The missing link: Tracing molecular gas in the outer filament of Centaurus A

We report the detection, using observations of the CO(2-1) line performed with the Atacama Pathfinder EXperiment (APEX), of molecular gas in the region of the outer filament of Centaurus A, a complex region known to show various signatures of an interaction between the radio jet, an H I cloud, and ionised gas filaments. We detect CO(2-1) at all observed locations, which were selected to represent regions with very different physical conditions. The H-2 masses of the detections range between 0.2 x 10(6) and 1.1 x 10(6) M-circle dot, for conservative choices of the CO to H-2 conversion factor. Surprisingly, the stronger detections are not coincident with the H I cloud, but instead are in the region of the ionised filaments. We also find variations in the widths of the CO(2-1) lines throughout the region, with broader lines in the region of the ionised gas, i.e. where the jet-cloud interaction is strongest, and with narrow profiles in the H I cloud. This may indicate that the molecular gas in the region of the ionised gas has the momentum of the jet-cloud interaction encoded in it, in the same way as the ionised gas does. These molecular clouds may therefore be the result of very efficient cooling of the down-stream gas photo-or shock-ionised by the interaction. On the other hand, the molecular clouds with narrower profiles, which are closer to or inside the H I cloud, could be pre-existing cold H-2 cores which manage to survive the effects of the passing jet.