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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. Wefirst confirm the result of Neufeld & McKee (1988) that this mechanism givesrise to extended blue wings which may have been observed in some radiogalaxies. Our Monte-Carlo radiative transfer calculations further show that ina minor modification of the shell-model, in which we add an additional staticshell of hydrogen, this process can naturally explain the small blue bumpsobserved in a subset of Ly{\alpha} emitting galaxies, which have been difficultto explain with conventional shell-models. Blue bumps can be produced with anadditional column density of static hydrogen as small as $N_{HI}^{static} \llN_{HI}^{shell}$, and typically occur at roughly the outflow velocity of theshell. In our model the spectra of so-called 'blue-bump objects' might reflectan evolutionary stage in which the outflows regulating the escape of Ly{\alpha}photons are still engulfed within a static interstellar medium.

Blue Wings and Bumps via Fermi-like Acceleration of Lyman-alpha photons across Shocks