The Subarcsecond Radio Structure in NGC 1068. II. Implications for the Central Engine and Unifying Schemes

Abstract

In this paper, we explore the nature of the subarcsecond radio components in the Seyfert nucleus of NGC 1068. The southernmost component (S) is the most likely candidate for the location of the central engine. The integrated Spectrum of this component is flat, and the radio brightness temperatures are T_b_ <~ 3 x 10^5^ K. The radio emission is probably dominated by thermal emission rather than self-absorbed synchrotron emission. The implication is that the central engine is either intrinsically radio silent or completely obscured at centimeter wavelengths. It seems likely that the thermal emission from the inverted spectrum subcomponent of S (S1) originates near the inner edge of the obscuring torus. The central component (C) is probably the location of a shock interface between the subarcsecond jet and a dense, molecular cloud. The bending of the radio jet through a projected angle of 20^deg^, the presence of H_2_O maser emission, and the local flattening of the radio spectrum lend support for this scenario. We demonstrate that a giant molecular cloud would have the size and mass sufficient `to divert the radio jet. The spectrum of the radio jet steepens very rapidly with distance northward from component S. Characteristic jet spectra are much steeper than those typically found in radio galaxies. Owing to the copious local infrared radiation field, inverse-Compton losses are estimated to be severe and are probably the dominant cause of the observed spectral aging. The Compton loss timescale constrains the velocity of the radio jet to be v_jet_ <~ 5 x 10^4^ km s^-1^. There is evidence for Faraday depolarization toward the subarcsecond radio structure. Inner narrow-line region (NLR) clouds alone might cause this depolarization. Unresolved polarization angle gradients and depolarization arising from other internal and external magnetoionic plasmas, such as a hot intercloud medium or a jet cocoon, presumably also contribute to the depolarization. We also consider the alignment between the radio (MERLIN and Very Large Array) and Hubble Space Telescope images. We chose a registration that aligns subcomponent 51 near the symmetry center of the optical/UV polarization and the mid-infrared peak. In this alignment there are striking correspondences between the NLR structure and the radio jet, suggesting that the jet has a strong impact on the NLR.