Subaru/HSC z-Broadband Excess Selection of Extreme Emission Line
  Galaxies at z<1
  We present a search for extreme emission line galaxies (EELGs) at z<1 in the
COSMOS and North Ecliptic Pole (NEP) fields with imaging from Subaru/Hyper
Suprime-Cam (HSC) and a combination of new and existing spectroscopy. We select
EELGs on the basis of substantial excess flux in the z broadband, which is
sensitive to H$\alpha$ at 0.3<z<0.42 and [OIII]$\lambda$5007 at 0.7<z<0.86. We
identify 10,470 galaxies with z excesses in the COSMOS dataset and 91,385 in
the NEP field. We cross-reference the COSMOS EELG sample with the zCOSMOS and
DEIMOS 10k spectral catalogs, finding 1395 spectroscopic matches. We made an
additional 71 (46 unique) spectroscopic measurements with Y<23 using the HYDRA
multi-object spectrograph on the WIYN 3.5m telescope, and 204 spectroscopic
measurements from the DEIMOS spectrograph on the Keck II telescope, providing a
total of 1441/10,470 spectroscopic redshifts for the EELG sample in COSMOS
(~14%). We confirm that 1418 (~98%) are H$\alpha$ or [OIII]$\lambda$5007
emitters in the above stated redshift ranges. We also identify 240 redshifted
H$\alpha$ and [OIII]$\lambda$5007 emitters in the NEP using spectra taken with
WIYN/HYDRA and Keck/DEIMOS. Using broadband selection techniques in g-r-i color
space, we distinguish between H$\alpha$ and [OIII]$\lambda$5007 emitters with
98.6% accuracy. We test our EELG selection by constructing H$\alpha$ and
[OIII]$\lambda$5007 luminosity functions and comparing to recent literature
results. We conclude that broadband magnitudes from HSC, the Vera C. Rubin
Observatory, and other deep optical multi-band surveys can be used to select
EELGs in a straightforward manner.
