Rest-frame UV spectroscopy of extreme [OIII] emitters at $1.3<z<3.7$:
  Toward a high-redshift UV reference sample for JWST
  Deep spectroscopy of galaxies in the reionization-era has revealed intense
CIII] and CIV line emission (EW $>15-20$ \r{A}). In order to interpret the
nebular emission emerging at $z>6$, we have begun targeting rest-frame UV
emission lines in galaxies with large specific star formation rates (sSFRs) at
$1.3<z<3.7$. We find that CIII] reaches the EWs seen at $z>6$ only in large
sSFR galaxies with [OIII]+H$\beta$ EW $>1500$ \r{A}. In contrast to previous
studies, we find that many galaxies with intense [OIII] have weak CIII]
emission (EW $=5-8$ \r{A}), suggesting that the radiation field associated with
young stellar populations is not sufficient to power strong CIII].
Photoionization models demonstrate that the spread in CIII] among systems with
large sSFRs ([OIII]+H$\beta$ EW $>1500$ \r{A}) is driven by variations in
metallicity, a result of the extreme sensitivity of CIII] to electron
temperature. We find that the strong CIII] emission seen at $z>6$ (EW $>15$
\r{A}) requires metal poor gas ($\simeq0.1\ Z_\odot$) whereas the weaker CIII]
emission in our sample tends to be found at moderate metallicities ($\simeq0.3\
Z_\odot$). The luminosity distribution of the CIII] emitters in our
$z\simeq1-3$ sample presents a consistent picture, with stronger emission
generally linked to low luminosity systems ($M_{\rm{UV}}>-19.5$) where low
metallicities are more likely. We quantify the fraction of strong CIII] and CIV
emitters at $z\simeq1-3$, providing a baseline for comparison against $z>6$
samples. We suggest that the first UV line detections at $z>6$ can be explained
if a significant fraction of the early galaxy population is found at large sSFR
($>200$ Gyr$^{-1}$) and low metallicity ($<0.1\ Z_\odot$).
