VLBA Continuum Observations of NGC 4258: Constraints onan Advection-dominated Accretion Flow

We report a 3 σ upper limit of 220 μJy on any 22 GHz continuum emission coincident with the central engine in NGC 4258. If NGC 4258 is powered by an advection-dominated accretion flow, this radio upper limit implies that the inner advection-dominated flow cannot extend significantly beyond ~102 Schwarzschild radii.