An extreme rotation measure in the high-redshift radio galaxy PKS B0529-549

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. The source is found to have a rest-frame Faraday rotation measure of −9600 rad m−2, 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−2, implying rotation measures as large as −15 400 rad m−2. Using supporting near-infrared (near-IR) imaging from the Very Large Telescope, 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, and propose that the emission-line 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.