The Spiderweb proto-cluster is being magnetized by its central radio jet

Abstract

We present deep broadband radio polarization observations of the Spiderweb radio galaxy (J1140-2629) in a galaxy proto-cluster at $z=2.16$. These yield the most detailed polarimetric maps yet made of a high redshift radio galaxy. The intrinsic polarization angles and Faraday Rotation Measures (RMs) reveal coherent magnetic fields spanning the $\sim 60$ kpc length of the jets, while $\sim 50$% fractional polarizations indicate these fields are well-ordered. Source-frame absolute RM values of $\sim 1,000$ rad/m/m are typical, and values up to $\sim 11,100$ rad/m/m are observed. The Faraday-rotating gas cannot be well-mixed with the synchrotron-emitting gas, or stronger-than-observed depolarization would occur. Nevertheless, an observed spatial coincidence between a localized absolute RM enhancement of $\sim 1,100$ rad/m/m, a bright knot of Ly$\hat{I}\pm$ emission, and a deviation of the radio jet provide direct evidence for vigorous jet-gas interaction. We detect a large-scale RM gradient totaling $\sim 1,000$s rad/m/m across the width of the jet, suggesting a net clockwise (as viewed from the AGN) toroidal magnetic field component exists at 10s-of-kpc-scales, which we speculate may be associated with the operation of a Poynting-Robertson cosmic battery. We conclude the RMs are mainly generated in a sheath of hot gas around the radio jet, rather than the ambient foreground proto-cluster gas. The estimated magnetic field strength decreases by successive orders-of-magnitude going from the jet hotspots ($\sim 90$ $\hat{I}¼$G) to the jet sheath ($\sim 10$ $\hat{I}¼$G) to the ambient intracluster medium ($\sim 1$ $\hat{I}¼$G). Synthesizing our results, we propose that the Spiderweb radio galaxy is actively magnetizing its surrounding proto-cluster environment, with possible implications for theories of the origin and evolution of cosmic magnetic fields.

12 pages, 7 figures, 2 tables. Accepted for publication in ApJ