Constraining C iii] emission in a sample of five luminous z = 5.7 galaxies

Recent observations have suggested that the C iii]$\lambda 1907/1909$ 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 (HST) Wide Field Camera 3 (WFC3) to search for C iii] 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 C iii] emission for the five galaxies with $J_{AB}$ ranging from 24.10 to 27.00 in our sample. For the brightest galaxy J132416.13+274411.6 in our sample (z = 5.70, $J_{AB}=24.10$), which has a significantly higher signal to noise, we report a C iii] flux of $3.34\pm 1.81\times {10}^{-18}$ $erg{s}^{-1}{cm}^{-2}$, which places a stringent 3σ upper limit of $5.43\times {10}^{-18}$ $erg{s}^{-1}{cm}^{-2}$ on C iii] flux and 6.57 Å on the C iii] equivalent width. Using the stacked image, we put a 3σ upper limit on the mean C iii] flux of $2.55\times {10}^{-18}erg{s}^{-1}{cm}^{-2}$ and a 3σ upper limit on the mean C iii] equivalent width of 4.20 Å for this sample of galaxies at z = 5.70. Combined with strong C iii] detection reported among high-z galaxies in the literature, our observations suggest that the equivalent widths of C iii] from galaxies at z > 5.70 exhibit a wide range of distribution. Our strong limits on C iii] emission could be used as a guide for future observations in the reionization epoch.