Broad Lyman-Alpha Emission from Supernova Remnants in Young Galaxies

Abstract

Charge transfer (or exchange) reactions between hydrogen atoms and protons in collisionless shocks of supernova remnants (SNRs) are a natural way of producing broad Balmer, Lyman and other lines of hydrogen. We wish to quantify the importance of shock-induced, non-thermal hydrogen emission from SNRs in young galaxies. We present a method to estimate the luminosity of broad ($\sim 1000$ km s$^{-1}$) Ly$α$, Ly$β$, Ly$γ$, H$β$ and P$α$ lines, as well as the broad and narrow luminosities of the two-photon (2$γ$) continuum, from existing measurements of the H$α$ flux. The expected broad Ly$α$ luminosity per object is at most $\sim 10^{36}$ erg s$^{-1}$. In principle, broad, ``non-radiative'' Ly$α$ from SNRs in young galaxies can be directly observed in the optical range of wavelengths. However, by taking into consideration the different rates between core collapse and thermonuclear supernovae, as well as the duration we expect to observe such Ly$α$ emission from SNRs, we expect their contribution to the total Ly$α$ luminosity from $z \sim 3$ to 5 galaxies to be negligibly small ($\sim 0.001 %$), compared to the radiative shock mechanism described by Shull & Silk (1979). Though broad, non-thermal Ly$α$ emission has never been observed, these photons are produced in SNRs and hence the non-radiative Ly$α$ luminosity is a part of the intrinsic Ly$α$ spectrum of young galaxies.