Evidence of Complex B-field Structures in the ICM surrounding Cygnus A

A new high sensitivity polarization study of Cygnus A using 2-18 GHz JVLA data shows significant depolarization below 6 GHz with 0.75" (750 pc) resolution, as well as complicated polarization structures. The newly measured rotation measures (RM) are consistent with those obtained from previous studies [1, 2]; with RMs ranging from -3000 to +1300 rad/m/m in the western lobe and -5500 to +3000 rad/m/m in the eastern lobe. Our preliminary analysis indicates multi-scale B-fields with scales > 120 kpc and < 300 pc in the vicinity of Cygnus A. There is also a strong evidence that these large RMs originate from the large-scale uniform B-fields in the ambient ICM -- in agreement with [1, 2]. We show that the observed depolarizations and polarization structures at our optimum resolution (750 pc) may result from small-scale fluctuations across the beam. However, it still remains a mystery whether all these complex B-fields are in the ambient ICM, or whether some are in compressed gas local to the source. But we are confident that internal effects due to a mixing of thermal and synchrotron gas are minor. Ideal observations for addressing this problem would be those that can achieve high resolutions < 0.3" while preserving the spectral resolution (2-18 GHz). Achieving this requires an instrument with a minimum baseline that is 3x longer than the current JVLA longest baseline. Such baseline lengths are typical of the ngVLA (180-300 km) and the SKA (200 km). We present the above ongoing analysis, and the different techniques utilised including RM Synthesis [3] and direct fitting to Stokes Q and U data. References [1] Perley R.A., & Carilli C.L. 1996, Cygnus A -- Study of a Radio Galaxy, 168 [2] Dreher et.al., 1987, APJ, 316, 611 [3] Brentjens M.A., & de Bruyn., A.G. 2005, A&A, 441, 1217