  We discuss the rest-frame optical emission line spectra of a large (~50)
sample of z=3.1 Lyman alpha emitters (LAEs) whose physical properties suggest
such sources are promising analogs of galaxies in the reionization era.
Reliable Lyman continuum escape fractions have now been determined for a large
sample of such LAEs from the Lyman Continuum Escape Survey (LACES) undertaken
via deep HST imaging in the SSA22 survey area reported in Fletcher et al.
(2019). Using new measures of [OII] emission secured from Keck MOSFIRE spectra
we re-examine, for a larger sample, earlier claims that Lyman continuum
leakages may correlate with the nebular emission line ratio [OIII]/[OII] as
expected for density-bound HII regions. We find that a large [OIII]/[OII] line
ratio is indeed a necessary condition for Lyman continuum leakage,
strengthening earlier claims made using smaller samples at various redshifts.
However, not all LAEs with large [OIII]/[OII] line ratios are leakers and
leaking radiation appears not to be associated with differences in other
spectral diagnostics. This suggests the detection of leaking radiation is
modulated by an additional property, possibly the viewing angle for porous HII
regions. We discuss our new results in the context of the striking bimodality
of LAE leakers and non-leakers found in the LACES program and the implications
for the sources of cosmic reionization.
