The nearby bright radio galaxy 3C 84 at the center of Perseus cluster is one of the ideal targets to explore the jet in active galactic nuclei ( AGNs ) and its parsec-scale environment .
A recent research of Fujita & Nagai revealed the existence of the northern counter-jet component ( N1 ) located at \sim 2 mas north from the central core in VLBI images at 15 and 43 GHz and they are explained by the free-free absorption ( FFA ) due to an ionized plasma foreground .
Here we report a new quasi-simultaneous observation of 3C 84 with the Korean VLBI Network ( KVN ) at 86 GHz and the KVN and VERA ( VLBI Exploration of Radio Astrometry ) Array ( KaVA ) at 43 GHz in 2016 February .
We succeeded the first detection of N1 at 86 GHz and the data show that N1 still has an inverted spectrum between 43 and 86 GHz with its spectral index \alpha ( S _ { \nu } \propto \nu ^ { \alpha } ) of 1.19 \pm 0.43 , while the approaching lobe component has the steep spectrum with the index of -0.54 \pm 0.30 .
Based on the measured flux asymmetry between the counter and approaching lobes , we constrain the averaged number density of the FFA foreground n _ { e } as 1.8 \times 10 ^ { 4 } ~ { } { cm ^ { -3 } } \lesssim n _ { e } \lesssim 1.0 \times 10 ^ { 6 } ~ { } % { cm ^ { -3 } } .
Those results suggest that the observational properties of the FFA foreground can be explained by the dense ionized gas in the circumnuclear disk and/or assembly of clumpy clouds at the central \sim 1 pc region of 3C 84 .