  The energy spectrum and mass composition of ultra-high energy cosmic rays
inferred at the Pierre Auger Observatory are used to derive a benchmark
scenario for the emission mechanisms at play in extragalactic accelerators as
well as for their energetics and for the abundances of elements in their
environments. Assuming a distribution of sources following the density of
stellar mass, the gradual increase of the cosmic ray mass number observed on
Earth from $\simeq$2\:EeV up to the highest energies is shown to call for
nuclei accelerated up to an energy proportional to their electric charge and
emitted with a hard spectral index. In addition, the inferred flux of protons
down to $\simeq$0.6\:EeV is shown to require for this population a spectral
index significantly softer than that of heavier nuclei. This is consistent with
in-source interactions that shape the energy production rate of injected
charged nuclei differently from that of the secondary neutrons escaping from
the confinement zone. Together with the inferred abundances of nuclei, these
results provide constraints on the radiation levels in the source environments.
Within this scenario, an additional component that falls off steeply with
increasing energy up to the ankle feature is necessary to make up the
all-particle flux in the sub-ankle energy range.
