A Young Blue Tidal Stream in NGC 5128

Abstract

Stellar streams in galaxy halos are the natural consequence of a history of merging and accretion. We present evidence for a blue tidal stream of young stars in the nearest giant elliptical galaxy, NGC 5128 (Centaurus A). Using optical UBVR color maps, unsharp masking, and adaptive histogram equalization, we detect a blue arc in the northwest portion of the galaxy that traces a partial ellipse with an apocenter of 8 kpc. We also report the discovery of numerous young star clusters that are associated with the arc. The brightest of these clusters is spectroscopically confirmed, has an age of ~350 Myr, and may be a proto-globular cluster. It is likely that this arc, which is distinct from the surrounding shell system and the young jet-related stars in the northeast, is a tidally disrupted stellar stream orbiting the galaxy. Both the age derived from the integrated optical colors of the stream and its dynamical disruption timescale have values of 200-400 Myr. We propose that this stream of young stars was formed when a dwarf irregular galaxy, or similar-sized gas fragment, underwent a tidally triggered burst of star formation as it fell into NGC 5128 and was disrupted ~300 Myr ago. The stars and star clusters in this stream will eventually disperse and become part of the main body of NGC 5128, suggesting that the infall of gas-rich dwarfs plays a role in the building of stellar halos and globular cluster systems.