An extreme rotation measure in the high - redshift radio galaxy PKS B 0529 − 549 ⋆

We present the results of a radio polarimetric study of the high-redshift radio galaxy PKS B0529−549 (z = 2.575), based on high-resolution 12 mm and 3 cm images obtained with the Australia Telescope Compact Array (ATCA). The source is found to have a rest-frame Faraday rotation measure of −9600 rad m, the largest seen thus far in the environment of a z > 2 radio galaxy. In addition, the rest-frame Faraday dispersion in the screen responsible for the rotation is calculated to be 5800 rad m, implying rotation measures as large as −15 400 rad m. Using supporting near-IR imaging from the Very Large Telescope (VLT), we suggest that the rotation measure originates in the Lyα halo surrounding the host galaxy, and estimate the magnetic field strength to be ∼10 μG. We also present a new optical spectrum of PKS B0529−549 obtained with the New Technology Telescope (NTT), and propose that the emissionline ratios are best described by a photoionization model. Furthermore, the host galaxy is found to exhibit both hot dust emission at 8.0 μm and significant internal visual extinction (∼1.6 mag), as inferred from Spitzer Space Telescope near/mid-IR imaging.