Issue |
A&A
Volume 465, Number 1, April I 2007
|
|
---|---|---|
Page(s) | 129 - 145 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20053555 | |
Published online | 02 January 2007 |
Modeling optical and UV polarization of AGNs
I. Imprints of individual scattering regions
1
Astronomical Institute of the Academy of Sciences, Bočni II 1401, 14131 Prague, Czech Republic e-mail: goosmann@astro.cas.cz
2
Observatoire de Paris – Meudon, 5 place Jules Janssen, 92190 Meudon, France
3
Department of Physics & Astronomy, University of Nebraska, Lincoln, NE 68588-0111, USA e-mail: mgaskell1@unl.edu
Received:
2
June
2005
Accepted:
22
December
2006
Context.Spectropolarimetry of AGNs is a powerful tool for studying the structure and kinematics of the inner regions of quasars.
Aims.We wish to investigate the effects of various AGN scattering region geometries on the polarized flux.
Methods.We introduce a new, publicly available Monte Carlo radiative transfer code, stokes, which models polarization induced by scattering off free electrons and dust grains. We model a variety of regions in AGNs.
Results.We find that the shape of the funnel of
the dusty torus has a significant impact on the polarization
efficiency. A compact torus with a steep inner surface scatters more
light toward type-2 viewing angles than a large torus of the same
half-opening angle, . For
, the
scattered light is polarized perpendicularly to the symmetry axis,
whilst for
it is polarized parallel to the
symmetry axis. In between these intervals the orientation of the
polarization depends on the viewing angle. The degree of polarization
ranges between 0% and 20% and is wavelength independent for a large
range of
. Observed wavelength-independent optical and
near-UV polarization thus does not necessarily imply electron
scattering. Spectropolarimetry at rest-frame wavelengths less than
2500 Å may distinguish between dust and electron scattering
but is not conclusive in all cases. For polar dust, scattering
spectra are reddened for type-1 viewing angles, and made bluer for
type-2 viewing angles. Polar electron-scattering cones are very
efficient polarizers at type-2 viewing angles, whilst the polarized
flux of the torus is weak.
Conclusions.We predict that the net polarization of Seyfert-2 galaxies decreases with luminosity, and conclude that the degree of polarization should be correlated with the relative strength of the thermal IR flux. We find that a flattened, equatorial, electron-scattering disk, of relatively low optical depth, reproduces type-1 polarization. This is insensitive to the exact geometry, but the observed polarization requires a limited range of optical depth.
Key words: galaxies: active / polarization / radiative transfer / scattering
© ESO, 2007
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.