  We discuss new Keck/MOSFIRE spectroscopic observations of four luminous
galaxies at z~7-9 selected to have intense optical line emission by
Roberts-Borsani et al. (2016). Previous follow-up has revealed Lyman-alpha in
two of the four galaxies. Our new MOSFIRE observations confirm that Lyman-alpha
is present in the entire sample. We detect Lyman-alpha emission in COS-zs7-1,
confirming its redshift as z=7.154, and we detect Lyman-alpha in EGS-zs8-2 at
z=7.477, verifying a tentative detection presented in an earlier study. The
ubiquity of Lyman-alpha in this sample is puzzling given that the IGM is likely
significantly neutral over 7<z<9. To investigate this result in more detail, we
have initiated a campaign to target UV metal emission in the four Lyman-alpha
emitters as a probe of both the radiation field and the velocity offset of
Lyman-alpha. Here we present the detection of intense CIII] emission in
EGS-zs8-1, a galaxy from this sample previously shown to have Lyman-alpha at
z=7.73. Photoionization models indicate that an intense radiation field and low
metallicity are required to reproduce the intense CIII] and optical line
emission. We argue that this extreme radiation field is likely to affect the
local environment, increasing the transmission of Lyman-alpha through the
galaxy. Moreover, the centroid of CIII] indicates that Lyman-alpha is
redshifted from the systemic value by 340 km/s. This velocity offset is larger
than that seen in less luminous systems, providing an additional explanation
for the transmission of Lyman-alpha emission through the IGM. Since the
transmission is further enhanced by the likelihood that such systems are also
situated in the densest regions with the largest ionized bubbles, the
visibility of Lyman-alpha at z>7 is expected to be strongly
luminosity-dependent, with the most effective transmission occurring in systems
with intense star formation.
