Anatomy of an ionized bubble at z=6.6: Which galaxies reionized the Universe?

Identifying the sources that drove cosmic reionization is a key goal of observational cosmology. Photons from these sources carved out ionized bubbles in the neutral intergalactic medium, and these bubbles gradually coalesced, resulting in a fully ionized Universe.

The luminous z=6.6 'COLA1' galaxy lies in the epoch of reionization and shows a remarkable, double-peaked Lyman-alpha (Lya) line, the only one confirmed by multiple teams with high SNR and resolution. The detection of Lya flux bluewards of the systemic velocity means COLA1 resides in an ionized bubble. The exact velocity at which the blue Lya light is cut-off constrains the bubble size. This bubble provides a unique fortuitous, controlled environment -- since the bubble size is constrained, so is the total ionizing flux required to power it. Did COLA1 produce this ionizing flux all by itself? Or is it surrounded by large numbers of bright galaxies? Is a significant contribution from the faintest galaxies necessary?

We propose to blindly identify emission-line galaxies within the ionized bubble and to obtain sensitive spectroscopy of COLA1 itself with slitless grism spectroscopy in the NIRCam F356W filter. The bubble size is well matched to the effective field of view for H-beta and the [OIII] doublet at z=6.6. We will obtain spectroscopic redshifts for all objects brighter than 0.1 L* (SFR>2 Msun/yr) and directly measure their ionizing photon production rate. We will then assess how much contribution from unseen galaxies is required. Through our detailed accounting of ionizing photons we will address the central question to reionization studies: was it bright or faint galaxies that reionized the universe?