CLASH: z ~ 6 young galaxy candidate quintuply lensed by the frontier
  field cluster RXC J2248.7-4431
  We present a quintuply lensed z ~ 6 candidate discovered in the field of the
galaxy cluster RXC J2248.7-4431 (z ~ 0.348) targeted within the Cluster Lensing
and Supernova survey with Hubble (CLASH) and selected in the deep HST Frontier
Fields survey. Thanks to the CLASH 16-band HST imaging, we identify the
quintuply lensed z ~ 6 candidate as an optical dropout in the inner region of
the cluster, the brightest image having magAB=24.81+-0.02 in the f105w filter.
We perform a detailed photometric analysis to verify its high-z and lensed
nature. We get as photometric redshift z_phot ~ 5.9, and given the extended
nature and NIR colours of the lensed images, we rule out low-z early type and
galactic star contaminants. We perform a strong lensing analysis of the
cluster, using 13 families of multiple lensed images identified in the HST
images. Our final best model predicts the high-z quintuply lensed system with a
position accuracy of 0.8''. The magnifications of the five images are between
2.2 and 8.3, which leads to a delensed UV luminosity of L_1600 ~ 0.5L*_1600 at
z=6. We also estimate the UV slope from the observed NIR colours, finding a
steep beta=-2.89+-0.38. We use singular and composite stellar population SEDs
to fit the photometry of the hiz candidate, and we conclude that it is a young
(age <300 Myr) galaxy with mass of M ~ 10^8Msol, subsolar metallicity
(Z<0.2Zsol) and low dust content (AV ~ 0.2-0.4).
