Indirect Evidence for Escaping Ionizing Photons in Local Lyman Break
  Galaxy Analogs
  A population of early star-forming galaxies is the leading candidate for the
re-ionization of the universe. It is still unclear what conditions and physical
processes would enable a significant fraction of the ionizing photons to escape
from these gas-rich galaxies. In this paper we present the results of the
analysis of HST COS far-UV spectroscopy plus ancillary multi-waveband data of a
sample of 22 low-redshift galaxies that are good analogs to typical
star-forming galaxies at high-redshift. We measure three parameters that
provide indirect evidence of the escape of ionizing radiation: (1) the residual
intensity in the cores of saturated interstellar low-ionization
absorption-lines. (2) The relative amount of blue-shifted Lyman alpha line
emission, and (3) the relative weakness of the [SII] optical emission lines. We
use these diagnostics to rank-order our sample in terms of likely leakiness,
noting that a direct measure of escaping Lyman continuum has recently been made
for one of the leakiest members of our sample. We then examine the correlations
between our ranking and other proposed diagnostics of leakiness and find a
correlation with the equivalent width of the Lyman alpha emission-line. Turning
to galaxy properties, we find the strongest correlations with leakiness are
with the compactness of the star-forming region and the speed of the galactic
outflow. This suggests that extreme feedback- a high intensity of ionizing
radiation and strong pressure from both radiation and a hot galactic wind-
combines to create significant holes in the neutral gas. These results not only
shed new light on the physical mechanisms that can allow ionizing radiation to
escape from intensely star-forming galaxies, they also provide indirect
observational indicators that can be used at high-redshift where direct
measurements of escaping Lyman continuum radiation are impossible.
