Acceleration of galactic supershells by Lyα radiation

Abstract

Scattering of Ly alpha photons by neutral hydrogen gas in a single outflowing 'supershell' around star-forming regions often explains the shape and offset of the observed Ly alpha emission line from galaxies. We compute the radiation pressure that is exerted by this scattered Ly alpha radiation on the outflowing material. We show that for reasonable physical parameters, Ly alpha radiation pressure alone can accelerate supershells to velocities in the range upsilon(sh) = 200-400 km s(-1). These supershells possibly escape from the gravitational potential well of their host galaxies and contribute to the enrichment of the intergalactic medium. We compute the physical properties of expanding supershells that are likely to be present in a sample of known high-redshift (z = 2.7-5.0) galaxies, under the assumption that they are driven predominantly by Ly alpha radiation pressure. We predict ranges of radii r(sh) = 0.1-10 kpc, ages t(sh) = 1-100 Myr and energies E(sh) = 10(53)-10(55) erg, which are in reasonable agreement with the properties of local galactic supershells. Furthermore, we find that the radius, r(sh), of a Ly alpha-driven supershell of constant mass depends uniquely on the intrinsic Lya luminosity of the galaxy, L(alpha), the H(I) column density of the supershell, N(HI), and the shell speed, upsilon(sh), through the scaling relation r(sh) alpha L(alpha)/(N(HI)upsilon(2)(sh)). We derive mass outflow rates in supershells that reach similar to 10-100 per cent of the star formation rates of their host galaxies.