Broad Lyα emission from supernova remnants in young galaxies

Abstract

Context. 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. Aims. We wish to quantify the importance of shock-induced, non-thermal hydrogen emission from SNRs in young galaxies. Methods. We present a method estimating the luminosity of broad (~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. We consider cases of $\beta=0.1$ and 1, where $\beta \equiv T_{\rm e}/T_{\rm p}$ is the ratio of electron-to-proton temperatures. We examine a modest sample of 8 proximate, Balmer-dominated SNRs from our Galaxy and the Large Magellanic Cloud. The expected broad Ly α luminosity per object is at most ~10 36 erg s -1 . The 2 γ continuum luminosities are comparable to the broad H α and Ly α ones. We restrict our analysis to homogenous and static media. Results. Differences in the Ly α /H α and Ly β /H α luminosity ratios between the $\beta=0.1$ and 1 cases are factors ~2 for shock velocities $1000 \lesssim v_{\rm s} \lesssim 4000$ km s -1 , thereby providing a direct and unique way to measure β . 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 the different rates between core collapse and thermonuclear supernovae into consideration, 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 (~0.001%), compared to the radiative shock mechanism described by Shull & Silk (1979). Although broad, non-thermal Ly α emission has never been observed, these photons are produced in SNRs. Hence, the non-radiative Ly α luminosity is a part of the intrinsic Ly α spectrum of young galaxies.