We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z 7 − 9 selected to have intense rest-frame optical line emission by Roberts-Borsani et al. (2016). Previous spectroscopic follow-up has revealed Lyα emission in two of the four galaxies. Our new MOSFIRE observations confirm that Lyα is present in the entire sample. We detect Lyα emission in the galaxy COS-zs7-1, confirming its redshift as z Lyα = 7.154, and we detect Lyα in EGS-zs8-2 at z Lyα = 7.477, verifying a tentative detection presented in an earlier study. The ubiquity of Lyα emission in this unique photometric sample is puzzling given that the IGM is expected to be significantly neutral over 7 < z < 9. To investigate this surprising result in more detail, we have initiated a campaign to target UV metal line emission in the four Lyα emitters as a probe of both the ionizing radiation field and the velocity offset of Lyα at early times. Here we present the detection of very large equivalent width [CIII], CIII] λλ1907,1909Å emission in EGS-zs8-1 (W CIII],0 = 22 ± 2Å), a galaxy from this sample previously shown to have Lyα emission at z = 7.73. Photoionization models indicate that an intense radiation field (log 10 ξ * ion [erg −1 Hz] 25.6) and moderately low metallicity (0.11 Z ) are required to reproduce the CIII] line emission and intense optical line emission implied by the broadband SED. We argue that this extreme radiation field is likely to affect the local environment, increasing the transmission of Lyα through the galaxy. Moreover, the centroid of CIII] emission indicates that Lyα is redshifted from the systemic value by 340 km sec −1 . This velocity offset is larger than that seen in less luminous systems and provides an additional explanation for the transmission of Lyα emission through the intergalactic medium. Since the transmission is further enhanced by the likelihood that such systems are also situated in the densest regions with accelerated evolution and the largest ionized bubbles, the visibility of Lyα at z > 7 is expected to be strongly luminosity-dependent, with the most effective transmission occurring in systems with intense star formation.

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z /2. Such tests are particularly important for interpreting the surprising high-ionization UV line emission detected at z > 6 in recent years. We present HST /COS ultraviolet spectra of ten nearby metal-poor star-forming galaxies selected to show He ii emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + log O/H < 8.5) and present uniformly large specific star formation rates (sSFR ∼ 10 2 Gyr −1 ). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C iii] equivalent widths comparable to those seen in systems at z ∼ 6 − 7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + log O/H 8.0 (Z/Z 1/5) presenting minimal stellar wind features and prominent nebular emission in He ii and C iv. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He + -ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C iv and He ii powered by these stars may provide useful metallicity constraints in the reionization era.