tokens: versus dust-corrected O 32 values (a proxy for ionization parameter) for our galaxies (blue circles), colour-coded by absolute i ′-band magnitude. We plot as black points the XMPs uncovered uniformly from the SDSS spectroscopic sample by Sánchez Almeida et al. ( 2016 ). The uncertainties (1σ) are plotted for our galaxies, and are hidden by the markers in several cases. The vast majority of SDSS XMPs are clustered at Hβ EWs 50 Å and O 32 3. Relatively few previously identified XMPs extend into the high-sSFR region of this diagram at Hβ EWs >50 where our objects reside. The gas in the XMPs presented here is also more highly ionized than in most XMPs from the SDSS spectroscopic sample, consistent with substantially different stellar populations and resulting ionizing radiation fields. The O 32 ratio is commonly used as a proxy for the ionization parameter, or the density of ionizing radiation relative to the gas density. Fig. 9 reveals that indeed, the XMPs presented here host more highly ionized gas than most XMPs from the SDSS spectroscopic sample. Our XMPs have O 32 ratios ranging as high as 10, with median O 32 = 4.1 (and an even higher median O 32 = 6.4 for the subset with Hβ equivalent widths above 100 Å). This is substantially higher than the majority of SDSS XMPs shown, which have median O 32 = 1.2. This indicates that the gas in the galaxies we have uncovered is more highly ionized than in most previously known XMPs, consistent with these systems hosting harder ionizing radiation fields. The offset towards higher Hβ equivalent widths and O 32 ratios in our XMPs suggests that these systems host significantly more prominent young massive stellar populations than typical previously known XMPs. Fig. 9 indicates that the average XMP from the SDSS spectroscopic sample has undergone a more extended star formation history, resulting in an optical spectrum with a substantial contribution from stars formed >10 Myr ago and less highly ionized gas. The EUV and X-ray radiation powered by a typical nearby XMP with a Hβ equivalent width of 30 Å (sSFR ≃ 1 under constant star formation) is potentially very different from that of a reionization-era galaxy which has formed essentially all of its stars in the last 100 Myr (sSFR ≃ 5–10 ). Future studies targeting these high-sSFR XMPs will provide a new window on to the massive stars and X-ray populations that likely dominate the early universe. As a first step towards constraining their EUV ionizing spectra, we examine high-ionization nebular He ii emission in our galaxies. The He ii λ4686 nebular emission line is a recombination line powered by extremely energetic photons >54.4 eV. This line is commonly observed in low-metallicity star-forming dwarf galaxies (e.g. Garnett et al. 1991 ; Brinchmann, Kunth Durret 2008 ; Shirazi Brinchmann 2012 ; Senchyna et al. 2017 ), but the origin of the necessary >54.4 eV photons is still a matter of debate. The commonly cited candidate sources of narrow He ii are very
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