The Lyman Continuum Escape Fraction of The Cosmic Horseshoe: A Test of
  Indirect Estimates
  High redshift star-forming galaxies are likely responsible for the
reionization of the Universe, yet direct detection of their escaping ionizing
(Lyman continuum) photons has proven to be extremely challenging. In this
study, we search for escaping Lyman continuum of the Cosmic Horseshoe, a
gravitationally lensed, star-forming galaxy at z=2.38 with a large
magnification of $\sim24$. Transmission at wavelengths of low ionization
interstellar absorption lines in the rest-frame ultraviolet suggest a patchy,
partially transparent interstellar medium. This makes it an ideal candidate for
direct detection of the Lyman continuum. We obtained a 10-orbit Hubble near-UV
image using the WFC3/UVIS F275W filter that probes wavelengths just below the
Lyman limit at the redshift of the Horseshoe in an attempt to detect escaping
Lyman continuum radiation. After fully accounting for the uncertainties in the
opacity of the intergalactic medium as well as accounting for the charge
transfer inefficiency in the WFC3 CCDs, we find a $3 \sigma$ upper-limit for
the relative escape fraction of $f_{esc,rel}<0.08$. This value is a factor of
five lower than the value (0.4) predicted by the 40\% transmission in the
low-ion absorption lines. We discuss the possible causes for this discrepancy
and consider the implications for future attempts at both direct Lyman
continuum detection as well as indirect estimates of the escape fraction.
