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 ∼ 60 kpc length of the jets, while ∼ 50% fractional polarizations indicate these fields are well-ordered. Source-frame |RM| values of ∼ 1, 000 rad m −2 are typical, and values up to ∼ 11, 100 rad m −2 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 |RM| enhancement of ∼ 1, 100 rad m −2 , a bright knot of Lyα emission, and a deviation of the radio jet provide direct evidence for vigorous jet-gas interaction. We detect a large-scale RM gradient totaling ∼ 1, 000s rad m −2 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 (∼ 90 µG) to the jet sheath (∼ 10 µG) to the ambient intracluster medium (∼ 1 µG). 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.

The 700 ks Chandra Spiderweb Field

“…The new set of data obtained in the last couple of years in the Spiderweb field, and in particular the deep Chandra observation presented in this work, allows us to better address all the science goals listed here. The presence of hot, diffuse baryons around the Spiderweb Galaxy and its relation to its nuclear activity are discussed in other papers based on the combination of Chandra X-ray and JVLA Radio data (Carilli et al 2022;Anderson et al 2022). Moreover, the ICM of the Spiderweb Galaxy is also investigated with ALMA SZ data (Di Mascolo et al in preparation).…”

“…A second explanation can be provided by the presence of an older population of relativistic electrons. In fact, this is the scenario that is discussed in much greater details in Carilli et al (2022) and Anderson et al (2022) where the synergy between the Chandra X-ray data and the JVLA radio data is fully exploited.…”

“…In Figure 13 we show the extraction regions used for our analysis of the diffuse emission, overplotted on the backgroundsubtracted images after removing the central AGN (we used n d = 4 for display). Consistently with the radio and X-ray combined analysis of the jets presented in Carilli et al (2022) and Anderson et al (2022), we select two box regions for the east and west jet regions. These regions cover all the radio emission observed at 10 GHz (see Carilli et al 2022), include most of the diffuse emission in the soft band (see Figure 13, left) and almost the entire diffuse emission in the hard band (see Figure 13, right two circular regions corresponding to the central AGN (a circle with a radius of 2 arcsec) and to the source XID 7 (a circle with a radius of 1.5 arcsec) that is found to be a companion quasar (see Paper I).…”