Extreme Emission Line Galaxies Observed with the James Webb Space Telescope's Medium-Band Filters

Since the beginning of the James Webb Space Telescope's science operations, there have been many observations dedicated to characterizing galaxies during the Epoch of Reionization (EoR). This thesis uses medium-band photometry from the JWST to search for Extreme Emission Line Galaxies (EELGs) in the EoR and analogues at lower redshifts. We present a sample of EELGs over 1.7 < z < 6.7 selected on their strong Halpha and [OIII] + Hbeta emission lines.  We measure the equivalent widths (EWs) and physical properties of the galaxies in our sample, finding high EWs (median EW(Halpha) = 893 Angstroms, EW([OIII] + Hbeta) = 1255 Angstroms) and properties typical of EELGs: mostly compact with low stellar mass, low metallicity, little dust, and high specific star formation rates. Follow-up NIRSpec spectroscopy of 15 of the EELGs in our sample confirm the nature and EWs of the EELGs measured from medium-band photometry, demonstrating the accuracy of our colour selections.