CLASH-VLT: spectroscopic confirmation of a z=6.11 quintuply lensed
  galaxy in the Frontier Fields Cluster RXC J2248.7-4431
  We present VIsible Multi-Object Spectrograph (VIMOS) observations of a z 6
galaxy quintuply imaged by the Frontier Fields galaxy cluster RXC J2248.7-4431
(z=0.348). This sub-L^*, high-z galaxy has been recently discovered by Monna et
al. (2013) using dropout techniques with the 16-band HST photometry acquired as
part of the Cluster Lensing And Supernova survey with Hubble (CLASH). Obtained
as part of the CLASH-VLT survey, the VIMOS medium-resolution spectra of this
source show a very faint continuum between ~8700A and ~9300A and a prominent
emission line at 8643A, which can be readily identified with Lyman-alpha at
z=6.110. The emission line exhibits an asymmetric profile, with a more
pronounced red wing. The rest-frame equivalent width of the line is EW=79+-10A.
After correcting for magnification, the star-formation rate (SFR) estimated
from the Lya line is SFR(Lya)=11 M_{sol}/yr and that estimated from the UV data
is SFR(UV)=3 M_{sol}/yr. We estimate that the effective radius of the source is
R_e<~0.4 kpc, which implies a star formation surface mass density Sigma_{SFR}>6
M_{sol}/yr/kpc^2 and, using the Kennicutt-Schmidt relation, a gas surface mass
density Sigma_{gas}>10^3 M_{sol}/pc^2. Our results support the idea that this
magnified, distant galaxy is a young and compact object with 0.4 L^* at z=6,
with comparable amount of mass in gas and stars. Future follow-up observations
with ALMA will provide valuable insight into the SFR and molecular gas content
of this source. In the spirit of the Frontier Fields initiative, we also
publish the redshifts of several multiply imaged sources and other background
objects which will help improving the strong lensing model of this galaxy
cluster.
