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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$α$ at 0.3<z<0.42 and [OIII]$λ$5007 at 0.7<z<0.86. We identify 10,470 galaxies with z excesses in the COSMOS dataset and 91,385 in the NEP field. We cross-reference the COSMOS EELG sample with the zCOSMOS and DEIMOS 10k spectral catalogs, finding 1395 spectroscopic matches. We made an additional 71 (46 unique) spectroscopic measurements with Y<23 using the HYDRA multi-object spectrograph on the WIYN 3.5m telescope, and 204 spectroscopic measurements from the DEIMOS spectrograph on the Keck II telescope, providing a total of 1441/10,470 spectroscopic redshifts for the EELG sample in COSMOS (~14%). We confirm that 1418 (~98%) are H$α$ or [OIII]$λ$5007 emitters in the above stated redshift ranges. We also identify 240 redshifted H$α$ and [OIII]$λ$5007 emitters in the NEP using spectra taken with WIYN/HYDRA and Keck/DEIMOS. Using broadband selection techniques in g-r-i color space, we distinguish between H$α$ and [OIII]$λ$5007 emitters with 98.6% accuracy. We test our EELG selection by constructing H$α$ and [OIII]$λ$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.