Mapping extreme-scale alignments of quasar polarization vectors

Abstract

Listen

Translate article

Based on a new sample of 355 quasars with significant optical p olarization and using complementary statistical methods, we confirm that quasar polarization vectors are not randomly oriented over the sky with a probability often in excess of 99.9%. The polarization vectors appear coherently oriented or aligned over huge (∼ 1 Gpc) regions of the sky located at both low (z∼ 0.5) and high (z∼ 1.5) redshifts and characterized by different preferred directions of the quasar polarization. In f act, there seems to exist a regular alternance along the line of si ght of regions of randomly and aligned polarization vectors with a typical comoving length scale of 1.5 Gpc. Furthermore, the mean polarization angle ¯ θ appears to rotate with redshift at the rate of∼ 30 ◦ per Gpc. The symmetry of the the ¯ θ− z relation is mirror-like, the mean polarization angle rotat ing clockwise with increasing redshift in North Galactic hemisphere and counter-clockwise in the South Galactic one. These characteristics make the alignment effect diffi cult to explain in terms of local mechanisms, namely a contamination by interstellar polarization in our Galaxy. While interpretations like a global rotation of the Universe can potentially explain the effect, the properties we observe qualitatively correspond to the dichroism and birefringence predicted by photon-pseudoscalar oscillation within a magnetic field. Interestingly, the alignment e ffect seems to be prominent along an axis not far from preferred directions tentatively identified in the Cosmic Microwave Background maps. Althoug h many questions and more particularly the interpretation of the effect remain open, alignments of quasar polarization vectors appear as a promising new way to probe the Universe and its dark components at extremely large scales.