The Magnetised Environments of Radiative-mode and Jet-mode AGN Shane P. O’Sullivan1, C. R. Purcell2, J. S. Farnes3, X. H. Sun2, C. S. Anderson2, B. M. Gaensler4, et al. Abstract: We present results from a broadband (1 to 10 GHz) radio polarisation analysis of 102 radio-loud AGN. We investigate the most likely mechanisms causing the observed differences in the integrated 1.4 GHz polarisation properties of radiative-mode and jet-mode AGN, as found in O’Sullivan et al. (2015). Our analysis shows that jet-mode sources with high integrated polarisation at 1.4 GHz have low RM dispersion, are dominated by a single polarised component, and have a steep spectral index. We find that while, on average, the radiative- mode AGN are in slightly more turbulent magnetised environments than the jet-mode AGN, it is mainly the intrinsically disordered magnetic field structures of the radiative-mode AGN that leads to the lower integrated polarisation seen at 1.4 GHz (O’Sullivan et al. 2016, in prep.). Results: There are ~40% of sources that have one dominant RM component, ~50% with two RM components and ~10% with three RM components. Therefore, in order to investigate the magnetised environments of all sources in a consistent manner we calculate the polarisation-weighted RM dispersion, σRM,wtd (eqn. 3). This provides a characterisation of the general amount of depolarisation in the local environment of each source. Figure 2b: Here we show a source with more complicated polarisation behaviour from 1 to 3 GHz, which is well fit by two complex-polarisation components. Interestingly, the high angular resolution data at 9 GHz show that the source is a double-lobed radio galaxy, which supports the results of our model-fitting. *Data from the Australia Telescope Compact Array (ATCA) References: O’Sullivan et al. (2012), MNRAS, 421, 3300 O’Sullivan et al. (2016), in prep. O’Sullivan et al. (2015), ApJ, 806, 83 Sokoloff et al. (1998), MNRAS, 299, 189 Summary & Future work: - Our study emphasises the necessity for broadband polarization observations to accurately determine the Faraday rotation measure properties of the integrated emission from radio-loud AGN. - The results show how we can distinguish between the intrinsic magnetic field properties of sources and the external magnetised environment that causes depolarisation, using our polarization model-fitting approach. - We find that the radiative-mode AGN are in more turbulent magnetised environments and have more intrinsically disordered magnetic field structures than the jet-mode AGN, on average. - Future all-sky radio polarisation surveys, with telescopes such as the Australian SKA Pathfinder (ASKAP) and the VLA, will provide much greater statistical power to investigate the detailed magnetic field properties of different classes of radio-loud AGN in different types of environments. Figure 3b: Here we see that all the highly polarized sources at 1.4 GHz are dominated by a single RM component (Blue circle: one component, Green diamond: Two components, Red triangle: Three components). The low degree of polarisation for sources with two or three RM components is possibly caused by interference between the components. Several, but not all, of the weakly polarised sources have flat spectral indices indicating that the polarised emission is likely emanating from a synchrotron-self-absorbed region, which limits the the intrinsic degree of polarisation to <~ 10%. The remaining optically-thin low-polarisation sources have intrinsically disordered magnetic field structures. Figure 3a: Here we plot the degree of polarisation at 1.4 GHz versus the polarisation-weighted RM dispersion for radiative-mode (blue) and jet-mode (red) AGN. There is a clear absence of sources with a high degree of polarisation at 1.4 GHz and high RM dispersions, suggesting the depolarisation caused by the magnetised environment local to the source is an important factor. However there are several sources with low RM dispersions that also