RELICS: Spectroscopy of gravitationally-lensed $z\simeq 2$
  reionization-era analogs and implications for CIII] detections at $z>6$
  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$_{\rm{[OIII]+H\beta}}$=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$_{\rm{[OIII]+H\beta}}\simeq 2000$ \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$_{\rm{[OIII]+H\beta}}\simeq 2000$ \r{A}, providing a method of
spectroscopic confirmation independent of Ly$\alpha$.
