As part of a larger project to study Luminous and Ultra-Luminous IR Galaxies in both H I absorption and radio continuum at high resolution, NGC 7674 was observed using the VLBA, the phased VLA, and the 305-m Arecibo radio telescope. These phase-referenced observations were carried out on September 15, 2001, and constituted the first scientific use of the Arecibo telescope with the VLBA after the installation and the successful testing of the VLBA4 recording system.

Figure 1 shows a moderate resolution continuum image of the innermost 1 kpc region of NGC 7674 at 1.38 GHz and 20 milliarcsecond (mas) resolution.  This resolution is comparable to the VLBI image of Unger et al. (1988). We distinguish six structures in this image, only three of them were previously known.

We identify these three structures, following Unger et al. (1988), as western (W), central (C), and eastern (E) components. In addition to these, we detect a clear collimated jet structure (J) connecting the C and W components, and two diffuse emission regions, the northeastern (NE) and southwestern (SW) components. The peak flux density in Figure 1 is 33.9 mJy/beam.

Figure 2 shows the C (top-left) and the W (top-right) components at the full resolution of the array, which is 11 x 5 mas.  The peak flux densities of the C and W continuum images at this resolution are 16.3 and 3.4 mJy/beam, respectively.

The continuum structures seen in the nuclear region of NGC 7674 seem to be related to AGN activity with no strong indication of a nuclear or circumnuclear starburst. The overall S-shaped pattern that the radio structures seem to form (Figure 1) 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.

The spectra shown in Figure 2 are of 21 cm neutral hydrogen absorption obtained toward the highest continuum peaks of the C and W components. While clear absorption features can be seen toward C, no absorption is detected toward W.

At a lower angular resolution (~100 mas: Figure 3), our results show several H I absorption features toward the components C, E, and NE. No absorption is detected toward the components J, W, and SW. The widest of these H I features has a full width of 165 km/s at half maximum, and may represent the rotating H I disk or torus associated with the AGN. The narrower H I features, which have full widths at half maximum less than 50 km/s, could represent clumpy neutral hydrogen structures in the H I torus. Figure 4 illustrates a possible model for the H I disk in the nuclear region of NGC 7674.

More details on these sensitive VLBI results are available here.