Discovery of the largest known lensed images formed by a critically convergent lensing cluster

We identify the largest known lensed images of a single spiral galaxy, lying close to the centre of the distant cluster MACS J1149.5+2223 ($z=0.544$). These images cover a total area of $\simeq 150 \Box\arcsec$ and are magnified $\simeq 200$ times. Unusually, there is very little image distortion implying the central mass distribution is almost uniform over a wide area ($r\simeq200 kpc$) with a surface density equal to the critical density for lensing, corresponding to maximal lens magnification. Many fainter multiply-lensed galaxies are also uncovered by our model, outlining a very large tangential critical curve, of radius $r\simeq 170 kpc$, posing a potential challenge for the standard LCDM-Cosmology. Because of the uniform central mass distribution a particularly clean measurement of the mass of the brightest cluster galaxy is possible here, for which we infer stars contribute most of the mass within a limiting radius of $\simeq 30 kpc$, with a mass-to-light ratio of $M/L_{B}\simeq 4.5(M/L)_{\odot}$. This cluster with its uniform and central mass distribution acts analogously to a regular magnifying glass, converging light without distorting the images, resulting in the most powerful lens yet discovered for accessing the faint high-$z$ Universe.