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.

We present Keck/MOSFIRE observations of UV metal lines in four bright (H=23.9-25.4) gravitationally-lensed z 6 − 8 galaxies behind the cluster Abell 1703. The spectrum of A1703-zd6, a highly-magnified star forming galaxy with a Lyα redshift of z = 7.045, reveals a confident (S/N=5.1) detection of the nebular CIVλ1548 emission line (unresolved with FWHM<125 km s −1 ). UV metal lines are not detected in the three other galaxies. At z 2 − 3, nebular CIV emission is observed in just 1% of UV-selected galaxies. The presence of strong CIV emission in one of the small sample of galaxies targeted in this paper may indicate hard ionizing spectra are more common at z 7. The total estimated equivalent width of the CIV doublet (W CIV 38Å) and CIV/Lyα flux ratio (f CIV /f Lyα 0.3) are comparable to measurements of narrow-lined AGNs. Photoionization models show that the nebular CIV line can also be reproduced by a young stellar population, with very hot metal poor stars dominating the photon flux responsible for triply ionizing carbon. Regardless of the origin of the CIV, we show that the ionizing spectrum of A1703-zd6 is different from that of typical galaxies at z 2, producing more H ionizing photons per unit 1500Å luminosity (log(ξ ion /erg −1 Hz)=25.68) and a larger flux density at 30-50 eV. If such extreme radiation fields are typical in UV-selected systems at z ∼ > 7, it would indicate that reionization-era galaxies are more efficient ionizing agents than previously thought. Alternatively, we suggest that the small sample of Lyα emitters at z ∼ > 7 may trace a rare population with intense radiation fields capable of ionizing their surrounding hydrogen distribution. Additional constraints on high ionization emission lines in galaxies with and without Lyα detections will help clarify whether hard ionizing spectra are common in the reionization era.

We present the Magellan/FIRE detection of highly ionized C IVλ1550 and O III]λ1666 in a deep infrared spectrum of the z=6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has previously known Lyα. No corresponding emission is detected at the expected location of He IIλ1640. The upper limit on He II, paired with detection of O III] and C IV, constrains possible ionization scenarios. Production of C IV and O III] requires ionizing photons of 2.5-3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (∼0.5 Ryd). As a pure recombination line, He II emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher He II flux. The conclusions based on ratios of metal line detections to helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general = z 2 3 -population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.

Large surveys of galaxy clusters with the Hubble and Spitzer Space Telescopes, including CLASH and the Frontier Fields, have demonstrated the power of strong gravitational lensing to efficiently deliver large samples of high-redshift galaxies. We extend this strategy through a wider, shallower survey named RELICS, the Reionization Lensing Cluster Survey. This survey, described here, was designed primarily to deliver the best and brightest high-redshift candidates from the first billion years after the Big Bang. RELICS observed 41 massive galaxy clusters with Hubble and Spitzer at 0.4-1.7µm and 3.0-5.0µm, respectively. We selected 21 clusters based on Planck PSZ2 mass estimates and the other 20 based on observed or inferred lensing strength. Our 188-orbit Hubble Treasury Program obtained the first high-resolution near-infrared images of these clusters to efficiently search for lensed highredshift galaxies. We observed 46 WFC3/IR pointings (∼200 arcmin 2 ) with two orbits divided among four filters (F105W, F125W, F140W, and F160W) and ACS imaging as needed to achieve single-orbit depth in each of three filters (F435W, F606W, and F814W). As previously reported by Salmon et al., we discovered 322 z ∼ 6 − 10 candidates, including the brightest known at z ∼ 6, and the most spatially-resolved distant lensed arc known at z ∼ 10. Spitzer IRAC imaging (945 hours awarded, plus 100 archival) has crucially enabled us to distinguish z ∼ 10 candidates from z ∼ 2 interlopers. For each cluster, two HST observing epochs were staggered by about a month, enabling us to discover 11 supernovae, including 3 lensed supernovae, which we followed up with 20 orbits from our program. We delivered reduced HST images and catalogs of all clusters to the public via MAST and reduced Spitzer images via IRSA. We have also begun delivering lens models of all clusters, to be completed before the JWST GO Cycle 1 call for proposals.