Sub-kiloparsec Imaging of Lyman-alpha Emission in a Low Mass, Highly Ionized, Gravitationally Lensed Galaxy at z = 1.84

Abstract

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.