The Epoch of Reionisation

The epoch of re-ionisation represents a dramatic period in our Universe’s history when the Universe transformed from a predominantly neutral to a predominantly ionised state. Understanding this era remains one of the key milestones for many upcoming experiments (e.g. JWST, Euclid, E-ELT). The depth and area of VEILS has been designed to probe both the bright-end of the galaxy luminosity function and the faint-end of the quasar luminosity function at the epoch of reionization and thus definitively establish the overlap between the two populations by connecting their luminosity distributions (Fig. 1). New results from Planck have now determined the redshift of instantaneous re-ionisation: zrein = 8.8±1.2 1.1. However, re-ionisation is an extended process and there is mounting evidence for patchy re-ionisation at z ⇠ 6 based on detailed studies of the Ly↵ forest transmission in high redshift quasars (e.g. Becker et al. 2015). Starforming galaxies at high-redshift are thought to be the primary sources responsible for reionization (Robertson et al. 2015) but the process is still not well understood with recent suggestions that quasars may play a non-negligible role (Madau & Haardt 2015). Indeed, Chardin et al. (2015 & in prep) find that a population of galaxies and quasars is required to explain the observed distribution of e↵ective optical depths of the Ly↵ forest at z ⇠ 6. Large numbers of bright z ⇠ 6 quasars have been found using wide-field NIR surveys such as UKIDSS, VHS and VIKING (Mortlock et al. 2011, Venemans et al. 2013). However, as shown in Fig. 1, the re-ionisation simulations also require a significant population of L⇤ quasars at M1500 ⇠ −24. Measurements of quasar numbers at these luminosities are still highly uncertain and VEILS will pin down this measurement thus directly constraining re-ionisation models. Based on the Willott et al. (2010) quasar luminosity function, we expect ⇠ 5 − 50 z ⇠ 6 quasars in our survey area (all at M1500 > −24 (AB)).