Spectroscopy from Photometry: A Population of Extreme Emission Line Galaxies at 1.7 ? z ? 6.7 Selected with JWST Medium Band Filters

We use JWST/NIRCam medium band photometry in a single pointing of the Canadian NIRISS Unbiased Cluster Survey to identify 118 extreme emission-line galaxies (EELGs) over 1.7 less than or similar to z less than or similar to 6.7, selected using a set of color cuts that target galaxies with extreme [O III] + H beta and H alpha emission. We show that our medium band color selections are able to select galaxies based on emission-line EW, which is advantageous to more commonly used selections since it does not require strong continuum emission, and can select galaxies with faint or red continuum fluxes. The median EWs of our sample is EW(H alpha) = 893 angstrom and EW ([O III] + H beta) = 1255 angstrom and includes some objects with EW ([O III] + H beta) similar to 3000 angstrom. These systems are mostly compact with low stellar mass (median log(M-star/M-circle dot) = 8.03), low metallicity (median Z = 0.14 Z(circle dot)), little dust (median A(V) = 0.18 mag), and high SSFR (median SSFR = 1.18 x 10(-8) yr(-1)). Additionally, galaxies in our sample show increasing EW(H alpha) and EW([O III] + H beta) with redshift, an anticorrelation of EW(H alpha) with stellar mass, and no correlation between EW([O III] + H beta) and stellar mass. Finally, we present NIRSpec spectroscopy of 15 of the EELGs in our sample. These spectra confirm the redshifts and EWs of the EELGs calculated from the medium bands, which demonstrates the accuracy and efficiency of our color selections. Overall, we show that there are significant advantages to using medium band photometry to identify and study EELGs at a wide range of redshifts.