Faraday rotation study of NGC 612 (PKS 0131−36): A hybrid radio source and its magnetized circumgalactic environment

We present a polarization and Faraday rotation study of the hybrid morphology radio galaxy NGC 612 (PKS 0131−36), using Australian Telescope Compact Array observations from 1 to 3 GHz. In general, the results are consistent with an external Faraday screen close to the radio source. In the eastern Fanaroff–Riley type II (FR II) lobe, the rotation measure (RM) of the hotspot increases in magnitude towards the leading edge, as well as changing sign (compared to the rest of the lobe). The Faraday depolarization is also ∼3 times larger at the hotspot than elsewhere. A plausible explanation for this is significant compression of ambient magnetized gas by the bow shock produced by the advancing hotspot. The western FR I lobe also exhibits some evidence of interaction with local magnetized gas, as a transverse band of high RM coincides with a distinct bend in the lobe. Previous observations of NGC 612 revealed an H I bridge of tidal debris along the direction of the eastern lobe towards the gas-rich companion NGC 619. We find no clear evidence that ionized gas associated with this bridge is either mixing with or lies in the foreground of the radio source. This is consistent with the absence of H I absorption against the hotspot, and indicates that the tidal debris must lie mostly behind the eastern lobe.