2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.( 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 km. Our observations reveal that the previously discovered compact source of emission is unresolved even at the near-infrared resolution of FWHM, which corresponds to about 7 pc at the distance of the galaxy. This is con-D0A.2 sistent with the source of emission being the region in the neighborhood of the purported 3.5] 107 M _black hole. After correcting for about 18 mag of visual extinction, the infrared data are consistent with an spectrum from 1.1 to 18 km, implying a nonthermal origin. Based on this spectrum,FlP l~1.4B0.1the total extinction-corrected infrared luminosity (1È20 km) of the central source is 2] 108 WeL _ 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 Ñow to be yr ~1. The agreement betweenM0 D 10~3 M _these observations and the theoretical spectral energy distribution for advection-dominated Ñows pro-vides evidence for the existence of an advection-dominated Ñow in this low-luminosity active galactic nucleus