Astro2020 Science White Paper: Exploration and characterization of the earliest epoch of galaxy formation: beyond the re-ionization era

Abstract

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 $z\sim 9-11$ and beyond. Recent HST observations unveiled the presence of a star-forming galaxy exhibiting the Lyman break at $\hat{I}{»}_{\mathrm{obs}}=1.47\pm 0.01$ $\hat{I}¼$m, i.e., a $z=11.09_{-0.12}^{+0.08}$ galaxy with a stellar mass of $\sim 10^9{M}_{\odot }$, 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 $z=9.1096\pm 0.0006$ by detecting the bright [OIII] 88~$\hat{I}¼$m nebular line, indicating the onset of star formation 250 million years after the Big Bang, i.e., corresponding to a redshift of $z\sim 15$. 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 $\hat{I}¼$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 ${\hat{I}}^3$-ray bursts.

Science White paper submitted to Astro2020 Decadal Survey