Ultra-High Energy Cosmic Rays from Centaurus A: Jet Interaction with
  Gaseous Shells
  Ultra high energy cosmic rays (UHECRs), with energies above ~6 x 10^19 eV,
seem to show a weak correlation with the distribution of matter relatively near
to us in the universe. It has earlier been proposed that UHECRs could be
accelerated in either the nucleus or the outer lobes of the nearby radio galaxy
Cen A. We show that UHECR production at a spatially intermediate location about
15 kpc northeast from the nucleus, where the jet emerging from the nucleus is
observed to strike a large star-forming shell of gas, is a plausible
alternative. A relativistic jet is capable of accelerating lower-energy heavy
seed cosmic rays (CRs) to UHECRs on timescales comparable to the time it takes
the jet to pierce the large gaseous cloud. In this model many CRs arising from
a starburst, with a composition enhanced in heavy elements near the knee region
around PeV, are boosted to ultra-high energies by the relativistic shock of a
newly oriented jet. This model matches the overall spectrum shown by the Auger
data and also makes a prediction for the chemical composition as a function of
particle energy. We thus predict an observable anisotropy in the composition at
high energy in the sense that lighter nuclei should preferentially be seen
toward the general direction of Cen A. Taking into consideration the magnetic
field models for the Galactic disk and a Galactic magnetic wind, this scenario
may resolve the discrepancy between HiRes and Auger results concerning the
chemical composition of UHECRs.
