Constraining C iii] Emission in a Sample of Five Luminous z = 5.7 Galaxies

Abstract

Recent observations have suggested that the CIII]$\lambda 1907/1909$ 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 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 $J_{\mathrm{AB}}$ ranging from 24.10 -- 27.00 in our sample. For the brightest galaxy J132416.13+274411.6 in our sample (z = 5.70, $J_{\mathrm{AB}}=24.10$), which has a significantly higher signal to noise, we report a CIII] flux of $3.34\pm 1.81\times 10^{-18}$ $\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}$, which places a stringent 3-$\ddot{\mathrm{I}}ƒ$ upper limit of $5.43\times 10^{-18}$ $\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}$ on CIII] flux and 6.57 à on the CIII] equivalent width. Using the stacked image, we put a 3-$\ddot{\mathrm{I}}ƒ$ upper limit on the mean CIII] flux of $2.55\times 10^{-18}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}$, and a 3-$\ddot{\mathrm{I}}ƒ$ upper limit on the mean CIII] equivalent width of 4.20 à 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.