BoRG postdoc Kasper Borello Schmidt published the latest determination of the z~8 BoRG luminosity function in the Astrophysical Journal.

The luminosity function measured by combining BoRG data with deeper data from the Hubble Ultradeep Field and ERS observations is shown here:

The luminosity function measure shows a very strong evolution in the few hundred Myr separating z~8 from z~6, and like our previous determination in 2012, it finds a steep faint end slope ($$\alpha$$ ~ -1.87 $$\pm$$ 0.26).

We also used the BoRG z~8 luminosity function to perform inference on the epoch of reionization, and in particular on the neutral fraction of hydrogen when the universe was only ~600 Myr old. The BoRG data prefer a ‘late reionization’ scenario, where a significant fraction of the IGM is neutral at z~8, i.e. the reionization was still underway at that time. This is in very good agreement with the results of our recent MOSFIRE follow-up campaign.

In addition to obtaining the largest area determination of the z~8 luminosity function published to date in a refereed journal, Kasper developed a new rigorous Bayesian approach to fit the luminosity function without resorting to binning of the data, commonly employed by previous studies.