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

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: morganti@astron.nl
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Department of Physics and Astronomy, University of Sheffield, Sheffield, S7 3RH, UK

Received: 17 May 2016
Accepted: 4 July 2016

Abstract

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 H₂ masses of the detections range between 0.2 × 10⁶ and 1.1 × 10⁶M_⊙, for conservative choices of the CO to H₂ 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 H₂ cores which manage to survive the effects of the passing jet.