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Characterizing the far-infrared properties of distant X-ray detected AGNs: evidence for evolution in the infrared–X-ray luminosity ratio

Mullaney, J. R. and Alexander, D. M. and Huynh, M. and Goulding, A. D. and Frayer, D. (2010) Characterizing the far-infrared properties of distant X-ray detected AGNs: evidence for evolution in the infrared–X-ray luminosity ratio. Monthly Notices of the Royal Astronomical Society, 401 (2). pp. 995-1012. ISSN 0035-8711.

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We investigate the far-infrared (FIR) properties of X-ray sources detected in the Chandra Deep Field-South (CDF-S) survey using the ultradeep 70 and 24 μm Spitzer observations taken in this field. Since only 30 (i.e. ≈ 10 per cent) of the 266 X-ray sources in the region of the 70 μm observations are detected at 70 μm, we rely on stacking analyses of the 70 μm data to characterize the average 70 μm properties of the X-ray sources as a function of redshift, X-ray luminosity and X-ray absorption. Using Spitzer-IRS data of the Swift-Burst Alert Telescope (BAT) sample of z ≈ 0 active galactic nuclei (AGNs), we show that the 70/24 μm flux ratio can distinguish between AGN-dominated and starburst-dominated systems out to z ≈ 1.5 . Among the X-ray sources detected at 70 μm, we note a large scatter in the observed 70/24 μm flux ratios, spanning almost a factor of 10 at similar redshifts, irrespective of object classification, suggesting a range of AGN:starburst ratios. From stacking analyses we find that the average observed 70/24 μm flux ratios of AGNs out to an average redshift of 1.5 are similar to z ≈ 0 AGNs with similar X-ray luminosities (L_X = 10^(42-44) erg s^(−1)) and absorbing column densities (N_H ≤ 10^(23) cm^(−2)) . Furthermore, both high-redshift and z ≈ 0 AGNs follow the same tendency towards warmer 70/24 μm colours with increasing X-ray luminosity (LX). From analyses of the Swift-BAT sample of z ≈ 0 AGNs, we note that the 70 μm flux can be used to determine the IR (8–1000 μm) luminosities of high-redshift AGNs. We use this information to show that L_X = 10^(42-43) erg s^(−1) AGNs at high redshifts (z = 1–2) have IR to X-ray luminosity ratios (L_(IR)/L_X) that are, on average, 4.7^(+10.2)_(−2.0) and 12.7+7.1−2.6 times higher than AGNs with similar X-ray luminosities at z = 0.5–1 and ≈0, respectively. By comparison, we find that the L_(IR)/L_X ratios of L_X= 10^(43-44) erg s^(−1) AGNs remain largely unchanged across this same redshift interval. We explore the consequences that these results may have on the identification of distant, potentially Compton-thick AGNs using L_(IR)/L_X ratios. In addition, we discuss possible scenarios for the observed increase in the L_(IR)/L_X ratio with redshift, including changes in the dust covering factor of AGNs and/or the star formation rates of their host galaxies. Finally, we show how deep observations to be undertaken by the Herschel Space Observatory will enable us to discriminate between these proposed scenarios and also identify Compton-thick AGNs at high redshifts.

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Additional Information:© 2009 The Authors. Journal compilation © 2009 RAS. Accepted 2009 September 18; received 2009 September 18; in original form 2009 July 3. We would like to thank David Elbaz for his useful comments on the paper and for providing us with the PACS filter response curves. We would also like to thank Chris Done for her useful comments on the bolometric luminosities of X-ray detected AGNs. Furthermore, we would like to thank the anonymous referee for their comments, particularly those concerning the X-ray to IR matching procedure. This work is based (in part) on observations made with the Spitzer Space Telescope and has made use of the NASA/IPAC Infrared Science Archive, which are both operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. We gratefully acknowledge support from the Leverhulme Trust (JRM; DMA) and the Royal Society (DMA).
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Leverhulme TrustUNSPECIFIED
Subject Keywords:galaxies: active; galaxies: high-redshift; infrared: galaxies; X-rays: galaxies
Record Number:CaltechAUTHORS:20100208-152238883
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17422
Deposited By: Jason Perez
Deposited On:09 Feb 2010 20:59
Last Modified:25 Mar 2014 19:29

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