Strong Lensing analysis of SPT-CLJ2325$-$4111 and SPT-CLJ0049$-$2440, two Powerful Cosmic Telescopes ($R_E > 40''$) from the SPT Clusters Sample

Abstract

We report the results from a study of two massive ($M_{500c} > 6.0 \times 10^{14} M_{\odot}$) strong lensing clusters selected from the South Pole Telescope cluster survey for their high Einstein radius ($R_E > 40''$), SPT-CLJ2325$-$4111 and SPT-CLJ0049$-$2440. Ground-based and shallow HST imaging indicated extensive strong lensing evidence in these fields, with giant arcs spanning 18\arcsec\ and 31\arcsec, respectively, motivating further space-based imaging followup. Here, we present multiband HST imaging and ground-based Magellan spectroscopy of the fields, from which we compile detailed strong lensing models. The lens models of SPT-CL\,J2325$-$4111 and SPT-CL\,J0049$-$2440 were optimized using 9, and 8 secure multiple-imaged systems with a final image-plane rms of 0\farcs63 and 0\farcs73, respectively. From the lensing analysis, we measure the projected mass density within 500~kpc of $M(<500 ~{\rm kpc}) = 7.30\pm0.07 \times 10^{14}$$M_{\odot}$, and $M(<500 ~{\rm kpc})=7.12^{+0.16}_{-0.19}\times 10^{14}$ $M_{\odot}$ for these two clusters, and a sub-halos mass ratio of $0.12\pm{0.01}$ and $0.21^{+0.07}_{-0.05}$, respectively. Both clusters produce a large area with high magnification ($\mu\geq 3$) for a source at $z=9$, $A^{lens}_{| \mu | \geq 3 }=4.93^{+0.03}_{-0.04} arcmin^2$, and $A^{lens}_{| \mu | \geq 3 }=3.64^{+0.14}_{-0.10} arcmin^2$ respectively, placing them in the top tier of strong lensing clusters. We conclude that these clusters are spectacular sightlines for further observations that will reduce the systematic uncertainties due to cosmic variance. This paper provides the community with two additional well-calibrated cosmic telescopes, as strong as the Frontier Fields, suitable for studies of the highly magnified background Universe.