Letter to the Editor MUSE discovers perpendicular arcs in Cen A inner filament

Context. Evidence of Active Galactic Nuclei (AGN) interaction with the intergalactic medium is observed in some galaxies and many cool core clusters. Radio-jets are suspected to dig large cavities into the surrounding gas. In most cases, very large optical filaments (several kpc) are also seen all around the central galaxy. The origin of these filaments is still not understood. Star forming regions are sometimes observed inside the filaments and are interpreted as an evidence of positive feedback (AGN-triggered star formation). Aims. Centaurus A is a very nearby galaxy with huge optical filaments aligned with AGN radio-jet direction. Here, we searched for line ratio variations along the filaments, kinematic evidences of shock-broaden line widths and large scale dynamical structures. Methods. We observed a 1′×1 ′ region around the so-called inner filament of Cen A with the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) during the Science Verification period. Results. (i) The brightest lines detected are the Hαλ6562.8, [NII]λ6583, [OIII]λ4959+5007 and [SII]λ6716+6731. MUSE shows that the fila-ments are made of clumpy structures inside a more diffuse medium aligned with the radio-jet axis. We find evidence of shocked shells surrounding the starforming clumps from the line profiles, suggesting the star formation in induced by shocks. The clumps line ratios are best explained by a composite of shocks and star formation illuminated by a radiation cone from the AGN. (ii) A previously undetected large arc-like structure: 3 streams running perpendicular to the main filament, kinematically, morphologically and excita-tionally distinct are also detected. The clear difference in the excitation of the arcs and clumps suggests that the arcs are very likel