Spectroscopic Confirmation of a z=6.740 Galaxy behind the Bullet Cluster
  We present the first results of our spectroscopic follow-up of 6.5 < z < 10
candidate galaxies behind clusters of galaxies. We report the spectroscopic
confirmation of an intrinsically faint Lyman break galaxy (LBG) identified as a
z 850LP-band dropout behind the Bullet Cluster. We detect an emission line at
{\lambda} = 9412 {\AA} at >5{\sigma} significance using a 16 hr long exposure
with FORS2 VLT. Based on the absence of flux in bluer broadband filters, the
blue color of the source, and the absence of additional lines, we identify the
line as Ly{\alpha} at z = 6.740 \pm 0.003. The integrated line flux is f = (0.7
\pm 0.1 \pm 0.3) \times 10^{-17} erg^{-1} s^{-1} cm^{-2} (the uncertainties are
due to random and flux calibration errors, respectively) making it the faintest
Ly{\alpha} flux detected at these redshifts. Given the magnification of {\mu} =
3.0 \pm 0.2 the intrinsic (corrected for lensing) flux is f^int = (0.23 \pm
0.03 \pm 0.10 \pm 0.02) \times 10^{-17} erg^{-1} s^{-1} cm^{-2} (additional
uncertainty due to magnification), which is ~2-3 times fainter than other such
measurements in z ~ 7 galaxies. The intrinsic H 160W-band magnitude of the
object is m^int(H_160W)=27.57 \pm 0.17, corresponding to 0.5 L* for LBGs at
these redshifts. The galaxy is one of the two sub-L* LBG galaxies
spectroscopically confirmed at these high redshifts (the other is also a lensed
z = 7.045 galaxy), making it a valuable probe for the neutral hydrogen fraction
in the early universe.
