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Session 76 - Star Formation in Galaxies.
Display session, Friday, January 09
Exhibit Hall,
We present the newly determined compact radio emission morphology of NGC4038/9 (``the Antennae") and discuss the implications of these observations on the overall study of star formation processes in the system. The galaxies are undergoing an extreme starburst, with star formation rates estimated at 5M_ødotyr^-1 or greater. The system is forming unusual compact blue clusters and appears to have an IMF weighted towards massive stars. NGC 4038/9 is also very dusty; estimates of extinction range from A_V\sim 0.5--70.
Radio observations are able to penetrate the dust and isolate compact radio sources in regions that are not visible at optical and UV wavelengths. We therefore used the VLA to make high resolution, high sensitivity 6cm and quick-look 3.5cm VLA measurements of NGC4038/9. In the deep 6cm image, a complex morphology of loops, shells, and tidal tails is traced by the radio emission. In this map, we detect more than 250 compact radio sources smaller than \sim 20pc above a threshold of 33\muJy/beam. In the 3.5cm map, we detect about 25 compact radio sources stronger than 90\muJy/beam, many of them coincident with the 6cm sources. Preliminary indications are that about half of the compact sources have steep radio spectra and are supernova remnants, while the other half have flat spectra and are probably HII regions.
If we assume that \sim 125 supernova remnants all have ages no greater than Cas A (\sim 400 yr), the supernova rate is at least 0.3 yr^-1, consistent with other estimates of 0.2--0.5yr^-1 for NGC 4038/9. Furthermore, if half the compact radio sources are HII regions, their integrated flux density of \sim 10 mJy implies that a total of at least 2\times 10^6 M_ødot in massive stars is required to keep those regions ionized. This number is consistent with estimates of at least 1.1\times 10^6 M_ødot in directly observed O stars, measured from UV observations of NGC4038/9.
