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 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_solar 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_solar. 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~10^-3 M_solar 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.