New Methods for Identifying Lyman Continuum Leakers and
  Reionization-Epoch Analogues
  Identifying low-redshift galaxies that emit Lyman Continuum radiation (LyC
leakers) is one of the primary, indirect methods of studying galaxy formation
in the epoch of reionization. However, not only has it proved challenging to
identify such systems, it also remains uncertain whether the low-redshift LyC
leakers are truly "analogues" of the sources that reionized the Universe. Here,
we use high-resolution cosmological radiation hydrodynamics simulations to
examine whether simulated galaxies in the epoch of reionization share similar
emission line properties to observed LyC leakers at $z\sim3$ and $z\sim0$. We
find that the simulated galaxies with high LyC escape fractions ($f_{\rm esc}$)
often exhibit high O32 and populate the same regions of the R23-O32 plane as
$z\sim3$ LyC leakers. However, we show that viewing angle, metallicity, and
ionisation parameter can all impact where a galaxy resides on the O32-$f_{\rm
esc}$ plane. Based on emission line diagnostics and how they correlate with
$f_{\rm esc}$, lower-metallicity LyC leakers at $z\sim3$ appear to be good
analogues of reionization-era galaxies. In contrast, local [SII]-deficient
galaxies do not overlap with the simulated high-redshift LyC leakers on the
SII-BPT diagram; however, this diagnostic may still be useful for identifying
leakers. We use our simulated galaxies to develop multiple new diagnostics to
identify LyC leakers using IR and nebular emission lines. We show that our
model using only [CII]$_{\rm 158\mu m}$ and [OIII]$_{\rm 88\mu m}$ can identify
potential leakers from non-leakers from the local Dwarf Galaxy Survey. Finally,
we apply this diagnostic to known high-redshift galaxies and find that
MACS1149_JD1 at $z=9.1$ is the most likely galaxy to be actively contributing
to the reionization of the Universe.
