Mapping extreme-scale alignments of quasar polarization vectors

Abstract

Based on a new sample of 355 quasars with significant optical polarization 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 fact, there seems to exist a regular alternance along the line of sight of regions of randomly and aligned polarization vectors with a typical comoving length scale of 1.5 Gpc. Furthermore, the mean polarization angleθ appears to rotate with redshift at the rate of ∼30• per Gpc. The symmetry of the theθ − z relation is mirror-like, the mean polarization angle rotating clockwise with increasing redshift in North Galactic hemisphere and counter-clockwise in the South Galactic one. These characteristics make the alignment effect difficult 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 effect seems to be prominent along an axis not far from preferred directions tentatively identified in the Cosmic Microwave Background maps. Although 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.

Optical circular polarization in quasars

“…We first modeled the distribution of the debiased polarization degree in the full quasar sample (from Hutsemékers et al 2005, including objects with p < 0.6%). We found that it was reasonably well reproduced by a half-gaussian distribution of zero mean and unitary variance.…”

“…To interpret the large-scale alignments of quasar optical polarization vectors observed at redshifts z ∼ 1 (Hutsemékers 1998;Hutsemékers & Lamy 2001;Hutsemékers et al 2005) polarization induced by photon-pseudoscalar mixing along the line of sight has been invoked (Hutsemékers 1998;Jain et al 2002). Photon-pseudoscalar mixing generates dichroism and birefringence, the latter transforming linear polarization into circular polarization and vice-versa along the line of sight.…”

Constraints on anisotropic cosmic expansion from supernovae

“…They found out that the low redshift regime (z < 0.06) of the data in the Union 2 catalogue (Amanullah et al 2010) is barely consistent with ΛCDM at 2−3σ because of a bulk flow towards the Shapley supercluster. Antoniou & Perivolaropoulos (2010) also applied the hemisphere comparison method to the SN Ia data of Union 2 and found a maximum anisotropy, which itself is statistically A&A 553, A56 (2013) not significant, but coincides with the bulk velocity flow axis (Feldman et al 2010;Kashlinsky et al 2009;Lavaux et al 2010), cosmic microwave background (CMB) low multipole moments (Bennett et al 2011;Copi et al 2010) and the quasar optical polarisation alignment axis (Hutsemekers et al 2008). Similar directions can be obtained by considering a dynamical dark energy fluid with an anisotropic equation of state in the wCDM model or in Chevallier-Polarski-Linder parametrisation (Cai & Tuo 2012).…”

Testing the isotropy of the Universe by using the JLA compilation of Type Ia supernovae

“…§ e-mail: lixin1981@cqu.edu.cn. Akrami et al 2014;Quartin & Notari 2015), the spatial variation of the electromagnetic fine-structure constant (Webb et al 2011;King et al 2012;Molaro et al 2013), the largescale alignment of the quasar polarization vectors (Hutsemekers & Lamy 2001;Hutsemekers et al 2005), the large-scale bulk flow beyond the prediction of ΛCDM model (Kashlinsky et al 2009(Kashlinsky et al , 2010Watkins, Feldman & Hudson 2009), and so on. All of these phenomena arouse us to rethink the validity of the cosmological principle.…”