The Parsec-scale radio structure of NGC 1068 and the nature of the nuclear radio source

Abstract

We present sensitive, multifrequency Very Long Baseline Array (VLBA2) images of the nuclear radio sources of NGC 1068. At 5 GHz and 8.4 GHz, the radio continuum source S1, argued to mark the location of the hidden active nucleus, resolves into an elongated, ~ 0.8 pc source oriented nearly at right angles to the radio jet axis but more closely aligned to the distribution of the nuclear H2O maser spots. S1 is detected at 5 GHz but not at 1.4 GHz, indicating strong freefree absorption below 5 GHz, and it has a flat spectrum between 5 GHz and 8.4 GHz. A 5—8.4 GHz spectral index map reveals an unresolved, inverted spectrum source at the center of the S1 structure which may mark the AGN proper. The average brightness temperature is too low for synchrotron self-absorption to impact the integrated spectrum significantly. In addition, a careful registration with the nuclear H2O masers argues that the S1 continuum source arises from the inner regions of the maser disk rather than a radio jet. The emission mechanism may be direct, thermal free-free emission from an X-ray-heated corona or wind arising from the molecular disk. We demonstrate that the hidden active nucleus is sufficiently luminous, to within the current estimates, to provide the requisite heating. The radio jet components C and S2 both show evidence for free-free absorption of a compact, steep-spectrum source. The free-free absorption might arise from a shock cocoon enveloping the compact radio sources. The presence of H2O masers specifically at component C supports the interpretation for the presence of a jet-ISM interaction. Component NE remains a steep-spectrum source on VLBA baselines and appears to be a local enhancement of the synchrotron emissivity of the radio jet. The reason for the enhancement is not clear; the region surrounding component NE is virtually devoid of narrow line region filaments, and so there is no clear evidence for interaction with the surrounding ISM. Component NE might instead arise in an internal shock, or perhaps denser jet plasma that broke away from an earlier interaction with the circumnuclear ISM.