Age, evolution, and dispersion of the loose groups of blue stars in the northeast radio lobe of Centaurus A

The association of shocked gas and loose groups of young stars in the northeast radio lobe of Centaurus A (NGC 5128) provides direct evidence for shock-induced star formation. In this paper, we present UBV photometry of the blue stars based on observations made with the 2.5 m du Pont Telescope at Las Campanas Observatory. In a held covering 8.'8 x 8.'8, three groups are identified. Color-magnitude and color-color diagrams are plotted for each group. Two-color photometry of the many foreground held stars belonging to our own Galaxy indicates a foreground reddening E(B-V) = 0.14 mag +/- 0.02 mag. The new photometry is compared with that carried out in a small section of the field with the Hubble Space Telescope. The HST frames, in conjunction with the ground-based material allow compact star clusters to be identified, some of which are exciting small H II regions. Positions, finding charts, and photometry are given for 50 stars with V < 23.0 mag and (B-V)< 0.4 mag. The brightest blue star, at V = 19.94 mag, has an estimated absolute visual magnitude M(V) = -8.2 mag, which is comparable to that of the brightest A-type supergiants in the Large Magellanic Cloud. Most of the blue stars lie just inside the 1.4 GHz contours of the large-scale jetlike feature recently detected by Morganti et al. It is suggested that rather than being a component of the jet that powers the radio lobes, this feature, which resembles a spiral arm of a normal galaxy, results from compression of ambient gas by the jet. Bolometric magnitudes and effective temperatures are derived for the bright blue stars and plotted. Isochrones based on the Geneva evolutionary tracks are fitted and indicate a significant age spread for the stars extending up to 15 million years. The older stars are distributed more widely than the younger stars indicating that star formation extends over time and that the blue stars eventually disperse into the extended stellar envelope of the galaxy.