Sensitive VLBI Continuum and H I Absorption Observations of NGC 7674: First Scientific Observations with the Combined Array VLBA, VLA, and Arecibo

We present phase-referenced VLBI observations of the radio continuum emission from, and the neutral hydrogen 21 cm absorption toward, the luminous infrared galaxy NGC 7674. The observations were carried out at 1380 MHz using the VLBA, the phased VLA, and the 305 m Arecibo radio telescope. These observations constitute the first scientific use of the Arecibo telescope in a VLBI observation with the VLBA. The high- and low-resolution radio continuum images reveal several new continuum structures in the nuclear region of this galaxy. At ~100 mas resolution, we distinguish six continuum structures extending over 1.″4 (742 pc), with a total flux density of 138 mJy. Only three of these structures were known previously. All these structures seem to be related to active galactic nucleus (AGN) activity. The overall S-shaped pattern that the radio structures seem to form could be the result of the interstellar medium diverting the outcoming jets from the central AGN. However, we cannot rule out the possibility of a black hole merger that could result in a similar structural pattern. At the full resolution of the array (11 × 5 mas), we detect only two of the six continuum structures. Both are composed of several compact components with brightness temperatures on the order of 107 K. While it is possible that one of these compact structures could host an AGN, they could also be shocklike features formed by the interaction of the jet with compact interstellar clouds in the nuclear region of this galaxy. Complex H I absorption is detected with our VLBI array at both high and low angular resolution. Assuming that the widest H I feature is associated with a rotating H I disk or torus feeding a central AGN, we estimate an enclosed dynamical mass of ~7 × 107 M☉, comparable to the value derived from the hidden broad Hβ emission in this galaxy. The narrower H I lines could represent clumpy neutral hydrogen structures in the H I torus. The detection of H I absorption toward some of the continuum components and its absence toward others suggest an inclined H I disk or torus in the central region of NGC 7674. The overall averaged H I spectrum toward the continuum structures with H I absorption is very consistent with the Arecibo single-dish H I absorption spectrum at 3.′3 resolution.