High-resolution infrared imaging of the compact nuclear source in NGC 4258

We present high-resolution imaging of the nucleus of NGC 4258 from 1 to 18 mu m. Our observations reveal that the previously discovered compact source of emission is unresolved even at the near-infrared resolution of similar to 0 ".2 FWHM, which corresponds to about 7 pc at the distance of the galax y. This is consistent with the source of emission being the region in the n eighborhood of the purported 3.5 x 10(7) Mo black hole. After correcting fo r about 18 mag of visual extinction, the infrared data are consistent with an F-v proportional to v(-1.4+/-0.1) spectrum from 1.1 to 18 mu m, implying a nonthermal origin. Based on this spectrum, the total extinction-correcte d infrared luminosity (1-20 mu m) of the central source is 2 x 10(8) L.. We argue that the infrared spectrum and luminosity of the central source obvi ates the need for a substantial contribution from a standard, thin accretio n disk at these wavelengths and calculate the accretion rate through an adv ection-dominated accretion flow to be (M) over dot similar to 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 gal actic nucleus.