Direct Detection of Lyman Continuum Escape from Local Starburst Galaxies
  with the Cosmic Origins Spectrograph
  We report on the detection of Lyman continuum radiation in two nearby
starburst galaxies. Tol 0440-381, Tol 1247-232 and Mrk 54 were observed with
the Cosmic Origins Spectrograph onboard the Hubble Space Telescopes. The three
galaxies have radial velocities of ~13,000 km/s, permitting a ~35 A window on
the restframe Lyman continuum shortward of the Milky Way Lyman edge at 912 A.
The chosen instrument configuration using the G140L grating covers the spectral
range from 912 to 2,000 {\AA}. We developed a dedicated background subtraction
method to account for temporal and spatial background variations of the
detector, which is crucial at the low flux levels around 912 A. This modified
pipeline allowed us to significantly improve the statistical and systematic
detector noise and will be made available to the community. We detect Lyman
continuum in all three galaxies. However, we conservatively interpret the
emission in Tol 0440-381 as an upper limit due to possible contamination by
geocoronal Lyman series lines. We determined the current star-formation
properties from the far-ultraviolet continuum and spectral lines and used
synthesis models to predict the Lyman continuum radiation emitted by the
current population of hot stars. We discuss the various model uncertainties
such as, among others, atmospheres and evolution models. Lyman continuum escape
fractions were derived from a comparison between the observed and predicted
Lyman continuum fluxes. Tol 1247-232, Mrk 54 and Tol 0440-381 have absolute
escape fractions of (4.5 +/- 1.2)%, (2.5 +/- 0.72)% and <(7.1 +/- 1.1)%,
respectively.
