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(l) = 0.957 with stellar velocity dispersion sigma(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 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(Ein) = 1.23 arcsec and a total mass within the Einstein radius of M-Ein = (5.55 +/- 0.24) x 10(11) M-circle dot corresponding to a velocity dispersion of sigma(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 similar to 6.5. In comparison with other lensed LBGs and typical z similar to 4LBG populations, HSC J0904-0102 is unusually compact, an outlier at > 2 sigma confidence. Together with a previously discovered SuGOIII lens, HSC J1152+0047, which is similarly compact, we believe that the HSC survey is extending LBG studies down to smaller galaxy sizes.