A&A 394, 435-442 (2002)
DOI: 10.1051/0004-6361:20021161
Unabsorbed Seyfert 2 galaxies
F. Panessa1, 2 and L. Bassani11 IASF/CNR, via Piero Gobetti 101, 40129 Bologna, Italy
2 Dipartimento di Astronomia, Universita' di Bologna, via Ranzani 1, 40127 Bologna, Italy
(Received 26 March 2002 / Accepted 5 August 2002)
Abstract
We present a sample of 17 type 2 Seyfert galaxies
which have an X-ray column density lower than 10
22 cm
-2.
The Compton thin nature of these sources is strongly suggested by
isotropic indicators. We estimate the fraction of these sources
to be in the range of 10%-30% of the population of
type 2 Seyfert galaxies. Furthermore,
this fraction appears to increase progressively at lower luminosities.
The simple formulation of the Unified Model for Seyfert galaxies
is not applicable in such sources since the pc-scale molecular torus is not
likely to be responsible for the low column density observed;
instead the absorption observed is likely to originate at larger scales.
According to this hypothesis, in these objects the broad line
regions are covered by some dusty obscuring material.
In particular, this could occur in objects
with dust lanes, patches or HII regions. However, we cannot rule out that
in the lowest luminosity sources the BLR is weak, absent or has faded away.
This last scenario is consistent with the predictions of some recent
theoretical models for low luminosity AGNs.
Key words: X-rays: galaxies -- galaxies: Seyfert
Offprint request: F. Panessa, panessa@bo.iasf.cnr.it
© ESO 2002