A Unified Origin of Faraday Rotation toward 3C 84: The Circumnuclear Ambient Medium within the Parsec-Scale Bondi Radius of the Host Galaxy NGC 1275

Abstract

We present multi-frequency polarimetric observations of 3C 84 obtained with the Korean VLBI Network at 43-141 GHz, the Very Long Baseline Array at 43 GHz, and the High Sensitivity Array at 8 GHz from 2015 to 2024. We find that the Faraday rotation measure (RM) decreases systematically with distance from the black hole over 1-8 pc, following a single power-law trend of RM proportional to r^{-2.7+/-0.2}. Notably, RM measurements from earlier studies across the same distance range follow the same relation. This consistency across epochs, frequencies, and independent datasets indicates a common and stable external Faraday screen. These results naturally identify the circumnuclear ambient medium within the parsec-scale Bondi radius of the host galaxy NGC 1275 as the origin of the Faraday rotation, thereby resolving a long-standing question about its physical origin. From the RM profile, we derive radial distributions of the electron density and magnetic-field strength in the circumnuclear ambient medium that are consistent with independent constraints. The derived density lies below that of the free-free absorption disk and, when extrapolated inward, remains below the density of the broad-line region. The magnetic-field strength gradually increases from 0.1-1.5 microgauss at the Bondi radius to milligauss-to-gauss levels toward the black hole, providing the first spatially resolved constraint on the magnetic-field strength at parsec-scale distances in an elliptical galaxy. Together, these results present a spatially resolved and physically consistent picture of the circumnuclear environment in NGC 1275.

Figures (22)

• Figure 1: The $(u,v)$ coverage of the KVN for 3C 84 at 86 GHz on 2024 November 8–9. Baselines involving the Pyeongchang (PC) are highlighted in magenta.
• Figure 2: BU images of 3C 84 from 2015 December to 2017 May. Contours show the total intensity, and colors represent the polarized intensity in units of mJy. The observing epochs, displayed above each image, are given in yyyymmdd format (e.g., 20151205).
• Figure 3: Calibration procedure for the relative EVPA (defined as the EVPA in each IF relative to the average EVPA across the four IFs) of C3 obtained with the BU data. Left: Relative EVPA values of the comparison sources in each IF, $\chi_{\rm cal,i}$, whose intrinsic RM are expected to be significantly smaller than $\rm 10^5\ rad\ m^{-2}$ at 43 GHz. Middle: The relative EVPA values of C3 in each IF, $\chi_{\rm C3,i}$, shown together with those of the comparison sources. Right: Relative EVPA values of C3 after subtracting the mean relative EVPA values from the comparison sources in each IF, $\chi'_{\rm C3,i} = \chi_{\rm C3,i} - \langle\chi_{\rm cal,i}\rangle$.
• Figure 4: Left: KVN images of 3C 84 at 86, 95, 129, 139, and 141 GHz obtained on 2018 February 13--21. Contours and colors represent total and polarized intensity, and bars show the EVPA. All images are convolved with the beam size at the lowest frequency (86.2 GHz) Right: The EVPA values at the polarized intensity peak within C3 are shown on the top. Blue dashed line shows the best fit to the EVPA values. The fractional polarization values at the same position are shown at the bottom.
• Figure 5: Same as Figure \ref{['fig:18feb']}. Left and middle: Observations at 86.6 and 94.0 GHz obtained on 2019 January 2--3 and May 2--3, respectively. Right: Observations at 86.3, 86.4, 92.6, 92.7, and 129.3 GHz obtained on 2021 December 1.