The jet-ISM interaction in the outer filament of Centaurus A

¹ ASTRON, Netherlands Institute for Radio Astronomy, PO 2, 7990 AA Dwingeloo, The Netherlands
e-mail: santoro@astro.rug.nl
² Kapteyn Astronomical Institute, University of Groningen, PO 800, 9700 AV Groningen, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 2 October 2014
Accepted: 17 November 2014

Abstract

Context. The interaction between the radio plasma ejected by the active nucleus of a galaxy and the surrounding medium is a key process that can have a strong impact on the interstellar medium (ISM) of the galaxy and hence on galaxy evolution. The closest laboratory where we can observe and investigate this phenomenon is the radio galaxy Centaurus A. About 15 kpc northeast of this galaxy, a particularly complex region is found: the so-called outer filament where it has been proposed that jet-cloud interactions occur.

Aims. We investigate the signatures of jet-ISM interaction by a detailed study of the kinematics of the ionized gas, expanding on previous results obtained from the H i.

Methods. We observed two regions of the outer filament with VLT/Visible MultiObject Spectrograph (VIMOS) in integral field unit (IFU) observing mode. Emission from Hβ and [O iii]λλ4959, 5007 Å is detected in both pointings.

Results. We found two distinct kinematical components of ionized gas that closely match the kinematics of the nearby H i cloud. One component follows the regular kinematics of the rotating gas, while the second shows similar velocities to those of the nearby H i component thought to be disturbed by an interaction with the radio jet.

Conclusions. We suggest that the ionized and atomic gas are part of the same dynamical gas structure stemming from the merger that shaped Centaurus A. It is regularly rotating around Centaurus A, as proposed by other authors. The gas (ionized and H i) with anomalous velocities traces the interaction of the large-scale radio jet with the interstellar medium, suggesting that the jet is still active although poorly collimated. However, we can exclude that a strong shock is driving the ionization of the gas. It is likely that a combination of jet entrainment and photoionization by the UV continuum from the central engine is needed to explain both the ionization and the kinematics of the gas in the outer filament.