First Results from the Lyman Alpha Galaxies in the Epoch of Reionization (LAGER) Survey: Cosmological Reionization at z ∼ 7

Abstract

We present the first results from the ongoing Lyman Alpha Galaxies in the Epoch of Reionization (LAGER) project, which is the largest narrowband survey for z ∼ 7 galaxies to date. Using a specially built narrowband filter NB964 for the superb large-area Dark Energy Camera (DECam) on the NOAO/CTIO 4 m Blanco telescope, LAGER has collected 34 hr NB964 narrowband imaging data in the 3 deg 2 COSMOS field. We have identified 23 Ly α Emitter candidates at z = 6.9 in the central 2-deg 2 region, where DECam and public COSMOS multi-band images exist. The resulting luminosity function (LF) can be described as a Schechter function modified by a significant excess at the bright end (four galaxies with L Ly α ∼ 10 43.4±0.2 erg s −1 ). The number density at L Ly α  ∼ 10 43.4±0.2 erg s −1 is little changed from z  = 6.6, while at fainter L Ly α it is substantially reduced. Overall, we see a fourfold reduction in Ly α luminosity density from z  = 5.7 to z  = 6.9. Combined with a more modest evolution of the continuum UV luminosity density, this suggests a factor of ∼3 suppression of Ly α by radiative transfer through the z  ∼ 7 intergalactic medium (IGM). It indicates an IGM neutral fraction of x H i  ∼ 0.4–0.6 (assuming Ly α velocity offsets of 100–200 km s −1 ). The changing shape of the Ly α LF between z ≲ 6.6 and z  = 6.9 supports the hypothesis of ionized bubbles in a patchy reionization at z ∼ 7.