A Semi-Analytical Line Transfer (SALT) model to interpret the spectra of
  galaxy outflows
  We present a Semi-Analytical Line Transfer model, SALT, to study the
absorption and re-emission line profiles from expanding galactic envelopes. The
envelopes are described as a superposition of shells with density and velocity
varying with the distance from the center. We adopt the Sobolev approximation
to describe the interaction between the photons escaping from each shell and
the remaining of the envelope. We include the effect of multiple scatterings
within each shell, properly accounting for the atomic structure of the
scattering ions. We also account for the effect of a finite circular aperture
on actual observations. For equal geometries and density distributions, our
models reproduce the main features of the profiles generated with more
complicated transfer codes. Also, our SALT line profiles nicely reproduce the
typical asymmetric resonant absorption line profiles observed in
star-forming/starburst galaxies whereas these absorption profiles cannot be
reproduced with thin shells moving at a fixed outflow velocity. We show that
scattered resonant emission fills in the resonant absorption profiles, with a
strength that is different for each transition. Observationally, the effect of
resonant filling depends on both the outflow geometry and the size of the
outflow relative to the spectroscopic aperture. Neglecting these effects will
lead to incorrect values of gas covering fraction and column density. When a
fluorescent channel is available, the resonant profiles alone cannot be used to
infer the presence of scattered re-emission. Conversely, the presence of
emission lines of fluorescent transitions reveals that emission filling cannot
be neglected.
