The VANDELS survey: the role of ISM and galaxy physical properties on
  the escape of Ly{\alpha} emission in z $\sim$ 3.5 star-forming galaxies
  We investigate the physical properties of a sample of 52 Ly{\alpha} emitting
galaxies in the VANDELS survey, with particular focus on the role of kinematics
and neutral hydrogen column density in the escape and spatial distribution of
Ly{\alpha} photons. For these galaxies, we derive different physical properties
and characterize the Ly{\alpha} emission in terms of kinematics, EW, FWHM and
spatial extension and then estimate the velocity of the neutral outflowing gas.
  We reproduce some of the well known correlations between Ly{\alpha} EW and
stellar mass, dust extinction and UV \b{eta} slope, in the sense that the
emission line appears brighter in lower mass, less dusty and bluer galaxies. We
do not find any correlation with the SED-derived star formation rate, while we
find that galaxies with brighter Ly{\alpha} tend to be more compact both in UV
and in Ly{\alpha}. Our data reveal a new interesting correlation between the
Ly{\alpha} velocity and the offset of the inter-stellar absorption lines with
respect to the systemic redshift, in the sense that galaxies with larger
inter-stellar medium (ISM) out-flow velocities show smaller Ly{\alpha} velocity
shifts. We interpret this relation in the context of the shell-model scenario,
where the velocity of the ISM and the HI column density contribute together in
determining the Ly{\alpha} kinematics. In support to our interpretation, we
observe that galaxies with high HI column densities have much more extended
Ly{\alpha} spatial profiles, a sign of increased scattering. However, we do not
find any evidence that the HI column density is related to any other physical
properties of the galaxies, although this might be due in part to the limited
range of parameters that our sample spans.
