Internal entrainment and the origin of jet-related broad-band emission in Centaurus A

The dimensions of Fanaroff-Riley class I jets and the stellar densities at galactic centres imply that there will be numerous interactions between the jet and stellar winds. These may give rise to the observed diffuse and 'knotty' structure of the jets in the X-ray, and can also mass load the jets. We performed modelling of internal entrainment from stars intercepted by Centaurus A's jet, using stellar evolution-and wind codes. From photometry and a code-synthesized population of 12 Gyr (Z = 0.004), 3 Gyr (Z = 0.008) and 0-60 Myr (Z = 0.02) stars, appropriate for the parent elliptical NGC 5128, the total number of stars in the jet is similar to 8 x 10(8). Our model is energetically capable of producing the observed X-ray emission, even without young stars. We also reproduce the radio through X-ray spectrum of the jet, albeit in a downstream region with distinctly fewer young stars, and recover the mean X-ray spectral index. We derive an internal entrainment rate of similar to 2.3 x 10(-3)M(circle dot) yr(-1) which implies substantial jet deceleration. Our absolute nucleosynthetic yields for the Asymptotic Giant Branch stellar population in the jet show the highest amounts for He-4, O-16, C-12, N-14 and Ne-20. If some of the events at >= 55 EeV detected by the Pierre Auger Observatory originate from internal entrainment in Centaurus A, we predict that their composition will be largely intermediate-mass nuclei with O-16, C-12 and N-14 the key isotopes.