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S.H. Teng (U.Md), J.S. Ulvestad (NRAO), S.G. Neff (NASA/GSFC), R.A. Sramek (NRAO)
We are investigating a sample of interacting starburst galaxies to determine the period of maximum star formation in a merger system. The seven galaxies in the program include systems that are representative of the various merger stages, ranging from young to remnant. The program galaxies include NGC 3690 and the famous ``Antennae'' (NGC 4038/9), both of which appear to be mergers in very early stages.
NGC 3690 has been imaged using data from the VLA archive that were acquired at 4cm and 6cm, in a variety of array configurations. Many unresolved radio sources were identified at the two frequencies, with a typical resolution near 200 pc. Most have steep spectra, indicating that they are likely to be complexes of supernova remnants. Only one source has a flat spectrum, indicating that it is dominated by thermal emission from HII regions, and may contain younger stars than the steep-spectrum sources. This differs from NGC4038/9, where a significant fraction of the sources have relatively flat spectra. Another source has a strongly inverted spectrum. It has been suggested in the literature that this is a background object, and it does not appear to coincide with a CO peak. If it is part of NGC 3690, it may be dominated by thermal emission with considerable free-free absorption. Energizing the radio source would require 1053 ionizing photons per second, requiring the equivalent of ~6000 O5 stars in a region likely to be only a few parsecs in diameter.
This research has been supported by the National Science Foundation, through its Research Experience for Undergraduates program. NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.