RELICS: Spectroscopy of gravitationally-lensed $z\simeq 2$ reionization-era analogs and implications for CIII] detections at $z>6$

Abstract

Recent observations have revealed the presence of strong CIII] emission (EW${}_{\mathrm{CIII}]}>20$ Ã ) 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${\hat{I}}^2$ emission (EW${}_{[\mathrm{OIII}]+\mathrm{H}{\hat{\mathrm{I}}}^2}$=500-2000 Ã ), 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${}_{\mathrm{CIII}]}\simeq 17-22$ Ã ) in the two sources with the strongest optical line emission (EW${}_{[\mathrm{OIII}]+\mathrm{H}{\hat{\mathrm{I}}}^2}\simeq 2000$ Ã ). 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${}_{[\mathrm{OIII}]+\mathrm{H}{\hat{\mathrm{I}}}^2}\simeq 2000$ Ã , providing a method of spectroscopic confirmation independent of Ly$\hat{I}\pm$.

Accepted for publication in MNRAS