Journal Article

An <i>XMM–Newton</i> spectral survey of 12 μm selected galaxies – II. Implications for AGN selection and unification

Murray Brightman and Kirpal Nandra

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 414, issue 4, pages 3084-3104
Published in print July 2011 | ISSN: 0035-8711
Published online July 2011 | e-ISSN: 1365-2966 | DOI:
An XMM–Newton spectral survey of 12 μm selected galaxies – II. Implications for AGN selection and unification

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We present a multi-waveband analysis of a 126-galaxy sub-sample of the 12 μm galaxy sample (12MGS), for which we have carried out a detailed X-ray spectral analysis in a previous paper. We determine the activity class of the galaxies by way of optical-line ratio diagnostics and characterize the optical classes by their X-ray, 12 μm and [O iii] luminosities and X-ray spectral properties. Our most interesting results from this investigation are as follows. (i) Seyfert (Sy) 1s and Sy2s show a significantly different X-ray luminosity distributions from each other. (ii) Sy2 galaxies with a detection of a hidden broad-line region show a significantly higher X-ray luminosity than those without a detection, supporting the findings of Tran. (iii) Sy1s also present a significantly different 12 μm luminosity distribution from both intermediate Sy types and Sy2s. (iv) The Sy2 fraction decreases towards high X-ray luminosities. (v) X-ray indications of active galactic nuclei (AGN) power agree well with the optical classifications. (vi) There is X-ray evidence for the presence of an AGN in 17 per cent of H ii/AGN composite galaxies and 40 per cent of LINERs. (vii) We advocate the use of a 2–10 keV X-ray luminosity of 1041 erg s−1 in the X-ray selection of AGN, rather than 1042 erg s−1, which we find gives a contamination rate of only 3 per cent from star-forming galaxies. (viii) From an analysis of the X-ray power-law index, Γ, we find that Sy1s and Sy2 have the same intrinsic distributions, implying that the central engines are the same, in support of AGN unification schemes. (ix) In 24 per cent of cases the absorption measured in the X-ray spectra does not correspond directly to that implied in the optical band from the visibility of the broad-line regions (BLRs), which is in conflict with AGN unification schemes. (x) We confirm the previous work showing that the obscured fraction in AGN declines at high X-ray luminosity, but also find a decrease at low luminosity having peaked at LX∼ 1042 erg s−1, suggesting that source luminosity has a large effect on the obscuring material, therefore also calling for a modification to unified schemes. (xi) The average obscured and Compton-thick fractions for this sample are 62 ± 5 per cent and 20 ± 4 per cent, respectively, which are higher than hard X-ray and optically selected samples, therefore supporting mid-infrared (MIR) selection as a relatively unbiased method of selecting AGN. (xii) We assess the use of the ‘T’ ratio () for selecting Compton-thick candidates. We conclude here that this quantity can often be unreliable due to uncertainties in the extinction corrections to the [O iii] flux. These results have important impacts on AGN selection and unification, and the results from the 12MGS are particularly useful as a local analogue to Spitzer/MIPS 24 μm samples selected at z= 1, as observed 24 μm emission originates at rest-frame 12 μm in sources at this redshift.

Keywords: galaxies: active; galaxies: Seyfert; infrared: galaxies; X-rays: galaxies

Journal Article.  13140 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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