Discovery of a Subparsec Jet 4000 Radii Away from the Central Schwarzschild Engine of NGC 4258

Abstract

We report the VLBI detection of compact continuum emission associated with the maser disk in NGC 4258. The strongest emission is located about 0.5 mas (0.015 pc) north of the dynamical center of the disk. It has an average flux density of about 3 mJy and varies by ~100% on timescales of weeks. We postulate that we have detected nonthermal synchrotron emission in the base of the northern jet, which is seen on parsec-to-kiloparsec scales in radio to X-ray emission. We also report a detection of emission from the southern jet which may be attenuated by thermal absorption in a layer of ionized gas above the molecular disk. The average flux density of the maser emission in the systemic velocity range is correlated with the flux density of the northern continuum emission. Together with the geometry of the disk, these data suggest that the masers amplify the southern continuum emission and that the southern and northern jet emission are correlated in strength. We discuss the results in the context of jet emission models and properties of other compact radio continuum sources.