2017ApJ...838L..22D

Recent observations have suggested that the C III]λ1907/1909 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±1.81×10^–18erg s^–1 cm^–2, which places a stringent 3σ upper limit of 5.43×10^–18erg s^–1 cm^–2 on C III] flux and 6.57 A on the C III] equivalent width. Using the stacked image, we put a 3σ upper limit on the mean C III] flux of 2.55×10^–18 erg s^–1 cm^–2 and a 3σ upper limit on the mean C III] equivalent width of 4.20 A 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.