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J.A. Graham (CIW-DTM)
The association of shocked gas and loose groups of young stars in the NE radio lobe of Centaurus A (NGC5128) provides direct evidence for shock induced star formation. Here, in the closest (3.5 Mpc) of the giant radio galaxies, a star forming process is apparently precipitated by the impact of the radio jet on an adjacent H I cloud (ApJ 502, 245, 1998). West of the ionized gas 15 arcmin NE of the NGC 5128 nucleus, 3 concentrations of blue stars have been identified and UBV photometry obtained for the individual stars using a CCD detector at the 2.5m du Pont telescope at Las Campanas Observatory. The photometry rests basically on the Graham (PASP 94, 244, 1982) E-region standards. Color-magnitude and two-color diagrams are presented. The brightest stars are close to magnitude 20 and are similar to the brightest stars in the Large Magellanic Cloud. The brightest and bluest stars are observed in the southernmost concentration. The northernmost concentration is very loose, older, and is on the verge of merging into the extended halo population of NGC 5128. Bolometric magnitudes and effective temperatures are derived for the brightest stars. Isochrones based based on the Geneva evolutionary tracks are fitted and indicate a significant age range for the stars, extending from less than a million to more than 15 million years. Stars whose photometry suggest colors of late B and A supergiants are the principal contributors to the older stars in the 3 groups. Two-color photometry of the many field stars belonging to our own galaxy indicates a foreground reddening (E(B-V)= 0.15±0.05 mag) for NGC 5128.
Examination of Hubble Space Telescope archival frames shows that most of the blue stars are indeed point sources. The HST frames allow several compact star clusters to be identified through their broader image profiles. Other tight clusters are seen directly on the ground-based images. A comparison will be made between the zero-points of the ground-based and the HST photometry.
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