Broad Lyα emission from supernova remnants in young galaxies ^*

¹ Max Planck Institut für Astrophysik, Karl-Schwarzschild-Strae 1, 85740 Garching, Germany e-mail: heng@ias.edu
² Max Planck Institut für extraterrestrische Physik, Giessenbachstrae, 85478 Garching, Germany
³ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 23 October 2007
Accepted: 18 December 2007

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) Lyα, Lyβ, Lyγ, Hβ and Pα lines, as well as the broad and narrow luminosities of the two-photon (2γ) continuum, from existing measurements of the Hα flux. We consider cases of and 1, where 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 Lyα luminosity per object is at most ~10³⁶ erg s^-1. The 2γ continuum luminosities are comparable to the broad Hα and Lyα ones. We restrict our analysis to homogenous and static media.

Results.Differences in the Lyα/Hα and Lyβ/Hα luminosity ratios between the and 1 cases are factors ~2 for shock velocities  km s^-1, thereby providing a direct and unique way to measure β. In principle, broad, “non-radiative” Lyα 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 Lyα emission from SNRs, we expect their contribution to the total Lyα luminosity from to 5 galaxies to be negligibly small (~0.001%), compared to the radiative shock mechanism described by Shull & Silk (1979). Although broad, non-thermal Lyα emission has never been observed, these photons are produced in SNRs. Hence, the non-radiative Lyα luminosity is a part of the intrinsic Lyα spectrum of young galaxies.