A systematic study of galactic outflows via fluorescence emission:
  implications for their size and structure
  Galactic outflows play a major role in the evolution of galaxies, but the
underlying physical processes are poorly understood. This is mainly because we
have little information about the outflow structure, especially on large
scales. In this paper, we probe the structure of galactic outflows in low-$z$
starburst by using a combination of ultra-violet spectroscopy and imaging of
the fluorescence emission lines (associated with transitions to excited
fine-structure levels) and spectroscopy of the corresponding strongly
blue-shifted resonance absorption lines. We find that in the majority of cases
the observed fluorescence emission lines are much weaker and narrower than the
absorption lines, originating in the star-forming interstellar medium and/or
the slowest-moving part of the inner outflow. In a minority of cases, the
outflowing absorbing material does make a significant contribution to the
fluorescence emission. These latter systems are characterized by both strong
Ly$\alpha$ emission lines and weak low-ionization absorption lines (both known
to be empirical signs of Lyman-continuum leakage). We argue that the observed
weakness of emission from the outflow seen in the majority of cases is due to
the missing emission arising on scales larger than those encompassed by the
aperture of the {\it{Hubble Space Telescope}}. This implies shallow radial
density profiles in these outflows, and suggests that most of the observed
absorbing material must be created/injected at radii much larger than that of
the starburst. This has important implications for our understanding of both
the physics of galactic outflows and for our estimation of their principal
properties.
