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Re-evaluating the local compton-thick AGN fraction

Re-evaluating the local compton-thick AGN fraction
Re-evaluating the local compton-thick AGN fraction
We study the NH distribution in a complete sample of 88 AGN selected in the 20-40 keV band from INTEGRAL/IBIS observations. We find that the fraction of absorbed (NH > 1022 cm-2) sources is 43% while Compton thick AGN comprise 7% of the sample. While these estimates are fully compatible with previous soft gamma-ray surveys, they would appear to be in contrast with results reported from an optically selected sample. This apparent difference can be explained as being due to a selection bias caused by the reduction in high energy flux in Compton thick objects rendering them invisible at our sensitivity limit. Taking this into account we estimate that the fraction of highly absorbed sources is actually in close agreement with the optically selected sample. Furthermore we show that the measured fraction of absorbed sources in our sample decreases from 80% to ~20-30% as a function of redshift with all Compton thick AGN having z<0.015. We conclude that in the low redshift bin we are seeing almost the entire AGN population, from unabsorbed to at least mildly Compton thick objects, while in the total sample we lose the heavily absorbed 'counterparts' of distant and therefore dim sources with little or no absorption. Taking therefore this low z bin as the only one able to provide the 'true' distribution of absorption in type 1 and 2 AGN, we estimate the fraction of Compton thick objects to be >24%.

1824-8039
1-8
Malizia, A.
efa63850-cf26-482c-ae3b-54b0c8fb8a87
Stephen, J.B.
c0b1afce-f3d9-4568-b2c1-51482f80cd9a
Bassani, L.
07e5602c-f97e-4334-a0fa-c2212431a61a
Bird, Anthony J.
045ee141-4720-46fd-a412-5aa848a91b32
Panessa, F.
ea57d0af-2ad5-4f65-a8f7-b001e70ebe13
Ubertini, P.
7035748e-152c-473e-8766-a2fa3ac49fcb
Malizia, A.
efa63850-cf26-482c-ae3b-54b0c8fb8a87
Stephen, J.B.
c0b1afce-f3d9-4568-b2c1-51482f80cd9a
Bassani, L.
07e5602c-f97e-4334-a0fa-c2212431a61a
Bird, Anthony J.
045ee141-4720-46fd-a412-5aa848a91b32
Panessa, F.
ea57d0af-2ad5-4f65-a8f7-b001e70ebe13
Ubertini, P.
7035748e-152c-473e-8766-a2fa3ac49fcb

Malizia, A., Stephen, J.B., Bassani, L., Bird, Anthony J., Panessa, F. and Ubertini, P. (2010) Re-evaluating the local compton-thick AGN fraction. Proceedings of Science, 1-8. (doi:10.22323/1.096.0012).

Record type: Article

Abstract

We study the NH distribution in a complete sample of 88 AGN selected in the 20-40 keV band from INTEGRAL/IBIS observations. We find that the fraction of absorbed (NH > 1022 cm-2) sources is 43% while Compton thick AGN comprise 7% of the sample. While these estimates are fully compatible with previous soft gamma-ray surveys, they would appear to be in contrast with results reported from an optically selected sample. This apparent difference can be explained as being due to a selection bias caused by the reduction in high energy flux in Compton thick objects rendering them invisible at our sensitivity limit. Taking this into account we estimate that the fraction of highly absorbed sources is actually in close agreement with the optically selected sample. Furthermore we show that the measured fraction of absorbed sources in our sample decreases from 80% to ~20-30% as a function of redshift with all Compton thick AGN having z<0.015. We conclude that in the low redshift bin we are seeing almost the entire AGN population, from unabsorbed to at least mildly Compton thick objects, while in the total sample we lose the heavily absorbed 'counterparts' of distant and therefore dim sources with little or no absorption. Taking therefore this low z bin as the only one able to provide the 'true' distribution of absorption in type 1 and 2 AGN, we estimate the fraction of Compton thick objects to be >24%.

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Submitted date: 25 February 2010
Published date: 11 June 2010
Additional Information: Imported from arXiv

Identifiers

Local EPrints ID: 144751
URI: http://eprints.soton.ac.uk/id/eprint/144751
ISSN: 1824-8039
PURE UUID: c70010ab-3f2f-469c-8d4f-f3c609ac98fb
ORCID for Anthony J. Bird: ORCID iD orcid.org/0000-0002-6888-8937

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Date deposited: 24 May 2010 12:23
Last modified: 20 Nov 2021 02:37

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Contributors

Author: A. Malizia
Author: J.B. Stephen
Author: L. Bassani
Author: Anthony J. Bird ORCID iD
Author: F. Panessa
Author: P. Ubertini

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