Is the Bremer Deep Field ionised, at z~7?

TL;DR

This study demonstrates that star-forming galaxies in the Bremer Deep Field at z~7 produce enough ionising photons to create and merge large ionised bubbles, supporting their role in cosmic reionisation.

Contribution

It provides the first detailed analysis showing that low luminosity galaxies in the BDF can reionise large regions at z~7, using observational data and cosmological models.

Findings

The BDF galaxies can ionise two large bubbles that are merging.

A low escape fraction of 10% is sufficient for reionisation.

Low luminosity sources are the main contributors to ionising photon production.

Abstract

We show herein that the population of star forming galaxies in the Bremer Deep Field (BDF) have enough ionising power to form two large ionised bubbles which could be in the process of merging into a large one with a volume of 14000 cMpc3. The sources identified in the BDF have been completed with a set of expected low luminosity sources at z $\approx$ 7. We have estimated the number of ionising photons per second produced by the different star forming galaxies in the BDF. This number has been compared with the number that would be required to ionise the bubbles around the two overdense regions. We have used, as reference, ionising emissivities derived from the AMIGA cosmological evolutionary model. We find that even using the most conservative estimates, with a Lyman continuum escape fraction of 10\% the two regions we have defined within the BDF would be reionised. Assuming more realistic estimates of the ionising photon production efficiency, both bubbles would be in the process of merging into a large reionised bubble, such as those that through percolation completed the reionisation of the universe by z = 6. The rather small values of the escape fraction required to reionise the BDF are compatible with the low fraction of faint Ly{\alpha} emitters identified in the BDF. Finally, we confirm that the low luminosity sources represent indeed the main contributors to the BDF ionising photon production.