Radio‐Optical Alignment and Recent Star Formation Associated with Ionized Filaments in the Halo of NGC 5128 (Centaurus A)

We used the direct CCD camera at the Magellan I telescope at Las CampanasObservatory and FORS1 at Antu VLT at ESO Paranal Observatory to image fieldscentered on the inner and outer optical filaments in the halo of NGC 5128. Inthe V vs. U-V color-magnitude diagrams we have identified young bluesupergiants associated with these line-emitting filaments located between theinner radio lobe and the northern middle lobe. Around the outer filament starsas young as 10 Myr were detected. They are principally aligned with thedirection of the radio jet, but a vertical north-east alignment along the edgeof the HI cloud is also present. Young stars in the inner filament field arefound inside the bright knots of photoionized gas and are strongly aligned inthe direction towards the center of the galaxy at the same position angle asthe inner radio jet. Fitting the Padova isochrones on UV color-magnitudediagrams we find that the blue stars around the inner filaments have agessimilar to the ones around the outer filaments ~10-15 Myr and the sameabundance of Z=0.004. The presence of young blue supergiants clearly shows thatthe bright blue knots in the north-eastern halo of NGC 5128 are associations ofyoung stars with photoionized gas. The temperature of the brightest stars is T\~ 12000-16000 K, insufficient to account alone for the high excitation linesobserved in the surrounding ionized gas. Thus the optical emission jet isprincipally seen due to its alignment with the radio structure of the AGN. Thehighly collimated star formation is present only in the north eastern halo ofthe galaxy, suggesting the interaction of the jet with the gas clouds depositedduring the last accretion event as the preferred triggering mechanism.(abridged)Comment: 10 pages, 5 figures; to be published in ApJ, v565 n1, January 20, 2002 issu