Preprint Logo P62 December

To be published in:
Astronomy & Astrophysics

Confirmation of the existence of coherent orientations of quasar polarization vectors on cosmological scales+.

D. Hutsemékers1,2,* and H. Lamy2

1 European Southern Observatory, Casilla 19001, Santiago, Chile
2 Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium

* Chercheur Qualifié FNRS, Belgium

+ Based in part on observations collected at the European Southern Observatory (ESO, La Silla)


     In order to verify the existence of coherent orientations of quasar polarization vectors on very large scales, we have obtained new polarization measurements for a sample of quasars located in a given region of the three-dimensional Universe, where the range of polarization position angles was predicted in advance. For this new sample, the hypothesis of uniform distribution of polarization position angles may be rejected at the 1.8% significance level on the basis of a simple binomial test. This result provides an independant confirmation of the existence of alignments of quasar polarization vectors on very large scales. In total, out of 29 polarized quasars located in this region of the sky, 25 have their polarization vectors coherently oriented. This alignment occurs at redshifts z~1-2 suggesting the presence of correlations in objects or fields on Gpc scales. More global statistical tests applied to the whole sample of polarized quasars distributed all over the sky confirm that polarization vectors are coherently oriented in a few groups of 20-30 quasars. Some constraints on the phenomenom are also derived. Considering more particularly the quasars in the selected region of the sky, we found that their polarization vectors are roughly parallel to the plane of the Local Supercluster. But the polarization vectors of objects along the same line of sight at lower redshifts are not accordingly aligned. We also found that the known correlations between quasar intrinsic properties and polarization are not destroyed by the alignment effect. Several possible mechanisms are discussed, but the interpretation of this orientation effect remains puzzling.

Key Words
Cosmology - large scale structure of the Universe - Quasars: general - Polarization

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