Spectral shapes of the Ly α emission from galaxies - II. The influence of stellar properties and nebular conditions on the emergent Ly α profiles

We demonstrate how the stellar and nebular conditions in star-forming galaxies modulate the emission and spectral profile of H i Ly alpha emission line. We examine the net Ly alpha output, kinematics, and in particular emission of blueshifted Ly alpha radiation, using spectroscopy from with the Cosmic Origins Spectrograph on Hubble Space Telescope (HST), giving a sample of 87 galaxies at redshift z = 0.05-0.44. We contrast the Ly alpha spectral measurements with properties of the ionized gas (from optical spectra) and stars (from stellar modelling). We demonstrate correlations of unprecedented strength between the Ly alpha escape fraction (and equivalent width) and the ionization parameter (p approximate to 10(-15)). The relative contribution of blueshifted emission to the total Ly alpha also increases from approximate to 0 to approximate to 40 per cent over the range of O-32 ratios (p approximate to 10(-6)). We also find particularly strong correlations with estimators of stellar age and nebular abundance, and weaker correlations regarding thermodynamic variables. Low ionization stage absorption lines suggest the Ly alpha emission and line profile are predominantly governed by the column of absorbing gas near zero velocity. Simultaneous multiparametric analysis over many variables shows we can predict 80 per cent of the variance on Ly alpha luminosity, and similar to 50 per cent on the EW. We determine the most crucial predictive variables, finding that for tracers of the ionization state and H beta luminosity dominate the luminosity prediction whereas the Ly alpha EW is best predicted by H beta EW and the H alpha/H beta ratio. We discuss our results with reference to high-redshift observations, focussing upon the use of Ly alpha to probe the nebular conditions in high-z galaxies and cosmic reionization.