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 ∼2.'8, filling the field of the Hubble Space Telescope/Advanced Camera for Surveys. The equivalent circular Einstein radius is θ e = 55'' ± 3'' (at an estimated source redshift of zs ∼ 2.5), corresponding to re ≅ 350 ± 20 kpc at the cluster redshift, nearly three times greater than that of A1689 (re ≅ 140 kpc for zs = 2.5). The mass enclosed by this critical curve is very large, 7.4 ± 0.5 x 1014 M sun and only weakly model dependent, with a relatively shallow mass profile within r < 250 kpc, 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 ≅3.5□' for sources with z ≅ 8, making MACS J0717.5+3745 a compelling target for accessing faint objects at high redshift. We calculate that only one such cluster, with θ e ≥ 55'', is predicted within ∼107 Universes with z ≥ 0.55, corresponding to a virial mass ≥3 x 1015 M sun, for the standard ΛCDM (WMAP5 parameters with 2σ uncertainties).