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

Abstract

We present the spectroscopic confirmation and lens modeling of HSC~J0233$-$0205, a strong-lensing system produced by a close pair of elliptical galaxies at $z_d=0.790\pm0.022$ that lenses a multi-component background source at $z_s=2.160\pm0.002$. The two deflectors are separated by only $0\arcsec.481\pm0\arcsec.004$ ($3.596\pm0.046$ kpc), making this system a compact galaxy-pair lens at relatively high redshift. Joint five-band HSC lens modeling requires two mass components, with Einstein radii of $0\arcsec.774\pm0\arcsec.011$ and $0\arcsec.767\pm0\arcsec.014$, and yields a circularized Einstein radius of $θ_{\rm E}=1\arcsec.549\pm0\arcsec.018$ for the overall system. The lensed source is reconstructed with three components: two extended components separated by $0\arcsec.462\pm0\arcsec.013$ ($3.830\pm0.108$ kpc), together with a compact component nearly aligned with one of them. Integrating the lensing convergence map within the critical curve gives a projected mass of $M_{\rm lensing,crit}=(9.626\pm0.010)\times10^{11}M_\odot$. Combining this with the stellar mass inferred from SED fitting, $M_{\ast,{\rm crit}}=(1.570\pm0.240)\times10^{11}M_\odot$, we obtain a projected dark-matter fraction within the critical curve of $f_{\rm dm}=83.7\pm2.5\%$. Within the $z$-band effective radii of the two deflectors, the corresponding dark-matter fractions are $82.1\pm4.5\%$ and $65.7\pm6.7\%$, respectively. HSC~J0233$-$0205 is therefore a compact, high-redshift galaxy-pair lens. Future high-resolution imaging and spatially resolved spectroscopy will enable detailed tests of merger signatures, the redistribution of stellar and dark matter, and possible light--mass offsets in the lens plane.