Constraining CIII] 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 JAB 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 +/- 1.81 x 10(-18) erg s(-1)cm(-2), which places a stringent 3 sigma upper limit of 5.43 x 10(-18) erg s(-1)cm(-2) on C III] flux and 6.57 angstrom on the C III] equivalent width. Using the stacked image, we put a 3 sigma upper limit on the mean C III] flux of 2.55 x 10(-18) erg s(-1) cm(-2) and a 3 sigma upper limit on the mean C III] equivalent width of 4.20 angstrom 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.