The nearby low-luminosity active galactic nucleus ( LLAGN ) NGC 4258 has a weak radio continuum component at the galactic center .
We investigate its radio spectral properties on the basis of our new observations using the Nobeyama Millimeter Array at 100 GHz and archival data from the Very Large Array ( VLA ) at 1.7–43 GHz and the James Clerk Maxwell telescope at 347 GHz .
The NGC 4258 nuclear component exhibits ( 1 ) an intra-month variable and complicated spectral feature at 5–22 GHz and ( 2 ) a slightly inverted spectrum at 5–100 GHz ( \alpha \sim 0.3 ; F _ { \nu } \propto \nu ^ { \alpha } ) in time-averaged flux densities , which are also apparent in the closest LLAGN M81 .
These similarities between NGC 4258 and M81 in radio spectral natures in addition to previously known core shift in their AU-scale jet structures produce evidence that the same mechanism drives their nuclei .
We interpret the observed spectral property as the superposition of emission spectra originating at different locations with frequency-dependent opacity along the nuclear jet .
Quantitative differences between NGC 4258 and M81 in terms of jet/counter jet ratio , radio loudness , and degree of core shift can be consistently understood by fairly relativistic speeds ( \Gamma \gtrsim 3 ) of jet and their quite different inclinations .
The picture established from the two closest LLAGNs is useful for understanding the physical origin of unresolved and flat/inverted spectrum radio cores that are prevalently found in LLAGNs , including Sgr A* , with starved supermassive black holes in the present-day universe .