Sub-kiloparsec Imaging of Lyman-alpha Emission in a Low Mass, Highly
  Ionized, Gravitationally Lensed Galaxy at z = 1.84
  Low mass, low metallicity galaxies at low to moderate ($z\lesssim3$)
redshifts offer the best opportunity for detailed examination of the interplay
between massive stars, ionizing radiation and gas in sources similar to those
that likely reionized the universe. We present new narrowband Hubble Space
Telescope observations of Ly$\alpha$ emission and the adjacent ultraviolet (UV)
continuum in the low mass ($M_{\star} = 2 \times 10^8$ M$_{\odot}$), low
metallicity ($Z\sim1/20$ Z$_{\odot}$) and highly ionized gravitationally lensed
galaxy SL2S J02176$-$0513 at $z=1.844$. The galaxy has strong Ly$\alpha$
emission with photometric equivalent width $W^{\rm phot}_{\rm{Ly}\alpha} = 218
\pm 12$ \AA, at odds with the Ly$\alpha$ escape fraction of 10%. However, the
spectroscopic Ly$\alpha$ profile suggests the presence of broad absorption
underlying the emission, and the total equivalent width is consistent with the
escape fraction once this underlying absorption is included. The Ly$\alpha$
emission is more spatially extended than the UV continuum, and the 0.14"
spatial resolution of HST coupled with the magnification of gravitational
lensing enables us to examine the distribution of Ly$\alpha$ and the UV
continuum on sub-kiloparsec scales. We find that the peaks of the Ly$\alpha$
emission and the UV continuum are offset by 650 pc, and there is no Ly$\alpha$
emission arising from the region with the strongest UV light. Our combined
spectroscopic and imaging data imply a significant range in neutral hydrogen
column density across the object. These observations offer indirect support for
a model in which ionizing radiation escapes from galaxies through channels with
low column density of neutral gas.
