The Largest Gravitational Lens: MACS J0717.5+3745 (z=0.546)

We identify 13 sets of multiply-lensed galaxies around MACS J0717.5+3745 ($z=0.546$), outlining a very large tangential critical curve of major axis $\sim 2.8\text{arcmin}$, filling the field of HST/ACS. The equivalent circular Einstein radius is ${\theta }_e=55\pm 3\text{arcsec}$ (at an estimated source redshift of $z_s\sim 2.5$), corresponding to $r_e\simeq 350\pm 20kpc$ at the cluster redshift, nearly three times greater than that of A1689 ($r_e\simeq 140 kpc$for$z_{s}=2.5$)\mathrm{.}Themassenclosedbythiscriticalcurveisverylarge,$7.4\pm 0.5 \times 10^{14}M_{\odot}$ and only weakly model dependent, with a relatively shallow mass profile within $r<250kpc$, reflecting the unrelaxed appearance of this cluster. This shallow profile generates a much higher level of magnification than the well known relaxed lensing clusters of higher concentration, so that the area of sky exceeding a magnification of $>10\times$, is $\simeq 3.5\text{sq}\text{arcmin}$ for sources with $z\simeq 8$, making MACS J0717.5+3745 a compelling target for accessing faint objects at high redshift. We calculate that only one such cluster, with $\theta_{e}\ge 55\arcsec$,ispredictedwithin$\sim 10^7$Universeswith$z\ge 0.55$, corresponding to a virial mass $\ge 3\times {10}^{15}{M}_{\odot }$, for the standard $\mathrm{\Lambda }CDM$ (WMAP5 parameters with $2\sigma$ uncertainties).