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.

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 results from the LymAn Continuum Escape Survey (LACES), a Hubble Space Telescope (HST) program designed to characterize the ionizing radiation emerging from a sample of Lyman alpha emitting galaxies at redshift z 3.1. As many show intense [O iii] emission characteristic of z > 6.5 star-forming galaxies, they may represent valuable low redshift analogs of galaxies in the reionization era. Using HST Wide Field Camera 3 / UVIS F 336W to image Lyman continuum emission, we investigate the escape fraction of ionizing photons in this sample. For 61 sources, of which 77% are spectroscopically confirmed and 53 have measures of [O iii] emission, we detect Lyman continuum leakage in 20%, a rate significantly higher than is seen in individual continuum-selected Lyman break galaxies. We estimate there is a 98% probability that ≤ 2 of our detections could be affected by foreground contamination. Fitting multi-band spectral energy distributions (SEDs) to take account of the varying stellar populations, dust extinctions and metallicities, we derive individual Lyman continuum escape fractions corrected for foreground intergalactic absorption. We find escape fractions of 15 to 60% for individual objects, and infer an average 20% escape fraction by fitting composite SEDs for our detected samples. Surprisingly however, even a deep stack of those sources with no individual F 336W detections provides a stringent upper limit on the average escape fraction of less than 0.5%. We examine various correlations with source properties and discuss the implications in the context of the popular picture that cosmic reionization is driven by such compact, low metallicity star-forming galaxies.

According to the current understanding of cosmic structure formation, the precursors of the most massive structures in the Universe began to form shortly after the Big Bang, in regions corresponding to the largest fluctuations in the cosmic density field 1-3 . Observing these structures during their period of active growth and assembly-the first few hundred million years of the Universe-is challenging because it requires surveys that are sensitive enough to detect the distant galaxies that act as signposts for these structures and wide enough to capture the rarest objects. As a result, very few such objects have been detected so far 4, 5 . Here we report observations of a far-infrared-luminous object at redshift 6.900 (less than 800 Myr after the Big Bang) that was discovered in a wide-field survey 6 . High-resolution imaging reveals this source to be a pair of extremely massive star-forming galaxies. The larger of these galaxies is forming stars at a rate of 2900 solar masses per year, contains 270 billion solar masses of gas and 2.5 billion solar masses of dust, and is more massive than any other known object at a redshift of more than 6. Its rapid star formation is probably triggered by its companion galaxy at a projected separation of just 8 kiloparsecs. This merging companion hosts 35

Galaxies in the reionization-era have been shown to have prominent [O III]+H β emission. Little is known about the gas conditions and radiation field of this population, making it challenging to interpret the spectra emerging at z 6. Motivated by this shortcoming, we have initiated a large MMT spectroscopic survey identifying restframe optical emission lines in 227 intense [O III] emitting galaxies at 1.3 < z < 2.4. This sample complements the MOSDEF and KBSS surveys, extending to much lower stellar masses (10 7 − 10 8 M ) and larger specific star formation rates (5 − 300 Gyr −1 ), providing a window on galaxies directly following a burst or recent upturn in star formation. The hydrogen ionizing production efficiency (ξ ion ) is found to increase with the [O III] EW, in a manner similar to that found in local galaxies by Chevallard et al. (2018). We describe how this relationship helps explain the anomalous success rate in identifying Lyα emission in z 7 galaxies with strong [O III]+H β emission. We probe the impact of the intense radiation field on the ISM using O32 and Ne3O2, two ionization-sensitive indices. Both are found to scale with the [O III] EW, revealing extreme ionization conditions not commonly seen in older and more massive galaxies. In the most intense line emitters, the indices have very large average values (O32 = 9.1, Ne3O2 = 0.5) that have been shown to be linked to ionizing photon escape. We discuss implications for the nature of galaxies most likely to have O32 values associated with significant LyC escape. Finally we consider the optimal strategy for JWST spectroscopic investigations of galaxies at z 10 where the strongest rest-frame optical lines are no longer visible with NIRSpec.