EDP Sciences
Free Access
Volume 444, Number 1, December II 2005
Page(s) 119 - 132
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20053643

A&A 444, 119-132 (2005)
DOI: 10.1051/0004-6361:20053643

X-ray obscuration and obscured AGN in the local universe

M. Guainazzi1, G. Matt2 and G. C. Perola2

1  XMM-Newton Science Operations Center, European Space Astronomy Center, ESA, Apartado 50727, 28080 Madrid, Spain
    e-mail: mguainaz@xmm.vilspa.esa.es
2  Università degli Studi "Roma Tre", Via della Vasca Navale 84, 00146, Roma, Italy

(Received 16 June 2005 / Accepted 4 August 2005)

We discuss the X-ray properties of 49 local (z<0.035) Seyfert 2 galaxies with HST/WFC2 high-resolution optical coverage. It includes the results of 26 still unpublished Chandra and XMM-Newton observations, which yield 25 (22) new X-ray detections in the 0.5-2 keV (2-10 keV) energy band. Our sample covers a range in the 2-10 keV observed flux, F2-10, from $3 \times 10^{-11}$ to $6 \times 10^{-15}$ erg cm-2 s-1. The percentage of objects that are likely obscured by Compton-thick matter (column density, $N_{\rm H} \ge \sigma_t^{-1}
\simeq 1.6 \times 10^{24}$ cm-2) is $\simeq$50%, and reaches $\simeq$80% for $\log\, (F_{2-10}) < 12.3$. Hence, K$_{\alpha}$ fluorescent iron lines with large Equivalent Width ( ${\it EW} > 0.6$ keV) are common in our sample (6 new detections at a confidence level $\ge$2$\sigma$). They are explained as due to reflection off the illuminated side of optically thick material. We confirm a correlation between the presence of a ~100-pc scale nuclear dust in the WFC2 images and Compton-thin obscuration. We interpret this correlation as due to the large covering fraction of gas associated with the dust lanes. The X-ray spectra of highly obscured AGN invariably present a prominent soft excess emission above the extrapolation of the hard X-ray component. This soft component can account for a very large fraction of the overall X-ray energy budget. As this component is generally unobscured - and therefore likely produced in extended gas structures - it may lead to a severe underestimation of the nuclear obscuration in $z \sim 1$ absorbed AGN, if standard X-ray colors are used to classify them. As a by-product of our study, we report the discovery of a soft X-ray, luminous ($\simeq$7 $\,\times\, 10^{40}$ erg s-1) halo embedding the interacting galaxy pair Mkn 266.

Key words: galaxies: active -- galaxies: nucleus -- galaxies: Seyfert -- X-ray: galaxies

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