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Session 118 - Radiogalaxies.
Oral session, Thursday, January 16
We present optical and infrared imaging polarimetry of a sample of ten high redshift (z>2) radio galaxies and one z\approx 1 radio galaxy. This investigation aims to elucidate the nature of the aligned continuum radiation in radio galaxies. The essential question is whether this component is starlight or scattered AGN light, and if the latter case is correct the variety of possible scattering media may be differentiated.
While no two sources appear distinctly similar in terms of polarization or morphology we can make several general statements about the sample's characteristics: polarizations are typically in the range of 10-30%, although two objects are show no significant (<5%) polarization, (2) rotations of the position angle with wavelength, P.A.(\lambda), are the norm for the sample, often as large as 90^\arcdeg, (3) the position angle generally is not tangential to the radio axis and, (4) there is no characteristic P(\lambda), some objects show an increase in polarization in the near-infrared, while others show a more or less flat P(\lambda).
With the exception of the two low polarization cases, we interpret these characteristics as resulting from the competition between two different mechanisms, scattering of beamed AGN light by dust, and polarization via extinction of this scattered light by aligned grains. This competition creates the observed rotation in position angle that allows us to strictly rule out electron scattering as the dominant polarization mechanism. The overall P(\lambda) is consistent with recent model results of dust scattering of beamed AGN light, which show a minimum in polarization around 4000ÅThe two component morphology observed is best explained by a scenario where two oppositely directed AGN beams of light are scattered into the line of sight by a dusty medium and obscured by a dusty torus near the core. We expect that with improved radio galaxy dust scattering models the dust properties in these objects may be constrained with polarimetric observations.
Program listing for Thursday