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

Abstract

We explore the impact of Fermi-like acceleration of Lyman-alpha (Lyα) photons across shock fronts on the observed Lyα 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 Lyα 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 Lyα photons are still engulfed within a static interstellar medium.

9 pages, 10 figures

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• A Spectroscopic Survey of Ly$\alpha$ Emitters at $z\approx 3.1$ over $\sim$1.2 Deg${}^2$
• Lyman Alpha Emitters and the 21cm Power Spectrum as Probes of Density-Ionization Correlation in the Epoch of Reionization
• Lyman-alpha resonant-line radiative transfer in expanding media