Nuclear Radio Jet from a Low-Luminosity Active Galactic Nucleus in NGC
  4258
  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 (a
spectral index of ~0.3) 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 (bulk Lorentz
factors of >~ 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.
