Broad-band selection, spectroscopic identification, and physical
  properties of a population of extreme emission line galaxies at 3<z<3.7
  We present the selection, spectroscopic identification, and physical
properties of extreme emission line galaxies (EELGs) at $3<z<3.7$ aiming at
studying physical properties of an analog population of star-forming galaxies
(SFGs) at the epoch of reionization. The sample is selected based on the excess
in the observed Ks broad band flux relative to the best-fit stellar continuum
model flux. By applying a 0.3 mag excess as a primary criterion, we select 240
EELG candidates with intense emission lines and estimated observed-frame
equivalent width (EW) of $\gtrsim 1000$ angstrom over the UltraVISTA-DR2
ultra-deep stripe in the COSMOS field. We then carried out a HK band follow-up
spectroscopy for 23 of the candidates with Subaru/MOIRCS, and find that 19 and
two of them are at $z>3$ with intense [OIII] emission, and H$\alpha$ emitters
at $z\simeq 2$, respectively. These spectroscopically identified EELGs at
$z\simeq 3.3$ show, on average, higher specific star formation rates (sSFR)
than the star-forming main sequence, low dust attenuation of $E(B-V) \lesssim
0.1$ mag, and high [OIII]/[OII] ratios of $\gtrsim 3$. We also find that our
EELGs at $z\simeq 3.3$ have higher hydrogen ionizing photon production
efficiencies ($\xi_\mathrm{ion}$) than the canonical value ($\simeq 10^{25.2}$
Hz/erg), indicating that they are efficient in ionizing their surrounding
interstellar medium. These physical properties suggest that they are low
metallicity galaxies with higher ionization parameters and harder UV spectra
than normal SFGs, which is similar to galaxies with Lyman continuum (LyC)
leakage. Among our EELGs, those with the largest [OIII]/[OII] and EW([OIII])
values would be the most promising candidates to search for LyC leakage.
