Discovery of an unusually compact lensed Lyman Break Galaxy from the Hyper Suprime-Cam Survey


We report the serendipitous discovery of HSC J0904$-$0102, a quadruply-lensed Lyman break galaxy (LBG) in the Survey of Gravitationally-lensed Objects in Hyper Suprime-Cam Imaging (SuGOHI). Owing to its point-like appearance, the source was thought to be a lensed active galactic nucleus. We obtained follow-up spectroscopic data with the Gemini Multi-Object Spectrographs on the Gemini South Telescope, which confirmed this to be a lens system. The deflecting foreground galaxy is a typical early-type galaxy at a high redshift of $z_ {\ell} = 0.957$ with stellar velocity dispersion $\sigma_v = 259 \pm 56$ km s$^{-1}$. The lensed source is identified as an LBG at $z_\mathrm{s} = 3.403$, based on the sharp drop bluewards of Ly$\alpha$ and other absorption features. A simple lens mass model for the system, assuming a singular isothermal ellipsoid, yields an Einstein radius of $\theta_ \mathrm{Ein} = 1. 23^{\prime\prime}$ and a total mass within the Einstein radius of $M_ \mathrm{Ein} = (5.55 \pm 0.24) \times 10^{11}\mskip3mu M_ \odot$ corresponding to a velocity dispersion of $\sigma_ \mathrm{SIE}= 283 \pm 3$ km s$^{-1}$, which is in good agreement with the value derived spectroscopically. The most isolated lensed LBG image has a magnification of $\sim 6.5$. In comparison with other lensed LBGs and typical $z \sim 4$ LBG populations, HSC J0904$-$0102 is unusually compact, an outlier at $\gt 2\sigma$ confidence. Together with a previously discovered SuGOHI lens, HSC J1152$+$0047, that is similarly compact, we believe that the HSC Survey is extending LBG studies down to smaller galaxy sizes.