The outer filament of Centaurus A as seen by MUSE

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
⁴ Yale University, Department of Physics, 217 Prospect St., New Haven, CT 06511, USA

Received: 12 December 2014
Accepted: 23 January 2015

Abstract

Context. Radio-loud active galactic nuclei (AGN) are known to inject kinetic energy into the surrounding interstellar medium (ISM) of their host galaxy via plasma jets. Understanding the impact that these flows can have on the host galaxy helps to characterize a crucial phase in their evolution. Because of its proximity, Centaurus A is an excellent laboratory in which the physics of the coupling of jet mechanical energy to the surrounding medium may be investigated. About 15 kpc northeast of this galaxy, a particularly complex region is found: the so-called outer filament, where jet-cloud interactions have been proposed to occur.

Aims. We investigate signatures of a jet-ISM interaction using optical integral-field observations of this region, expanding on previous results that were obtained on a more limited area.

Methods. Using the Multi Unit Spectroscopic Explorer (MUSE) on the VLT during the science verification period, we observed two regions that together cover a significant fraction of the brighter emitting gas across the outer filament. Emission from a number of lines, among which Hβλ4861 Å, [ O iii ] λλ4959,5007 Å, Hαλ6563 Å, and [ N ii ] λλ6548,6584 Å, is detected in both regions.

Results. The ionized gas shows a complex morphology with compact blobs, arc-like structures, and diffuse emission. Based on the kinematics, we identified three main components of ionized gas. Interestingly, their morphology is very different. The more collimated component is oriented along the direction of the radio jet. The other two components exhibit a diffuse morphology together with arc-like structures, which are also oriented along the radio jet direction. Furthermore, the ionization level of the gas, as traced by the [O iii]λ5007/Hβ ratio, is found to decrease from the more collimated component to the more diffuse components.

Conclusions. The morphology and velocities of the more collimated component confirm the results of our previous study, which was limited to a smaller area, implying that both the outer filament and the nearby H i cloud are probably partially shaped by the lateral expansion of the jet. The arc-like structures embedded within the two remaining components are the clearest evidence of a smooth jet-ISM interaction along the jet direction. We thus find signs of a jet-ISM interaction across all identified gas components. This suggests that, although poorly collimated, the large-scale radio jet is still active and affects the surrounding gas. This result indicates that the effect on the ISM of even low-power radio jets should be considered when studying the influence AGN can have on their host galaxy.