High Resolution Infrared Imaging of the Compact Nuclear Source in NGC4258

We present high resolution imaging of the nucleus of NGC4258 from 1 μm 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 3.5×10 M⊙ black hole. After correcting for about 18 mags of visual extinction, the infrared data are consistent with a Fν ∝ ν −1.4±0.1 spectrum from 1.1 μm to 18 μm, implying a non-thermal origin. Based on this spectrum, the total extinction corrected infrared luminosity (1−20 μm) of the central source is 2×10 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 Ṁ ∼ 10 M⊙/yr. 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 AGN. Division of Astronomy & Astrophysics, University of California, Los Angeles, CA 90095-1562 Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125 Palomar Observatory, California Institute of Technology, Pasadena, CA 91125 Jet Propulsion Laboratory, Pasadena, CA 91109 Present Address: Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064; [email protected]