Zusammenfassung
We present $U_336V_606J_125H_160$ follow-up $HST$ observations of 16
$z\sim3$ candidate LyC emitters in the HS1549+1933 field. With these data, we
obtain high spatial-resolution photometric redshifts of all sub-arcsecond
components of the LyC candidates in order to eliminate foreground contamination
and identify robust candidates for leaking LyC emission. Of the 16 candidates,
we find one object with a robust LyC detection that is not due to foreground
contamination. This object (MD5) resolves into two components; we refer to the
LyC-emitting component as MD5b. MD5b has an observed 1500\AA\ to 900\AA\
flux-density ratio of $(F_UV/F_LyC)_obs=4.0\pm2.0$, compatible with
predictions from stellar population synthesis models. Neglecting IGM
absorption, this ratio corresponds to lower limits to the relative (absolute)
escape fraction of $f_esc,rel^MD5b=75\%\pm38\%$
($f_esc,abs^MD5b=14\%\pm7\%$). The stellar population fit to MD5b indicates
an age of $łesssim50$Myr, which is in the youngest 10% of the $HST$ sample and
the youngest third of typical $z\sim3$ Lyman break galaxies, and may be a
contributing factor to its LyC detection. We obtain a revised,
contamination-free estimate for the comoving specific ionizing emissivity at
$z=2.85$, indicating (with large uncertainties) that star-forming galaxies
provide roughly the same contribution as QSOs to the ionizing background at
this redshift. Our results show that foreground contamination prevents
ground-based LyC studies from obtaining a full understanding of LyC emission
from $z\sim3$ star-forming galaxies. Future progress in direct LyC searches is
contingent upon the elimination of foreground contaminants through high
spatial-resolution observations, and upon acquisition of sufficiently deep LyC
imaging to probe ionizing radiation in high-redshift galaxies.
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