with late reionisation; however such evolution is inconsistent with observations at z ∼ 4 − 6 and poorly motivated physically. These results arise because AGN are more biased towards massive halos than typical reionising galaxies, resulting in stronger clustering and later formation times. AGN-dominated models produce larger ionising bubbles that are reflected in ∼ ×2 more 21cm power on all scales. A model with equal parts galaxies and AGN contribution is still (barely) consistent with observations, but could be distinguished using next-generation 21cm experiments HERA and SKA-low. We conclude that, even with recent claims of more faint AGN than previously thought, AGN are highly unlikely to dominate the ionising photon budget for reionisation.

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Faint Active Galactic Nuclei and the Ionizing Background

Cited by 77 publications

References 146 publications

Faint AGNs atz>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe

Faint AGNs atz>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe

Dark-ages reionization and galaxy formation simulation – X. The small contribution of quasars to reionization

Constraining the contribution of active galactic nuclei to reionization

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