We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z \simeq 7 - 9 selected to have intense rest-frame optical line emission by .
Previous spectroscopic follow-up has revealed Ly \alpha emission in two of the four galaxies .
Our new MOSFIRE observations confirm that Ly \alpha is present in the entire sample .
We detect Ly \alpha emission in the galaxy COS-zs7-1 , confirming its redshift as z _ { Ly \alpha } = 7.154 , and we detect Ly \alpha in EGS-zs8-2 at z _ { Ly \alpha } = 7.477 , verifying a tentative detection presented in an earlier study .
The ubiquity of Ly \alpha 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 Ly \alpha emitters as a probe of both the ionizing radiation field and the velocity offset of Ly \alpha at early times .
Here we present the detection of very large equivalent width [ CIII ] , CIII ] \lambda \lambda 1907,1909 Å emission in EGS-zs8-1 ( W _ { CIII ] , 0 } = 22 \pm 2 Å ) , a galaxy from this sample previously shown to have Ly \alpha emission at z = 7.73 .
Photoionization models indicate that an intense radiation field ( log _ { 10 } \xi ^ { \ast } _ { ion } [ erg ^ { -1 } Hz ] ~ { } \simeq 25.6 ) and moderately low metallicity ( 0.11 Z _ { \odot } ) 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 Ly \alpha through the galaxy .
Moreover , the centroid of CIII ] emission indicates that Ly \alpha 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 Ly \alpha 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 Ly \alpha at z > 7 is expected to be strongly luminosity-dependent , with the most effective transmission occurring in systems with intense star formation .