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Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys

Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys
Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys
The cosmological process of hydrogen (H i) reionization in the intergalactic medium is thought to be driven by UV photons emitted by star-forming galaxies and ionizing active galactic nuclei (AGN). The contribution of quasars (QSOs) to H i reionization at z > 4 has been traditionally believed to be quite modest. However, this view has been recently challenged by new estimates of a higher faint-end UV luminosity function (LF). To set firmer constraints on the emissivity of AGN at z < 6, we here make use of complete X-ray-selected samples including deep Chandra and new Cosmic Evolution Survey data, capable to efficiently measure the 1 Ryd comoving AGN emissivity up to z ∼ 5–6 and down to 5 mag fainter than probed by current optical surveys, without any luminosity extrapolation. We find good agreement between the logNH ≲ 21-22 cm−2 X-ray LF and the optically selected QSO LF at all redshifts for M1450 ≤ −23. The full range of the logNH ≲ 21-22 cm−2 LF (M1450 ≤ −17) was then used to quantify the contribution of AGN to the critical value of photon budget needed to keep the Universe ionized. We find that the contribution of ionizing AGN at z = 6 is as small as 1–7 per cent, and very unlikely to be greater than 30 per cent, thus excluding an AGN-dominated reionization scenario.
0035-8711
1915-1925
Ricci, Federica
8d69561c-2eb1-4af1-84d7-1c13286f32d5
Marchesi, Stefano
82bbf999-4609-4e6a-a7e2-422135c09f51
Shankar, Francesco
b10c91e4-85cd-4394-a18a-d4f049fd9cdb
La Franca, Fabio
abbb3d3c-c4e3-4e73-920a-c6c183698ed2
Civano, Francesca
c5bd5318-f93c-4458-af95-adb4d60d04c5
Ricci, Federica
8d69561c-2eb1-4af1-84d7-1c13286f32d5
Marchesi, Stefano
82bbf999-4609-4e6a-a7e2-422135c09f51
Shankar, Francesco
b10c91e4-85cd-4394-a18a-d4f049fd9cdb
La Franca, Fabio
abbb3d3c-c4e3-4e73-920a-c6c183698ed2
Civano, Francesca
c5bd5318-f93c-4458-af95-adb4d60d04c5

Ricci, Federica, Marchesi, Stefano, Shankar, Francesco, La Franca, Fabio and Civano, Francesca (2017) Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys. Monthly Notices of the Royal Astronomical Society, 465 (2), 1915-1925. (doi:10.1093/mnras/stw2909).

Record type: Article

Abstract

The cosmological process of hydrogen (H i) reionization in the intergalactic medium is thought to be driven by UV photons emitted by star-forming galaxies and ionizing active galactic nuclei (AGN). The contribution of quasars (QSOs) to H i reionization at z > 4 has been traditionally believed to be quite modest. However, this view has been recently challenged by new estimates of a higher faint-end UV luminosity function (LF). To set firmer constraints on the emissivity of AGN at z < 6, we here make use of complete X-ray-selected samples including deep Chandra and new Cosmic Evolution Survey data, capable to efficiently measure the 1 Ryd comoving AGN emissivity up to z ∼ 5–6 and down to 5 mag fainter than probed by current optical surveys, without any luminosity extrapolation. We find good agreement between the logNH ≲ 21-22 cm−2 X-ray LF and the optically selected QSO LF at all redshifts for M1450 ≤ −23. The full range of the logNH ≲ 21-22 cm−2 LF (M1450 ≤ −17) was then used to quantify the contribution of AGN to the critical value of photon budget needed to keep the Universe ionized. We find that the contribution of ionizing AGN at z = 6 is as small as 1–7 per cent, and very unlikely to be greater than 30 per cent, thus excluding an AGN-dominated reionization scenario.

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Accepted/In Press date: 8 November 2016
e-pub ahead of print date: 10 November 2016
Published date: 21 February 2017
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 402946
URI: http://eprints.soton.ac.uk/id/eprint/402946
ISSN: 0035-8711
PURE UUID: 8f80ea29-d6dd-43db-b584-6d4f70722df3

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Date deposited: 18 Nov 2016 15:33
Last modified: 26 Nov 2021 06:02

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Contributors

Author: Federica Ricci
Author: Stefano Marchesi
Author: Fabio La Franca
Author: Francesca Civano

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