5 Conclusions

In order to verify the existence of very large-scale coherent orientations of quasars polarization vectors, we have obtained new polarization measurements for quasars located in a given region of the sky where the range of polarization position angles was predicted in advance. The statistical analysis of this new sample provides an independent confirmation of the existence of alignments of quasar polarization vectors at high redshifts. In total, out of 29 polarized quasars located in this region, 25 have their polarization vectors coherently oriented. Moreover, global statistical tests applied to the whole sample indicate that, with the increased size of the sample, the detection of the effect is stable and even more significant. The increased size of the sample also allowed us to put some constraints on the phenomenon. We namely found that the polarization vectors of quasars in region A1 are apparently parallel to the plane of the Local Supercluster, while those of quasars at lower redshifts are not accordingly aligned. Furthermore, we found that the well-known correlations between quasar intrinsic properties and polarization also prevail for those quasars located in that region of alignment. Mechanisms modifying the polarization somewhere along the line of sight could explain some results, but cannot easily fit all constraints, namely the fact that correlations between quasar intrinsic properties and polarization are not washed away. Another possibility is that the quasar structural axes themselves are coherently oriented. However, given the scales involved, a reasonable physical mechanism is far from obvious. Interestingly, in region A1, nine quasars with aligned polarization vectors are radio-loud, such that this hypothesis could be tested by simply mapping their radio core. The interpretation of this orientation effect therefore remains puzzling. Nevertheless, the presence of coherent orientations at such large scales seems to indicate the existence of a new interesting effect of cosmological importance.

Acknowledgements

We are grateful to the referee for useful comments. The SIMBAD and NED databases have been consulted namely to classify some quasars in Table 2.