Contribution à l’Etude du Modèle d’Unification des Quasars à l’Aide de la Polarimétrie et de l’Effet de Microlentille Gravitationnelle

Quasars are among the most luminous and the most distant objects in the Universe. Consequently they are particularly interesting to probe its originand to understand its evolution. However, the huge distances at which theseobjects are generally found prevent us from resolving their central regionsso that we cannot directly check the validity of the geometrical as well asthe dynamical models accounting for their observational properties (spectralenergy distribution, line profiles, presence or absence of radio jets etc). Inour thesis, we use two indirect observational techniques in order to constrainthe existing models.These techniques which are particularly sensitive to the geometrical structureof the quasar emission regions are polarimetry and gravitational microlensing.In the first part of our thesis we study the correlation betweenthe direction of the linear polarization and the orientation of the host galaxy/extended emission that we determined on the basis of high resolutionHST images. We show how this study enables us to bring new clues favoringthe existence of a unification model for the Type 1 and Type 2 quasars.In the second part, we show how gravitational microlensing allows toconstrain the geometry and size of the regions at the origin of the broadabsorption lines observed in the spectrum of 10 to 20 % of quasars. Forthis purpose we build a radiative transfer code allowing to simulate the lineprofiles produced in a variety of realistic wind models. These models arethen used to study the variations of line profiles induced by the transit of agravitational microlens. This technique is finally applied to the case of thequasar H1413+117 in order to determine the geometry of the regions whichproduce the broad absorption lines

Evidence of a type 1/type 2 dichotomy in the correlation between quasar optical polarization and host-galaxy/extended emission position angles

peer reviewedAims.For Seyfert galaxies, the AGN unification model provides a simple and well-established explanation of the type 1/type 2 dichotomy through orientation-based effects. The generalization of this unification model to the higher luminosity AGNs that quasars are remains a key question. The recent detection of type 2 radio-quiet quasars seems to support such an extension. We propose a further test of this scenario. Methods: On the basis of a compilation of quasar host-galaxy position angles consisting of previously published data and of new measurements performed using HST Archive images, we investigate the possible existence of a correlation between the linear polarization position angle and the host-galaxy/extended emission position angle of quasars. Results: We find that the orientation of the rest-frame UV/blue extended emission is correlated to the direction of the quasar polarization. For type 1 quasars, the polarization is aligned with the extended UV/blue emission, while these two quantities are perpendicular in type 2 objects. This result is independent of the quasar radio loudness. We interpret this (anti-)alignment effect in terms of scattering in a two-component polar+equatorial model that applies to both type 1 and type 2 objects. Moreover, the orientation of the polarization -and then of the UV/blue scattered light- does not appear correlated to the major axis of the stellar component of the host-galaxy measured from near-IR images

New Insights into the Quasar Type 1/Type 2 Dichotomy from Correlations between Quasar Host Orientation and Polarization

We investigate correlations between the direction of the optical linear polarization and the orientation of the host galaxy/extended emission for type 1 and type 2 radio-loud and radio-quiet quasars. We have used high resolution Hubble Space Telescope data and a deconvolution process to obtain a good determination of the host galaxy/extended emission (EE) position angle. With these new measurements and a compilation of data from the literature, we find a significant correlation, different for type 1 and type 2 objects, between the linear polarization position angle and the orientation of the EE, suggesting scattering by an extended UV/blue region in both types of objects. Our observations support the extension of the Unification Model to the higher luminosity AGNs like the quasars, assuming a two component scattering model

QSO Type1/Type2 dichotomy (Borguet+, 2008)

This table contains the measurements/compilation of host galaxy/ extended emission position angle in the visible and near-IR domain, polarimetric data, and radio axis orientation for the 135 quasars of our sample. The angles are given in degrees east of north. The table gives the spectral and the radio-loud/radio-quiet classification of each object according to the references. (2 data files)

Large-Scale Alignments of Quasar Polarization Vectors: Evidence at Cosmological Scales for Very Light Pseudoscalar Particles Mixing with Photons ?

Based on a sample of 355 quasars with significant optical polarization, we found that quasar polarization vectors are not randomly oriented over the sky as naturally expected. The probability that the observed distribution of polarization angles is due to chance is lower than 0.1%. The polarization vectors of the light from quasars are aligned although the sources span huge regions of the sky (˜ 1 Gpc). Groups of quasars located along similar lines of sight but at different redshifts (typically z ≍ 0.5 and z ≍ 1.5) are characterized by different preferred directions of polarization. These characteristics make the observed alignment effect difficult to explain in terms of a local contamination by interstellar polarization in our Galaxy. Interpreted in terms of a cosmological-size effect, we show that the dichroism and birefringence predicted by a mixing between photons and very light pseudoscalar particles within a magnetic field can qualitatively reproduce the observations. We find that circular polarization measurements could help constrain this mechanism