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

Abstract

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 $M_{\mathrm{Ein}}=(5.55\pm 0.24)\times 10^{11}{M}_{\odot }$ 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 $>2\sigma$ 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.

Accepted for publication on MNRAS, 10 pages, 6 figures

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