Constraining C iii] Emission in a Sample of Five Luminous z = 5.7
  Galaxies
  Recent observations have suggested that the CIII]$\lambda1907/1909$ emission
lines could be alternative diagnostic lines for galaxies in the reionization
epoch. We use the F128N narrowband filter on the Hubble Space Telescope's
($\it{HST}$) Wide Field Camera 3 (WFC3) to search for CIII] emission in a
sample of five galaxies at z = 5.7 in the Subaru Deep Field and the
Subaru/XMM-Newton Deep Field. Using the F128N narrowband imaging, together with
the broadband imaging, we do not detect CIII] emission for the five galaxies
with $J_{\rm{AB}}$ ranging from 24.10 -- 27.00 in our sample. For the brightest
galaxy J132416.13+274411.6 in our sample (z = 5.70, $J_{\rm{AB}} = 24.10$),
which has a significantly higher signal to noise, we report a CIII] flux of
$3.34\pm1.81 \times 10^{-18}$ $\mathrm{erg\ s^{-1}\ cm^{-2}}$, which places a
stringent 3-$\rm\sigma$ upper limit of $5.43\times 10^{-18}$ $\mathrm{erg\
s^{-1}\ cm^{-2}}$ on CIII] flux and 6.57 \AA\ on the CIII] equivalent width.
Using the stacked image, we put a 3-$\rm\sigma$ upper limit on the mean CIII]
flux of $\mathrm{2.55\times10^{-18}\ erg\ s^{-1}\ cm^{-2}}$, and a
3-$\rm\sigma$ upper limit on the mean CIII] equivalent width of 4.20 {\AA} for
this sample of galaxies at z = 5.70. Combined with strong CIII] detection
reported among high-z galaxies in the literature, our observations suggest that
the equivalent widths of CIII] from galaxies at z $>$ 5.70 exhibit a wide range
of distribution. Our strong limits on CIII] emission could be used as a guide
for future observations in the reionization epoch.
