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 |$\sigma_v=259\pm56$|  km s⁻¹. The lensed source is identified as an LBG at |$z_{\rm s}=3.403$|⁠ , based on the sharp drop bluewards of Ly α and other absorption features. A simple lens mass model for the system, assuming a singular isothermal ellipsoid, yields an Einstein radius of |$\theta_{\rm Ein}=1.23$| arcsec and a total mass within the Einstein radius of |$M_{\rm Ein}=(5.55\pm0.24)\times10^{11}\rm M_{\odot}$| corresponding to a velocity dispersion of |$\sigma_{\rm SIE}=283\pm3$|  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 |$z\sim4$| LBG populations, HSC J0904–0102 is unusually compact, an outlier at |$>2\sigma$| 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. [ABSTRACT FROM AUTHOR]