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Extremely metal-poor star-forming galaxies (XMPs) represent one of our onlylaboratories for study of the low-metallicity stars we expect to encounter atearly epochs. But as our understanding of the $z>6$ universe has improved, ithas become clear that the majority of known XMPs within 100 Mpc hostsignificantly less prominent massive star populations than theirreionization-era counterparts, severely limiting their utility as testbeds forinterpreting spectral features found at the highest redshifts. Here we presenta new photometric selection technique designed to identify nearby XMPsdominated by young stellar populations comparable to those expected in thereionization era. We apply our technique to uncover candidate XMPs in SDSSimaging at magnitudes $16<i'<23$, extending significantly below thecompleteness limits of the SDSS spectroscopic survey. Spectroscopicobservations with the MMT confirm that 32 of the 53 uniformly metal-poor andhigh specific star formation rate targets we observed have gas-phase oxygenabundances $12+\log\mathrm{O/H}<7.7$ ($Z/Z_\odot<0.1$), including two in therange of the lowest-metallicity galaxies known, $Z/Z_\odot<0.05$. Ourobservations shed new light onto the longstanding mystery of He II emission instar-forming galaxies: we find that the equivalent width of the He II $\lambda4686$ high-ionization emission line does not scale with that of H$\beta$ in oursample, suggesting that binary evolution or other processes on $>10$ Myrtimescales contribute substantially to the $\mathrm{He^+}$-ionizing photonbudget in this metallicity regime. Applying such selection techniques coupledwith deep spectroscopy to next-generation photometric surveys like LSST mayeventually provide a basis for an empirical understanding of metal-poor massivestars.

monthly notices of the royal astronomical society

Photometric identification and MMT spectroscopy of new extremely  metal-poor galaxies: towards a better understanding of young stellar  populations at low metallicity