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tokens: For Ion2 , the observed ratio is 15 making Ion2 an outlier in the oxygen abundance vs. the [O iii ]/[O ii ] ratio relation and the EW(H) (i.e., sSFR) vs. the [O iii ]/[O ii ] ratio (Fig. 2, Stasińska et al. 2015 ); the Ion2 ratio is higher by 0.6 dex for the observed EW(H) (Table 1 ) and the ratio is higher by 1 dex than galaxies in the SDSS sample with similar metallicities. The Ion2 [O iii ]/[O ii ] ratio is also higher than expected for the derived stellar mass, SFR, and sSFR ( Nakajima Ouchi 2014 ). Therefore, we conclude that the extreme [O iii ]/[O ii ] ratio is due to unusual physical conditions (density-bounded nebula), which imply a low column density of neutral gas, and so favor leakage of ionizing photons ( Nakajima Ouchi 2014 ). 3.3. Physical properties derived from the photometry While the emission lines highlight Ion2 ISM physical properties, we also use the photometry to confirm findings such as the large [O iii ] emission and derive other physical parameters. From Fig. 8 , the jump in the -band is evident, while the continuum level in the adjacent bands, , and IRAC Channel 1 is 24.15. The magnitude in the HAWKI -band corresponds to an additional flux of erg/s/cm, which corresponds to an observed equivalent width of 6700 Å (1600 Å rest-frame). The total flux derived for H+[O iii ] from the MOSFIRE spectrum (see Table 1 ) is fully compatible with the flux estimated from the -band excess (after correcting for slit losses for the three lines, see above). The derived + [OIII] equivalent width is one of the largest ever observed at . The measured line ratios among the H+ [O iii ] suggest the [O iii ] rest-frame equivalent width is 1100 Å, with a similar value to the few extreme cases reported in the literature at ( Atek et al. 2011 ; Amorín et al. 2014b ; Maseda et al. 2014 ). The [O iii ]/H ratio is also higher than the value reported in Holden et al. (2014) for LBGs at , while the ratio is higher in this sample than in the local Universe ( Brinchmann et al. 2004 ). At large equivalent widths of H+[O iii ] have also been reported, measured through photometric excess detected in the Spitzer /IRAC channels (e.g., Finkelstein et al. 2013 ; Smit et al. 2014 ; Oesch et al. 2015 ). While these cases can be spectroscopically investigated only with future facilities, such as the James Webb Space Telescope (JWST) and Extremely Large Telescope class, in our case the MOSFIRE near-infrared spectroscopy has allowed us to access the details of the lines. Fig. 7 Comparison of the metallicity and ionization parameter () between the mean sample properties from Nakajima Ouchi (2014) and Ion2 with 1 uncertainties. Ion2 is compared to local galaxies from the SDSS survey, a galaxy sample at , a Lyman break galaxy (LBG) sample at , a Lyman alpha emitter (LAE) sample at , local Lyman break analogues (LBAs), LyC emitters, and Green Peas (GPs). See Nakajima Ouchi (2014) and references therein for a complete description of these samples. We derive the SFR