The impact of ISM kinematics and geometry on Lyman-α escape in the Lyman Alpha Reference Sample
The Lyman-α line between the ground and first excited states of neutral Hydrogen is a crucial tool in high-redshift astronomy and observational cosmology. However, the resonant nature of the line and the resulting strong radiative transfer effects complicate the interpretation of the observed Ly-α radiation. Among other effects, the escape fraction and spectral shape of the intrinsic Ly-α feature, computed from recombination theory, seem to be governed by a complicated interplay of a number of properties in the intervening ISM, including gas column density, dust content, bulk outflow velocity, clumpiness and geometrical configuration.
As part of the Lyman Alpha Reference Sample, we here present results of high-resolution UV spectroscopy of a sample of 14 nearby star forming galaxies. Based on absorption lines arising in the neutral ISM and high-precision velocity zero-points computed from optical emission lines in the SDSS, we map the outflow velocity, line widths and other basic properties of the warm and neutral ISM along the line of sight to the brightest star forming knots in the galaxies, and compare our findings to global properties obtained through imaging and radio observations. Furthermore, the high resolution of the spectra, the presence of a number of SiII absorption lines all arising from the same energy level, allow us to map gas covering fraction to velocity and thus disentangle the question of whether residual flux at maximum absorption stems from part opacity of a fully covering gas, or partial covering of a fully opaque gas. We sketch how this mapping, in concert with a well resolved Ly-α line profile, can help distinguish a situation in which clumps of varying velocity together fully cover the background source, from a "picket fence" model with direct sight lines to the background source, and thus can help estimate the probability of Lyman Continuum escape. We also find a strong anticorrelation between H-α EW and maximum velocity-binned covering fraction; something we interpret as feedback from strong star formation giving rise to Rayleigh-Taylor instabilities and subsequent fragmentation of the outflowing medium.”