2023-01-06 11:00  P5A-1/Online

Observational signatures and effects of magnetic fields in astrophysical systems

Dr. Alvina Yee Lian On


Magnetic fields are ubiquitous, permeating across all scales from interstellar space to cosmic voids. Yet their origins and evolution remain as open questions. On galactic scales and beyond, Faraday rotation measure (RM) at radio wavelengths is commonly used to diagnose large-scale magnetic fields. Often, the magnetic field length scales are inferred from correlations in the observed RM. RM is a quantity derived from the polarised radiative transfer equations under restrictive conditions. In this talk, I will assess the usage of rotation measure fluctuation (RMF) analyses for magnetic field diagnostics in the framework of polarised radiative transfer. I will demonstrate how density fluctuations affect the correlation length of magnetic fields inferred from the conventional RMF analyses. Notably, the spatial correlations are generally dissimilar along the line-of-sight and across the sky plane, hence the context of RMF must always be clarified when inferring from radio observations. Magnetic fields also imprint observational signatures in the radio synchrotron emission, whereby the total intensity reveals the field strength, and the polarisation traces the field orientation. A point-by-point comparison between the X-ray and radio emissions of a simulated galaxy cluster follows a linear best-fit slope of almost unity, indicating that the magnetic field scales with density locally. On smaller scales, magnetic fields are believed to have an important role during the formation and evolution of molecular clouds. The effects of magnetic fields on the ionisation and heating rates of cosmic rays in IC 5146 are quantified, assuming that the fields are traceable via optical and near-infrared starlight polarisations. While the ionisation rate is fairly constant across the cloud, cosmic-ray heating is capable of raising temperatures by order of 1 K in a galactic environment or even higher in actively star-forming regions. This may lead to an increase in the Jeans mass and consequently affect the onset of star formation.