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# Résumé du preprint DAPNIA/SAp/00-30

SAp/00-30
The optical polarization of spiral galaxies
John F. L. Simmons, Edouard Audit
Abstract:
========
Scattering of starlight  by dust, molecules and  electrons in spiral galaxies
will produce a modification of the direct intensity and a polarization in the
observed light. We treat the case where the distribution of scatterers can be
considered to be optically thin, and derive semi-analytic expressions for the
resolved  intensity and polarized intensity  for  Thomson, Rayleigh, and more
general scattering mechanisms. These  expressions are applied to a parametric
model spiral galaxies.  It is further  shown that in  the case of Thomson and
Rayleigh scattering, and   when  scatterers and  stars are  distributed  with
rotational  symmetry, the total   polarized flux depends  on the inclination,
$i$, of the galactic axis to the line of sight according to a simple $\sin ^2 i$ law.   This generalises the well  known result for pointlike and spherical
light sources. By using a method based  on spherical harmonics, we generalise
this law  for more general mechanisms, and  show that  to good approximation,
the $\sin ^2 i$ law still holds for the class of models considered.