HST Imaging of the Ionizing Radiation from a Star-forming Galaxy at z =
  3.794
  We report on the HST detection of the Lyman-continuum (LyC) radiation emitted
by a galaxy at redshift z=3.794, dubbed Ion1 (Vanzella et al. 2012). The LyC
from Ion1 is detected at rest-frame wavelength 820$\sim$890 \AA with HST
WFC3/UVIS in the F410M band ($m_{410}=27.60\pm0.36$ magnitude (AB), peak SNR =
4.17 in a circular aperture with radius r = 0.12'') and at 700$\sim$830 \AA
with the VLT/VIMOS in the U-band ($m_U = 27.84\pm0.19$ magnitude (AB), peak SNR
= 6.7 with a r = 0.6'' aperture). A 20-hr VLT/VIMOS spectrum shows low- and
high-ionization interstellar metal absorption lines, the P-Cygni profile of CIV
and Ly$\alpha$ in absorption. The latter spectral feature differs from what
observed in known LyC emitters, which show strong Ly$\alpha$ emission. An HST
far-UV color map reveals that the LyC emission escapes from a region of the
galaxy that is bluer than the rest, presumably because of lower dust
obscuration. The F410M image shows that the centroid of the LyC emission is
offset from the centroid of the non-ionizing UV emission by 0.12''$\pm$0.03'',
corresponding to 0.85$\pm$0.21 kpc (physical), and that its morphology is
likely moderately resolved. These morphological characteristics favor a
scenario where the LyC photons produced by massive stars escape from low HI
column-density "cavities" in the ISM, possibly carved by stellar winds and/or
supernova. We also collect the VIMOS U-band images of a sample of 107
Lyman-break galaxies with spectroscopic redshifts at $3.40<z<3.95$, i.e.
sampling the LyC, and stack them with inverse-variance weights. No LyC emission
is detected in the stacked image, resulting in a 32.5 magnitude (AB) flux limit
(1$\sigma$) and an upper limit of absolute LyC escape fraction $f_{esc}^{abs} <
0.63\%$. LyC emitters like Ion1 are very likely at the bright-end of the LyC
luminosity function.
