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We report on the HST detection of the Lyman-continuum (LyC) radiation emittedby a galaxy at redshift z=3.794, dubbed Ion1 (Vanzella et al. 2012). The LyCfrom Ion1 is detected at rest-frame wavelength 820$\sim$890 \AA with HSTWFC3/UVIS in the F410M band ($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$830 \AAwith the VLT/VIMOS in the U-band ($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- andhigh-ionization interstellar metal absorption lines, the P-Cygni profile of CIVand Ly$\alpha$ in absorption. The latter spectral feature differs from whatobserved in known LyC emitters, which show strong Ly$\alpha$ emission. An HSTfar-UV color map reveals that the LyC emission escapes from a region of thegalaxy that is bluer than the rest, presumably because of lower dustobscuration. The F410M image shows that the centroid of the LyC emission isoffset 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 islikely moderately resolved. These morphological characteristics favor ascenario where the LyC photons produced by massive stars escape from low HIcolumn-density "cavities" in the ISM, possibly carved by stellar winds and/orsupernova. We also collect the VIMOS U-band images of a sample of 107Lyman-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 emissionis 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 LyCluminosity function.

HST Imaging of the Ionizing Radiation from a Star-forming Galaxy at z = 3.794