Triggered star formation in the inner filament of Centaurus A

We present recent Hubble Space Telescope observations of the inner filament of Centaurus A, using the new Wide Field Camera 3 (WFC3) F225W, F657N and F814W filters. We find a young stellar population near the south-west tip of the filament. Combining the WFC3 data set with archival Advanced Camera for Surveys (ACS) F606W observations, we are able to constrain the ages of these stars to ≲10Myr, with best-fitting ages of 1-4Myr. No further recent star formation is found along the filament. Based on the location and age of this stellar population, and the fact that there is no radio lobe or jet activity near the star formation, we propose an updated explanation for the origin of the inner filament. Sutherland et al. suggested that radio jet-induced shocks can drive the observed optical line emission. We argue that such shocks can naturally arise due to a weak cocoon-driven bow shock (rather than from the radio jet directly), propagating through the diffuse interstellar medium from a location near the inner northern radio lobe. The shock can overrun a molecular cloud, triggering star formation in the dense molecular cores. Ablation and shock heating of the diffuse gas then give rise to the observed optical line and X-ray emission. Deeper X-ray observations should show more diffuse emission along the filament.