Science

POSSUM observations will provide completely new insights into the magnetic field properties of the Milky Way Galaxy over an immense breadth of scales. The high-quality POSSUM RM Grid, covering the entire Southern sky, will probe the large-scale topology of the magnetic field, including at intermediate Galactic latitude and toward the Galactic halo. At the same time, the unprecedented angular density of the POSSUM RM Grid will illuminate the magnetic field on smaller physical scales than ever before, and allow us to quantify the turbulent cascade of magnetic energy dissipated to larger scales after being injected through the star formation process. We will use sensitive maps of diffuse polarization (coupled with complementary single-dish data from the Parkes radio telescope’s PEGASUS survey) to perform “Faraday tomography” and chart magnetic structure in 3D through the nearby ISM.

The distribution of Faraday rotation measure across the sky, from Hutschenreuter et al. (2021). Red areas indicate regions where the overall magnetic field points toward Earth, whereas the field points away from us in blue regions. POSSUM’s RM Grid will be about 30 times denser than the data used to develop this image and will thus provide far finer details.

We will also be able to study the magnetic fields associated with individual objects within the Milky Way, such as supernova remanants, planetary nebulae, and HII regions. As with the overall Milky Way ISM, this will be achieved through a combination of background RMs from extragalactic radio galaxies, and diffuse emission from the objects of interest themselves.

Diffuse polarized emission from the supernova remnant SN1006 (centre) and the Milky Way’s ISM, from Vanderwoude et al. (in prep). Also visible are the hundreds of distant radio galaxies that will be used to form the POSSUM RM Grid, noticeable here as specks of polarized light sprinkled across the image.

ASKAP observations will allow us to measure the total magnetic field strength of nearby galaxies from EMU imaging. In many cases, we will also image the structure of the regular component of the magnetic field and its directionality, through diffuse polarization and its associated RM. The magnetic size of distant star-forming galaxies, and their regular field structure on scales beyond that of their stellar disk, will be probed in statistical samples of nearby galaxies via the POSSUM RM Grid.

Diffuse polarization as detected by the ATCA at 13cm in the nearby galaxy M83 (Frick et al. 2016). POSSUM observations will map diffuse polarization from tens of nearby galaxies, along with the distribution of associated Faraday rotation measure.

POSSUM observations of individual radio galaxies will provide important new insights into their structure and evolution. The broadband polarization properties of unresolved cores will be used to constrain the small-scale magnetic structure in the regions where jets are launched. On intermediate scales, we will trace the magnetic structure of jets, while on the largest scales we will trace the impact of galactic outflows on the surrounding intergalactic medium.

Image of radio galaxy 2144-5637 (courtesy Emma Alexander; PhD thesis, University of Manchester).

Magnetic fields are a crucial component in setting the physical condition of the intracluster medium in galaxy clusters. The origin and evolution of such fields remain poorly understood. POSSUM will uncover the strength, morphology and extent of magnetic fields in multiple individual galaxy clusters, as well as within statistical samples. Recent work by Anderson et al. (2021) has demonstrated that POSSUM will also be a powerful probe of otherwise undetectable ionized gas filling the very extended intracluster medium.

Left: An RM grid towards the Fornax cluster and surrounds from POSSUM commissioning data (Anderson et al. 2021). Right: Equivalent RM grid from NVSS data (Taylor et al. 2009). The sky density of the ASKAP RM grid is about 25 RMs per square degree, less than will be achieved for the full POSSUM survey, but far superior to the NVSS density of about 1 RM per square degree. Red (blue) circles indicate positive (negative) RMs. POSSUM RMs are enhanced within the inner dashed circle, revealing a massive and previously hidden reservoir of ionised gas in the Fornax cluster. The outer dotted circle denotes the virial radius of the Fornax cluster. The grayscale is an optical image from the Digitized Sky Survey.

POSSUM will also provide the means to detect and study the elusive cosmic web that forms the backbone of the Universe on the very largest scales. Magnetic fields threading the Warm-Hot Intergalactic Medium (WHIM) in cosmic web filaments will impose a small but detectable fingerprint in POSSUM’s RM Grid. The densely sampled, high-quality network of RMs cataloged by POSSUM will allow us to identify this fingerprint and uncover the properties of the cosmic web.