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

Abstract

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 ~2farcm8, 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 sime 350 ± 20 kpc at the cluster redshift, nearly three times greater than that of A1689 (r_e sime 140 kpc for z_s = 2.5). The mass enclosed by this critical curve is very large, 7.4 ± 0.5 × 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 sime3.5squ' for sources with z sime 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 × 10¹⁵ M _sun, for the standard ΛCDM (WMAP5 parameters with 2σ uncertainties).