A new technique for finding galaxies leaking Lyman-continuum radiation:
  [SII]-deficiency
  The source responsible for the reionization of the Universe is believed to be
the population of star-forming galaxies at $z\sim6$ to 12. The biggest
uncertainty concerns the fraction of Lyman-continuum photons that actually
escape from the galaxies. In recent years, several relatively small samples of
"leaky" galaxies have been uncovered, and clues have begun to emerge as to both
the indirect signposts of leakiness and of the conditions/processes that enable
the escape of ionizing radiation. In this paper we present the results of a
pilot program aimed to test a new technique for finding leaky galaxies---using
the weakness of the [SII] nebular emission-lines relative to typical
star-forming galaxies as evidence that the interstellar medium is
optically-thin to the Lyman continuum. We use the Cosmic Origins Spectrograph
on the Hubble Space Telescope to detect significant emerging flux below the
Lyman edge in two out of three [SII]-weak star-forming galaxies at $z\sim0.3$.
We show that these galaxies differ markedly in their properties from the class
of leaky "Green-Pea" galaxies at similar redshifts: our sample galaxies are
more massive, more metal-rich, and less extreme in terms of their stellar
population and the ionization state of the interstellar medium. Like the Green
Peas, they have exceptionally high star-formation rates per unit area. They
also share some properties with the known leaky galaxies at $z\sim3$, but are
significantly dustier. Our results validate a new way to identify local
laboratories for exploring the processes that made it possible for galaxies to
reionize the Universe.
