Strong-lensing analysis of MS 1358.4+6245: New multiple images and implications for the well-resolved z=4.92 galaxy

We present a strong-lensing analysis of the galaxy cluster MS 1358.4+6245 (z = 0.33), in deep six-band HST/ACS imaging. In addition to the well-studied system at z = 4.92, our modelling method uncovers 19 new multiply lensed images belonging to seven background sources, so that a total of 23 images and their redshifts are used to accurately constrain the inner mass distribution. We derive a relatively shallow inner mass profile, d log Sigma/d log r similar or equal to -0.33 +/- 0.05 (r < 200 kpc), with a much higher magnification than estimated previously by models constrained only by the z = 4.92 system. Using these many new images we can apply a non-parametric adaptive-grid method, which also yields a shallow mass profile without prior assumptions, strengthening our conclusions. The total magnification of the z(s) = 4.92 galaxy is high, about a similar to 100 times over its four images, so that the inferred source size, luminosity and star formation rate are about similar to 5 times smaller than previous estimates, corresponding to a dwarf-sized galaxy of radius similar or equal to 1 kpc. A detailed image of the interior morphology of the source is generated with a high effective resolution of only similar or equal to 50 pc, thanks to the high magnification and to the declining angular diameter distance above z similar to 1.5 for the standard cosmology, so that this image apparently represents the best resolved object known at high redshift.