Evidence for cloud-to-cloud variations in the ratio of polarized thermal dust emission to starlight polarization

¹ South African Astronomical Observatory, PO Box 9, Observatory, 7935, Cape Town, South Africa
² Department of Astronomy, University of Cape Town, Private Bag, Rondebosch, 7701, Cape Town, South Africa
³ Department of Space, Earth and Environment, Chalmers University of Technology, 412 93, Göteborg, Sweden
⁴ Institute of Theoretical Astrophysics, University of Oslo, Blindern, Oslo, Norway
⁵ Université Libre de Bruxelles, Science Faculty CP230, B-1050, Brussels, Belgium
⁶ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191, Gif-sur-Yvette, France
⁷ Department of Physics, University of Crete, Vasilika Vouton, 70013, Heraklion, Greece
⁸ Institute of Astrophysics, Foundation for Research and Technology – Hellas, 100 Nikolaou Plastira, Vassilika Vouton, 70013, Heraklion, Greece
⁹ University of Oxford, Denys Wilkinson Building, Department of Physics, Oxford, OX1 3RH, UK
¹⁰ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
¹¹ Department of Physics, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa
¹² Inter-University Centre for Astronomy and Astrophysics, Post bag 4, Ganeshkhind, Pune, 411007, India
¹³ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA, 91125, USA
¹⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109-8099, USA
¹⁵ TAPIR, California Institute of Technology, MC 350-17, Pasadena, CA, 91125, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 14 October 2025
Accepted: 15 February 2026

Abstract

The correlation between optical starlight polarization and polarized thermal dust emission can be used to infer intrinsic dust properties. This correlation is quantified by the ratio R_P/p, which has been measured by the Planck Collaboration to be 5.42 ± 0.05 MJy sr⁻¹ at 353 GHz when averaged over large areas of the sky. We investigated this correlation using newly published stellar polarimetric data densely sampling a continuous sky region of about four square degrees at intermediate Galactic latitude. We combined RoboPol optical polarization measurements for 1430 stars with submillimeter data from the Planck satellite at 353 GHz. We performed linear fits between the Planck (Q_s, U_s) and optical (q_v, u_v) Stokes parameters, taking into account the differences in resolution between the two datasets as well as the distribution of clouds along the line of sight. We find in this region of the sky that the R_P/p value is 3.67 ± 0.05 MJy sr⁻¹, indicating a significantly shallower slope than that found previously using different stellar samples. We also find significant differences in the fitted slopes when fitting the Q_s–q_v and U_s–u_v data separately. We explore two explanations using mock data: the miscalibration of the polarization angle and the variations in R_P/p along the line of sight due to multiple clouds. We show that the former can produce differences in the correlations of Q_s–q_v and U_s–u_v, but large miscalibration angles would be needed to reproduce the magnitude of the observed differences. Our simulations favor the interpretation that R_P/p differs between the two dominant clouds that overlap on the sky in this region. The difference in R_P/p suggests that the two clouds may have distinct dust polarimetric properties. With knowledge from the tomographic decomposition of the stellar polarization, we find that one cloud appears to dominate the correlation of U_s–u_v, while both clouds contribute to the correlation of the Q_s–q_v data.