Magnetic Field Tomography in Nearby Galaxies with the Square Kilometre Array

Abstract

Magnetic fields play an important role in shaping the structure and evolution of the interstellar medium (ISM) of galaxies, but the details of this relationship remain unclear. With SKA1, the 3D structure of galactic magnetic fields and its connection to star formation will be revealed. A highly sensitive probe of the internal structure of the magnetoionized ISM is the partial depolarization of synchrotron radiation from inside the volume. Different configurations of magnetic field and ionized gas within the resolution element of the telescope lead to frequency-dependent changes in the observed degree of polarization. The results of spectro-polarimetric observations are tied to physical structure in the ISM through comparison with detailed modeling, supplemented with the use of new analysis techniques that are being actively developed and studied within the community such as Rotation Measure Synthesis. The SKA will enable this field to come into its own and begin the study of the detailed structure of the magnetized ISM in a sample of nearby galaxies, thanks to its extraordinary wideband capabilities coupled with the combination of excellent surface brightness sensitivity and angular resolution.Advancing Astrophysics with the Square Kilometre Array

SKA Deep Polarization and Cosmic Magnetism

“…This may reflect either a smaller overall number of polarized sources, perhaps arising from a decreasing fraction of Active Galactic Nuclei (AGN), and/or an on-average smaller fractional polarization for fainter sources. The latter is consistent with integrated polarization properties of disk galaxies in which internal depolarization effects from thermal plasma decreases the fractional polarization at frequencies below a few GHz (Stil et al 2009, Heald et al 2015. Faint polarized counts are thus expected to be frequency dependent and broad-band data are required to unravel the underlying astrophysics.…”

“…Band 3 will sample the regime where internal Faraday depolarization effects are negligible and alignment of polarization and geometric properties of the galaxy are preserved. Band 2 samples the depolarization regime and allows measurement of internal Faraday processes from galaxy structure (Heald et al 2015). The significant depolarization in band 2 for nearby galaxies will be substantially reduced at z >1.…”

Structure, dynamical impact and origin of magnetic fields in nearby galaxies in the SKA era

“…The resolution in Faraday space (given by the half-power width of the RM Spread Function) for SKA1-MID Band 4 is about 400 rad/m 2 , so that only the average Faraday depth can be measured and multiple components cannot be resolved. 3-D Faraday tomography should be done at lower frequencies (Heald et al 2015) and perfectly complements this project. SKA1 observations of HI emission in nearby galaxies (de Blok et al 2015) and in particular in the Magellanic Clouds (McClure-Griffith et al 2015) will provide data on the properties and dynamics of neutral gas.…”

Using SKA Rotation Measures to Reveal the Mysteries of the Magnetised Universe

“…A complementary approach at somewhat lower frequencies is to investigate the polarised emission and its variation across a wide frequency band. The resulting Faraday rotation measure and frequency-dependent depolarisation can be used to examine the detailed internal structure of the magnetoionic medium, both across the galaxy and along the line-of-sight via Faraday tomography (Heald et al 2015). Details that can be studied include the coherence length of the line-of sight magnetic field, the large-scale three-dimensional magnetic field structure including the vertical structure of magnetic fields and the properties and dynamical impact of magnetic fields in the disk-halo interface of star forming galaxies.…”