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 starbursts using a combination of ultraviolet spectroscopy and imaging of the fluorescence emission lines (associated with transitions to excited fine-structure levels) and spectroscopy of the corresponding strongly blueshifted 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 Cosmic Origins Spectrograph on board the 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 both for our understanding of the physics of galactic outflows and for our estimation of their principal properties.