Physical conditions in Centaurus A's northern filaments I: APEX mid-J CO
  observations of CO-bright regions
  NGC 5128 (Centaurus A) is one of the best targets to study AGN-feedback in
the local Universe. Optical filaments located at 16 kpc from the galaxy along
the radio jet direction show recent star formation, likely triggered by the
interaction of the jet with an HI shell. A large reservoir of molecular gas has
been discovered outside the HI. In this reservoir, lies the Horseshoe complex:
a filamentary structure seen in CO with ALMA and in Halpha with MUSE. The
ionised gas is mostly excited by shocks, with only a minor contribution of star
formation. We used the Atacama Pathfinder EXperiment (APEX) to observe the
12CO(3-2) and 12CO(4-3) transitions, as well as dense gas tracers in the
Horseshoe complex. 12CO(3-2) and 12CO(4-3) are detected for the first time in
the northern filaments of Centaurus A, with integrated intensity line ratios
R32~0.2 and R43~0.1, compared to the 12CO(1-0) emission. We also derived a line
ratio R21~0.6, based on the previous 12CO(2-1) observations of Salom\'e et al.
(2016). We used the non-LTE radiative transfer code RADEX and determined that
the molecular gas in this region has a temperature of 55-70 K and densities
between 2-6x10^2 cm^-3. Such densities are also in agreement with results from
the Paris-Durham shock code that predicts a post-shock density of a few 100
cm^-3. However, we need more observations of emission lines at a better angular
resolution in order to place tighter constraints on our radiative models,
whether they are used as a stand-alone tool (LVG codes) or combined with a
shock model.
