ASKAP image heralds new era of polarisation studies

Preliminary ASKAP observations of linear polarisation in the southern lobe of Centaurus A have revealed extraordinarily detailed structure, showing that ASKAP will be a powerful instrument for widefield polarisation studies.

We’ve known for a decade that the radio lobes of Centaurus A contribute to the Faraday rotation of background radio sources (as do the lobes of other radio galaxies). Where the Faraday-rotating plasma lies – throughout the lobes or on their surface – is still an unsettled question. The distribution of the plasma may give clues as to the lobes’ history and dynamics.

Determining the distribution needs high-resolution polarimetric imaging. This is observationally expensive, because observations must be made over both a wide area of sky and a wide range of frequencies. Traditionally, such data would have taken thousands of hours to collect. But Craig Anderson (CSIRO) and his collaborators were able to begin a detailed study of the Cen A lobes, in full polarisation at sub-arcminute resolution, using just 12 hours of data taken with the 16-antenna commissioning array.

The observations were made over a 240 MHz band centred on 913  MHz and achieved an angular resolution of 26 x 33 arcseconds. The resulting maps of polarisation and Faraday rotation are among the most detailed ever made for radio lobes, with ~105 resolution elements covering the source. This is ~13 times better spatial resolution than any comparable published image, at any radio frequency, and ~40 times better than for any image made at a similar frequency.

ASKAP 913-MHz hue–intensity map of the southern lobe of Cen A. The hue channel represents Faraday depth (a measure of the amount of magnetised plasma along a sight-line); intensity is the magnitude of the polarisation vector; white contours are Stokes I (total flux density) levels. From Anderson et al. (2018)

These images are complex. Determining the structure and location of the magnetised plasma in Cen A’s lobes will need both advanced analysis and more, highly detailed observations. But these first observations show that ASKAP had an excellent capability for imaging polarisation even in the commissioning phase. The quality of the raw ASKAP data has improved even further since then.

Anderson et al. plan to observe the whole of Cen A with ASKAP–36 (the full array) and the new Ultra Wideband Low receiver on Parkes telescope.