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 ~8700 Å and ~9300 Å and a prominent emission line at 8643 Å, which can be readily identified with Lyman-α at z = 6.110 ± 0.002. The emission line exhibits an asymmetric profile, with a more pronounced red wing. The rest-frame equivalent width of the line is EW = 79 ± 10 A, relatively well constrained thanks to the detection of the UV continuum, which is rarely achieved for a sub-L* galaxy at this redshift. After correcting for magnification, the star formation rate (SFR) estimated from the Lyα line is SFR(Lyα) = 11 M⊙yr^-1 and that estimated from the UV data is SFR(UV) = 3 M⊙yr^-1. We estimate that the effective radius of the source is R_e (Formula presented.) 0.4 kpc, which implies a star formation surface mass density ∑_SFR > 6 M⊙yr^-1 kpc^-2 and. using the Kennicutt-Schmidt relation, a gas surface mass density ∑^gas > 103 M⊙pc^-2. Our results support the idea that this magnified, distant galaxy is a young and compact object with luminosity 0.4 L* at z= 6, when the Universe was just 1 Gyr old. with a similar amount of mass in gas and stars. 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.