Recent observations have revealed the presence of strong CIII] emission (EW$_{rm{CIII]}}>20$ r{A}) in $z>6$ galaxies, the origin of which remains unclear. In an effort to understand the nature of these line emitters, we have initiated a survey targeting CIII] emission in gravitationally-lensed reionization era analogs identified in HST imaging of clusters from the RELICS survey. Here we report initial results on four galaxies selected to have low stellar masses (2-8$times$10$^7$ M$_odot$) and J$_{125}$-band flux excesses indicative of intense [OIII]+H$beta$ emission (EW$_{rm{[OIII]+Hbeta}}$=500-2000 r{A}), similar to what has been observed at $z>6$. We detect CIII] emission in three of the four sources, with the CIII] EW reaching values seen in the reionization era (EW$_{rm{CIII]}}simeq 17-22$ r{A}) in the two sources with the strongest optical line emission (EW$_{rm{[OIII]+Hbeta}}simeq 2000$ r{A}). We have obtained a Magellan/FIRE near-infrared spectrum of the strongest CIII] emitter in our sample, revealing gas that is both metal poor and highly ionized. Using photoionization models, we are able to simultaneously reproduce the intense CIII] and optical line emission for extremely young (2-3 Myr) and metal poor (0.06-0.08 Z$_odot$) stellar populations, as would be expected after a substantial upturn in the SFR of a low mass galaxy. The sources in this survey are among the first for which CIII] has been used as the primary means of redshift confirmation. We suggest that it should be possible to extend this approach to $z>6$ with current facilities, using CIII] to measure redshifts of objects with IRAC excesses indicating EW$_{rm{[OIII]+Hbeta}}simeq 2000$ r{A}, providing a method of spectroscopic confirmation independent of Ly$alpha$.
Download