Ultra-strong Emission Line Galaxies in the North Ecliptic Pole Field

We present initial results from a spectroscopic follow-up campaign on candidate intermediate-redshift ultra-strong emission line galaxies (USELs) in the North Ecliptic Pole (NEP) field. Using ~30 sq. deg. of deep grizy and narrowband (NB) imaging at 8160 and 9210 A from Subaru/Hyper SuprimeCam, we selected sources with a >1 magnitude "excess" in each NB relative to the underlying broadband, which is caused by high equivalent width emission lines falling into the NB. We then followed up our sources using the multi-fiber positioner Hydra on the 3.5m WIYN telescope to identify the excess-causing line (and redshift) and determine basic properties. [OIII] 4959,5007 emitters at z ~ 0.63 (0.85) dominate our NB816 (NB921) sample, with only a handful of NB excesses caused by Ha at z ~ 0.24 (0.4) or [OII] at z ~ 1.2 (1.4). We derive line luminosity functions for [OIII], [OII], and Balmer lines in each of our redshift sheets; these allow us to compute the contribution of our USELs to the cosmic star formation rate density as a function of redshift. Many of our galaxies have large rest-frame Hb equivalent widths (~30 A or greater) in emission, which has been shown to correlate with significant emission in the [OIII] 4363 auroral line and hence extremely low metallicity, though further follow-up with Keck/DEIMOS is required to detect it. Our observations in the NEP place special emphasis on the JWST NEP Time Domain field and the AKARI NEP deep and wide fields, enhancing their legacy value. These data will also provide great synergy with the recently-launched eROSITA X-ray mission, which will achieve its greatest depths in the ecliptic poles.