Subkiloparsec Imaging of Lyα Emission in a Low-mass, Highly Ionized, Gravitationally Lensed Galaxy at z = 1.84

Low-mass, low-metallicity galaxies at low to moderate (z ≲ 3) 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}α emission and the adjacent ultraviolet (UV) continuum in the low-mass (M _⋆ = 2 × 10⁸ {M}_⊙ ), low-metallicity (Z ∼ 1/20 Z_⊙), and highly ionized gravitationally lensed galaxy SL2S J02176-0513 at z = 1.844. The galaxy has strong {Ly}α emission with photometric equivalent width {W}_Lyα }^phot}=218+/- 12 \mathringA , at odds with the {Ly}α escape fraction of 10%. However, the spectroscopic {Ly}α 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}α 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}α and the UV continuum on subkiloparsec scales. We find that the peaks of the {Ly}α emission and the UV continuum are offset by 650 pc, and there is no {Ly}α 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.

Based on observations with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Support for program number 14632 was provided through a grant from the STScI under NASA contract NAS5-26555.