Physical conditions in Centaurus A’s northern filaments

I. APEX mid-J CO observations of CO-bright regions^⋆,,^⋆⋆

¹ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, 58089 Morelia, Michoacán, Mexico
e-mail: q.salome@irya.unam.mx
² LERMA, Observatoire de Paris, Ecole Normale Supérieure, PSL University, CNRS, Sorbonne Université, Paris, France
³ Collège de France, 11 place Marcelin Berthelot, 75005 Paris, France

Received: 16 July 2018
Accepted: 21 May 2019

Abstract

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 H I shell. A large reservoir of molecular gas has been discovered outside the H I. In this reservoir, lies the Horseshoe complex: a filamentary structure seen in CO with ALMA and in Hα 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 previous ¹²CO(2-1) observations. 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 2−6 × 10² 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.