Polarization of fluorescence lines: tracing magnetic field from circumstellar medium to early universe

TL;DR

This paper proposes using polarization of fluorescence emission lines as a novel method to trace magnetic fields across various cosmic environments, including the circumstellar medium, supernova remnants, and distant galaxies.

Contribution

It introduces the concept of using ground state atomic alignment in fluorescence lines to measure three-dimensional magnetic field directions in diffuse media.

Findings

Polarization degree of fluorescence lines exceeds 10%.

Primary fluorescence reveals magnetic polar angle along the line-of-sight.

Forbidden lines trace the plane-of-sky magnetic direction.

Abstract

Fluorescence emission lines are broadly applied in observation for diffuse medium in the universe. They are normally observed around strong pumping source, tracing the gas in circumstellar medium, reflection nebula, and H\,{\sc ii} regions, etc. They reside in UV/optical and infrared bands and hence could be directly observed with ground-base telescopes. In this letter, we demonstrate the polarization of fluorescence lines as a magnetic field tracer arising from ground state atomic alignment in diffuse medium, including our solar system, supernova remnants (SNRs), as well as quasi-stellar object (QSO) host galaxies. Two types of fluorescence emissions are considered: the primary fluorescence from the excited states; and the secondary fluorescence from the metastable state (forbidden lines). We find that the synergy of these lines could measure three-dimensional magnetic direction: the polarizations of the primary fluorescence lines could reveal the magnetic polar angle along the line-of-sight, whereas the polarization of forbidden lines traces the plane-of-sky magnetic direction. The expected degree of polarization is $P>10\%$. Polarizations of both types of fluorescence emissions have shown strong potential for observations, and are applicable to measure magnetic field within and beyond our galaxy.