Discovery of a Strong-lens Galaxy Pair with the Smallest Projected Separation

Close galaxy-pair lenses provide a unique opportunity to investigate how gravitational interactions reshape the stellar and dark-matter distributions of galaxies during the early stages of merging. In this talk, I will present the spectroscopic confirmation and lens modeling of HSC J0233−0205, a strong-lensing system produced by two closely separated early-type galaxies at zd=0.790, lensing a multi-component background source atzs=2.160. The two deflectors have a projected separation of only 0.481 arcsec, corresponding to 3.60 kpc, making HSC J0233−0205 the strong-lens galaxy pair with the smallest measured projected separation currently known. Joint modeling of the five-band HSC imaging requires two comparable mass components and yields a circularized Einstein radius of 1.549 arcsec. Combining the lensing mass with stellar masses inferred from spectral energy distribution fitting, we measure a projected dark-matter fraction of 83.7% within the critical curve, while the dark-matter fractions within the effective radii of the two deflectors are 82.1% and 65.7%, respectively. The reconstructed source contains two extended components separated by approximately 3.8 kpc and an additional compact component. These results establish HSC J0233−0205 as a rare laboratory for studying mass redistribution in close galaxy pairs. Future high-resolution imaging and spatially resolved spectroscopy will enable searches for tidal structures, merger kinematics, and possible offsets between the stellar and dark-matter distributions.