Blue Wings and Bumps via Fermi-like Acceleration of Lyman-alpha photons
  across Shocks
  We explore the impact of Fermi-like acceleration of Lyman-alpha (Ly{\alpha})
photons across shock fronts on the observed Ly{\alpha} spectral line shape. We
first confirm the result of Neufeld & McKee (1988) that this mechanism gives
rise to extended blue wings which may have been observed in some radio
galaxies. Our Monte-Carlo radiative transfer calculations further show that in
a minor modification of the shell-model, in which we add an additional static
shell of hydrogen, this process can naturally explain the small blue bumps
observed in a subset of Ly{\alpha} emitting galaxies, which have been difficult
to explain with conventional shell-models. Blue bumps can be produced with an
additional column density of static hydrogen as small as $N_{HI}^{static} \ll
N_{HI}^{shell}$, and typically occur at roughly the outflow velocity of the
shell. In our model the spectra of so-called 'blue-bump objects' might reflect
an evolutionary stage in which the outflows regulating the escape of Ly{\alpha}
photons are still engulfed within a static interstellar medium.
