MUSE discovers perpendicular arcs in the inner filament of Cen A
  Evidence of AGN interaction with the IGM 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 are seen
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. Cen A is a nearby galaxy
with huge optical filaments aligned with the AGN radio-jet direction. We
searched for line ratio variations along the filaments, kinematic evidence of
shock-broadend line widths, and large-scale dynamical structures. We observed a
1'x1' region around the inner filament of Cen A with MUSE on the VLT during
Science Verification. The brightest lines detected 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 that the star formation is induced by shocks. The clump line ratios
are best explained by a composite of shocks and star formation illuminated by a
radiation cone from the AGN. We also report a previously undetected large
arc-like structure: three streams running perpendicular to the main filament;
they are kinematically, morphologically, and excitationally distinct. The clear
difference in the excitation of the arcs and clumps suggests that the arcs are
very likely located outside of the radiation cone and match the position of the
filament only in projection. The three arcs are most consistent with neutral
material swept along by a backflow of the jet plasma from the AGN outburst that
is ionised through a diffuse radiation field with a low-ionisation parameter
that continues to excite gas away from the radiation cone.
