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The X-ray luminosity function of active galactic nuclei in the redshift interval z=3-5
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Georgakakis, A., Aird, J., Buchner, J., Salvato, M., Menzel, M-L., Brandt, W. N., McGreer, I. D., Dwelly, T., Mountrichas, G., Koki, Constandina, Georgantopoulos, I., Hsu, L.-T., Merloni, A., Liu, Z., Nandra, K. and Ross, N. P. (2015) The X-ray luminosity function of active galactic nuclei in the redshift interval z=3-5. Monthly Notices of the Royal Astronomical Society, 453 (2). pp. 1946-1964. doi:10.1093/mnras/stv1703 ISSN 0035-8711.
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Official URL: http://dx.doi.org/10.1093/mnras/stv1703
Abstract
We combine deep X-ray survey data from the Chandra observatory and the wide-area/shallow XMM-XXL field to estimate the active galactic nuclei (AGN) X-ray luminosity function in the redshift range z = 3–5. The sample consists of nearly 340 sources with either photometric (212) or spectroscopic (128) redshift in the above range. The combination of deep and shallow survey fields also provides a luminosity baseline of three orders of magnitude, LX(2–10 keV) ≈ 1043–1046 erg s− 1 at z > 3. We follow a Bayesian approach to determine the binned AGN space density and explore their evolution in a model-independent way. Our methodology properly accounts for Poisson errors in the determination of X-ray fluxes and uncertainties in photometric redshift estimates. We demonstrate that the latter is essential for unbiased measurement of space densities. We find that the AGN X-ray luminosity function evolves strongly between the redshift intervals z = 3–4 and z = 4–5. There is also suggestive evidence that the amplitude of this evolution is luminosity dependent. The space density of AGN with LX(2–10 keV) < 1045 erg s− 1 drops by a factor of 5 between the redshift intervals above, while the evolution of brighter AGN appears to be milder. Comparison of our X-ray luminosity function with that of ultraviolet (UV)/optical selected quasi-stellar objects at similar redshifts shows broad agreement at bright luminosities, LX(2–10 keV) > 1045 erg s− 1. At fainter luminosities X-ray surveys measure higher AGN space densities. The faint-end slope of UV/optical luminosity functions, however, is steeper than for X-ray selected AGN. This implies that the Type I AGN fraction increases with decreasing luminosity at z > 3, opposite to trends established at lower redshift. We also assess the significance of AGN in keeping the hydrogen ionized at high redshift. Our X-ray luminosity function yields ionizing photon rate densities that are insufficient to keep the Universe ionized at redshift z > 4. A source of uncertainty in this calculation is the escape fraction of UV photons for X-ray selected AGN.
Item Type: | Journal Article | ||||||
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Divisions: | Faculty of Science, Engineering and Medicine > Science > Mathematics | ||||||
Journal or Publication Title: | Monthly Notices of the Royal Astronomical Society | ||||||
Publisher: | Royal Astronomical Society | ||||||
ISSN: | 0035-8711 | ||||||
Official Date: | 21 October 2015 | ||||||
Dates: |
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Volume: | 453 | ||||||
Number: | 2 | ||||||
Page Range: | pp. 1946-1964 | ||||||
DOI: | 10.1093/mnras/stv1703 | ||||||
Status: | Peer Reviewed | ||||||
Publication Status: | Published | ||||||
Description: | Free access |
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