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The fraction of compton-thick sources in an integral complete AGN sample

The fraction of compton-thick sources in an integral complete AGN sample
The fraction of compton-thick sources in an integral complete AGN sample
We study the NH distribution in a complete sample of 88 active galactic nuclei (AGN) selected in the 20-40 keV band from INTEGRAL/Imager on Board the Integral Satellite (IBIS) observations. We find that the fraction of absorbed (NH ≥ 1022 cm2) sources is 43 per cent while the Compton thick AGN comprise 7 per cent 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 by Risaliti, Maiolino & Salvati using 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 per cent as a function of redshift with all Compton thick AGN having z ≤ 0.015 .

If we limit our analysis to this distance and compare only the type 2 objects in our sample with the Risaliti et al. objects below this redshift value, we find a perfect match to their NH distribution. 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 types 1 and 2 AGN, we estimate the fraction of Compton thick objects to be ≥24 per cent.
galaxies, active, nuclei, x-rays
0035-8711
944-951
Malizia, A.
efa63850-cf26-482c-ae3b-54b0c8fb8a87
Stephen, J.B.
c0b1afce-f3d9-4568-b2c1-51482f80cd9a
Bassani, L.
07e5602c-f97e-4334-a0fa-c2212431a61a
Bird, A.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, A.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, A.J., Panessa, F. and Ubertini, P. (2009) The fraction of compton-thick sources in an integral complete AGN sample. Monthly Notices of the Royal Astronomical Society, 399 (2), 944-951. (doi:10.1111/j.1365-2966.2009.15330.x).

Record type: Article

Abstract

We study the NH distribution in a complete sample of 88 active galactic nuclei (AGN) selected in the 20-40 keV band from INTEGRAL/Imager on Board the Integral Satellite (IBIS) observations. We find that the fraction of absorbed (NH ≥ 1022 cm2) sources is 43 per cent while the Compton thick AGN comprise 7 per cent 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 by Risaliti, Maiolino & Salvati using 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 per cent as a function of redshift with all Compton thick AGN having z ≤ 0.015 .

If we limit our analysis to this distance and compare only the type 2 objects in our sample with the Risaliti et al. objects below this redshift value, we find a perfect match to their NH distribution. 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 types 1 and 2 AGN, we estimate the fraction of Compton thick objects to be ≥24 per cent.

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More information

Published date: October 2009
Keywords: galaxies, active, nuclei, x-rays
Organisations: Astronomy and Space Science

Identifiers

Local EPrints ID: 144775
URI: http://eprints.soton.ac.uk/id/eprint/144775
ISSN: 0035-8711
PURE UUID: 7c3452de-349f-417c-901b-49e348737919
ORCID for A.J. Bird: ORCID iD orcid.org/0000-0002-6888-8937

Catalogue record

Date deposited: 24 May 2010 14:26
Last modified: 26 Nov 2021 02:36

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Contributors

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

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