Cosmological-scale coherent orientations of quasar optical polarization vectors in the $Planck$ era Investigating the Galactic dust contamination scenario

TL;DR

This study uses Planck satellite data to assess Galactic dust contamination in quasar optical polarization measurements, finding low contamination levels for most data and suggesting that dust cannot fully explain observed polarization alignments.

Contribution

The paper provides an independent, high-quality analysis of Galactic dust contamination in quasar polarization data using Planck data, offering new criteria for data quality assessment.

Findings

Approximately 30% of quasar lines of sight show dust contamination above 2 sigma.

70% of the data shows negligible dust contamination, supporting the robustness of polarization alignments.

Galactic dust alone cannot fully explain the observed quasar polarization alignments.

Abstract

Gigaparsec scale alignments of the quasar optical polarization vectors have been proven to be robust against a scenario of contamination by the Galactic interstellar medium (ISM). This claim has been established by means of optical polarization measurements of the starlight surrounding the lines of sight of the 355 quasars for which reliable optical polarization measurements are available. In this paper, we take advantage of the full-sky and high quality polarization data released by the \textit{Planck} satellite to provide an independent, complementary, and up-to-date estimation of the contamination level of the quasar optical polarization data by the Galactic dust. Our analysis reveals signatures of Galactic dust contamination at the two sigma level for about 30 per cent of the quasar optical polarization data sample. The remaining 70 per cent of the lines of sight do not show Galactic dust contamination above the two sigma level, suggesting low to negligible contamination of the quasar optical polarization signal. We further found arguments suggesting that Galactic thermal dust cannot fully account for the reported quasar optical polarization alignments. Based on the measurements of the ratio of the polarized intensity of the dust in the submillimeter to the degree of linear polarization of the quasar in the optical, we provide a new and independent quality criteria to apply to the quasar optical polarization sample. We argue that, unless correction is applied, such a criterion should be imposed on the data for future investigations in the framework of the cosmological-scale correlations of quasar optical polarization vector orientations that still could compete with the isotropic principle of the cosmological paradigm.