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S. H. Teng (U. Md), J. S. Ulvestad (NRAO), S. G. Neff (NASA/GSFC), R. A. Sramek (NRAO)
We present the results from our deep radio ({\lambda} = 6 & 4 cm) imaging of the interacting system NGC~3690. The system has been imaged with a resolution of ~200 pc using data from the VLA archive and recent observations, in a variety of array configurations. Many compact radio sources were identified. Most of these sources have steep spectra implying that they may be knots of supernova remnants with non-thermal emission or complex regions with a mixture of thermal and non-thermal emission.
One source, which we have named source D following the notation in Gehrz et al. (1983), has a rising spectral index (\alpha ~0.99 where S\nu \propto {\nu}\alpha). It has been suggested in the literature that this source may be a background AGN because of its spectrum and because its position does not coincide with a CO peak. However, it is well within the region of interaction delineated by ISO mid-infrared observations. Archived 2, 13, and 20~cm data show that source D continues to have a rising spectrum as frequency increases. This suggests that the source is part of the system and it must be dominated by thermal free-free emission. Source D is unresolved in our study. If we assume it has a brightness temperature of 8000 K, the emission region has a linear diameter of ~13 pc. The ionizing flux required to maintain the HII region corresponds to ~4 \times 104 O7 stars. Based on these observations, source D is most likely a powerful young super star cluster formed as a result of the merger.
NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.