2000ApJ...531..756C

We present high-resolution imaging of the nucleus of NGC 4258 from 1 to 18 µm. Our observations reveal that the previously discovered compact source of emission is unresolved even at the near-infrared resolution of ∼0".2 FWHM, which corresponds to about 7 pc at the distance of the galaxy. This is consistent with the source of emission being the region in the neighborhood of the purported 3.5x10⁷ M_☉ black hole. After correcting for about 18 mag of visual extinction, the infrared data are consistent with an F_ν∝ν^–1.4±0.1 spectrum from 1.1 to 18 µm, implying a nonthermal origin. Based on this spectrum, the total extinction-corrected infrared luminosity (1-20 µm) of the central source is 2x10⁸ L_☉. We argue that the infrared spectrum and luminosity of the central source obviates the need for a substantial contribution from a standard, thin accretion disk at these wavelengths and calculate the accretion rate through an advection-dominated accretion flow to be M{dot}∼10^–3 M_☉.yr^–1. The agreement between these observations and the theoretical spectral energy distribution for advection-dominated flows provides evidence for the existence of an advection-dominated flow in this low-luminosity active galactic nucleus.