An extreme rotation measure in the high-redshift radio galaxy PKSB0529-549

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 Ly alpha halo surrounding the host galaxy, and estimate the magnetic field strength to be similar to 10 mu G. 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 mu m and significant internal visual extinction (similar to 1.6 mag), as inferred from Spitzer Space Telescope near/mid-IR imaging.