Astro2020 Science White Paper: Exploration and characterization of the
  earliest epoch of galaxy formation: beyond the re-ionization era
  State-of-the-art rest-frame UV and FIR photometric and spectroscopic
observations are now pushing the redshift frontiers of galaxy formation studies
up to $z\sim9-11$ and beyond. Recent HST observations unveiled the presence of
a star-forming galaxy exhibiting the Lyman break at $\lambda_{\rm
obs}=1.47\pm0.01$ $\mu$m, i.e., a $z=11.09^{+0.08}_{-0.12}$ galaxy with a
stellar mass of $\sim10^9 M_\odot$, demonstrating that galaxy build-up was well
underway early in the epoch of reionization (EoR) at $z>10$. Targeted
spectroscopy of a lensed Lyman break galaxy uncovers the earliest metals known
to date up to $z=9.1096\pm0.0006$ by detecting the bright [OIII] 88~$\mu$m
nebular line, indicating the onset of star formation 250 million years after
the Big Bang, i.e., corresponding to a redshift of $z\sim15$. These latest
findings lead us to a number of key questions: How and when metal enrichment
happened in the EoR? What was the nature of the earliest-epoch star-forming
galaxies at $z=10-15$? What was the spatial distribution of such galaxies, and
what was the relation to the putative large-scale ionization bubbles during the
EoR? What were the dark-halo masses of such earliest-epoch star-forming
galaxies? To address all these questions, we need to uncover a statistically
large number of $z=10-15$ galaxies in the pre-reionization era. Here we argue
two possible pathways: (1) a wide-area, sensitive blind spectroscopic survey of
[OIII] 88 $\mu$m line-emitting galaxies at submillimeter wavelengths, and (2)
an ultra-wide-area, high-cadence photometric survey of transient sources at
radio-to-(sub)millimeter wavelengths, together with the immediate follow-up
spectroscopy with an ultra-wide-band spectrograph, to catch the pop-III
$\gamma$-ray bursts.
