2020MNRAS.494.3156J

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_ℓ=0.957 with stellar velocity dispersion σ_v=259 ±56 km s^–1. The lensed source is identified as an LBG at z_ s_=3.403, based on the sharp drop bluewards of Lyα and other absorption features. A simple lens mass model for the system, assuming a singular isothermal ellipsoid, yields an Einstein radius of θ_Ein=1.23 arcsec and a total mass within the Einstein radius of M_ Ein_=(5.55 ±0.24) x 10¹¹ M_☉ corresponding to a velocity dispersion of σ_SIE=283 ±3 km s^–1, which is in good agreement with the value derived spectroscopically. The most isolated lensed LBG image has a magnification of ∼6.5. In comparison with other lensed LBGs and typical z ∼ 4 LBG populations, HSC J0904-0102 is unusually compact, an outlier at >2σ confidence. Together with a previously discovered SuGOHI lens, HSC J1152+0047, which is similarly compact, we believe that the HSC survey is extending LBG studies down to smaller galaxy sizes.