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 (VLBA) images of the nuclear radio sources of NGC 1068. At 5 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 H 2 O maser spots. S1 is detected at 5 GHz but not at 1.4 GHz, indicating strong free-free absorption below 5 GHz, and it has a flat spectrum between 5 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 that 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 H 2 O 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 H 2 O 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 in denser jet plasma that broke away from an earlier interaction with the circumnuclear ISM.

Mid-infrared imaging- and spectro-polarimetric subarcsecond observations of NGC 1068

“…It shows a complex geometry in the inner few parsecs. Through 5 and 8.5 GHz radio observations using the very long baseline array (VLBA), Gallimore et al (2004) detected the location of the hidden active nucleus (labeled as S1 in their paper) as a parsec-sized (∼0.8 pc diameter) structure with a major axis at P.A. ∼105 • .…”

An original interferometric study of NGC 1068 with VISIR BURST mode images

“…3.1.2 and the deconvolved image obtained from VISIR standard-mode observations (Galliano et al 2005). Right: same comparison with the contour plot of the 12.5 µm deconvolved image obtained at Keck 2 (Bock et al 2000) and the 5GHz radio map of Gallimore et al (2004) (grey scale). Green dashed lines correspond to the more extended component.…”

“…• (Gallimore et al 2004). This would indicate a direct collision between the radio jet and a dense NLR cloud -labeled HST-C in UV (Capetti et al 1997), NE1 in MIR (Galliano et al 2005), and IR-1b in the NIR (Gratadour et al 2006).…”

Discovery of millimetre-wave excess emission in radio-quiet active galactic nuclei

“…The physical origin of the radio emission in RQ AGN, whether a down-⋆ E-mail: behar@physics.technion.ac.il scaled version of the RL, collimated jets (Barvainis et al 1996;Gallimore et al 2006), or coronal emission from magnetic activity above the accretion disk (Field & Rogers 1993), remains to be resolved. In some sources, a combination of several spectral components may be present around the nucleus (Barvainis et al 1996;Gallimore et al 2004). For the sake of clarity of discussion, in this paper we distinguish a jet that is both highly relativistic and wellcollimated, from coronal emission that arises from hot gas, and perhaps an outflow that is neither relativistic nor wellcollimated.…”