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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
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DOI：10.3847/1538-4357/ae547e.png)
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Abstract
En 中文
We present multifrequency 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 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 to 1.5 mu G 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.
Keywords:
ACTIVE GALACTIC NUCLEI
BROAD-LINE REGION
ADVECTION-DOMINATED ACCRETION
BLACK-HOLE MASSES
MAGNETIC-FIELD STRUCTURE
SAGITTARIUS-A-ASTERISK
OPTICAL-EMISSION LINE
X-RAY OBSERVATIONS
POLARIZED EMISSION
MULTIFREQUENCY POLARIZATION
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