RSAA Colloquia / Seminars / Feast-of-Facts: Tuesday, 14 August 2018, 11:00-12:00; Duffield Lecture Theatre

Yik Ki (Jackie) Ma

"Breaking the Ambiguity -- The Taylor RM Catalogue and the Magnetic Universe"

The magnetic universe can be unveiled by measurements of Rotation Measure (RM) of polarised extragalactic sources in radio wavelengths. Such RM-grid experiments have allowed studies of magnetic fields in a wide range of astrophysical objects such as Galactic HII regions, the Milky Way, nearby galaxies like the Magellanic Clouds and M31, and distant galaxies. As the largest RM catalogue to date with measurements towards 37,543 sightlines, the Taylor RM catalogue is a powerful dataset for cosmic magnetism studies. Their RM values were derived from polarisation measurements at two frequencies only, hence susceptible to npi-ambiguity. Despite efforts to minimise this effect, some sources in the catalogue could still have their RM values deviate from the true values by +- 652.9 rad m-2. In this talk, I will present our work using new broadband spectro-polarimetric observations of 23 npi-ambiguity candidates in the Taylor catalogue with the Jansky VLA at L-band (1-2 GHz). We revealed 9 sources suffering from npi-ambiguity. From this, we estimated the number of sources in the entire catalogue which may suffer from the same RM ambiguity. Our study further revealed that off-axis polarisation leakage affects the Taylor catalogue, leading to unpolarised sources mistakenly listed as polarised, as well as deviations of their RM values from ours even after correcting for the npi-ambiguity. We also exploited the broadband capability of the Jansky VLA to investigate Faraday complexities of our sample, including preliminary study of broadband polarisation time variabilities.