MUSE discovers perpendicular arcs in Cen A inner filament

Stephen Hamer, Philippe Salomé, Francoise Combes, Quentin Salomé

Published 2014-09-26Version 1

Evidence of 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 evidence of positive feedback (AGN-triggered star formation). Cen A is a very nearby galaxy with huge optical filaments aligned with AGN radio-jet direction. Here, we search for line ratio variations along the filaments, kinematic evidence of shock-broadend line widths and large scale dynamical structures. We observe a 1'x1' region around the inner filament of Cen A with MUSE on the VLT during the Science Verification period. The brightest lines are the Halpha, [NII], [OIII] and [SII]. MUSE shows that the filaments are made of clumpy structures inside a more diffuse medium aligned with the radio-jet axis. We find evidence of shocked shells surrounding the star forming 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. A previously undetected large arc-like structure: 3 streams running perpendicular to the main filament, kinematically, morphologically and excitationally distinct are also detected. The clear difference in the excitation of the arcs and clumps suggests that the arcs are likely outside of the radiation cone, matching the position of the filament only in projection. The 3 arcs are thus most consistent with neutral material swept along by a backflow of the jet plasma from the AGN outburst and ionised through slow shocks which continues to excite gas away from the main jet axis.