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 zℓ = 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) × 10¹¹M☉ 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.