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 z_s ∼ 2.5), corresponding to r_e ≃ 350 ±20 kpc at the cluster redshift, nearly three times greater than that of A1689 (r_e ≃ 140 kpc for z_s = 2.5). The mass enclosed by this critical curve is very large, 7.4± 0.5 × 10¹⁴ M _⊙ 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 10 ⁷ Universes with z ≥ 0.55, corresponding to a virial mass ≥3 × 10¹⁵ M _⊙, for the standard ΛCDM (WMAP5 parameters with 2σ uncertainties).