We present the first results from the ongoing LAGER project (Lyman Alpha Galaxies in the Epoch of Reionization), 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 4m Blanco telescope, LAGER has collected 34 hours NB964 narrowband imaging data in the 3 deg 2 COSMOS field. We have identified 23 Lyman Alpha Emitter (LAE) 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 can be described as a Schechter function modified by a significant excess at the bright end (4 galaxies with L Lyα ∼ 10 43.4±0.2 erg s −1 ). The number density at L Lyα ∼ 10 43.4±0.2 erg s −1 is little changed from z = 6.6, while at fainter L Lyα it is substantially reduced. Overall, we see a fourfold reduction in Lyα luminosity density from z = 5.7 to 6.9. Combined with a more modest evolution of the continuum UV luminosity density, this suggests a factor of ∼ 3 suppression of Lyα by radiative transfer through the z ∼ 7 intergalactic medium (IGM). It indicates an IGM neutral fraction x HI ∼ 0.4-0.6 (assuming Lyα velocity offsets of 100-200 km s −1 ). The changing shape of the Lyα luminosity function between z 6.6 and z = 6.9 supports the hypothesis of ionized bubbles in a patchy reionization at z ∼ 7.
We present the largest-ever sample of 79 Lyα emitters (LAEs) at z ∼ 7.0 selected in the COSMOS and CDFS fields of the LAGER project (the Lyman Alpha Galaxies in the Epoch of Reionization). Our newly amassed ultradeep narrowband exposure and deeper/wider broadband images have more than doubled the number of LAEs in COSMOS, and we have selected 30 LAEs in the second field CDFS. We detect two large-scale LAE-overdense regions in the COSMOS that are likely protoclusters at the highest redshift to date. We perform injection and recovery simulations to derive the sample incompleteness. We show significant incompleteness comes from blending with foreground sources, which however has not been corrected in LAE luminosity functions in the literature. The bright end bump in the Lyα luminosity function in COSMOS is confirmed with 6 (2 newly selected) luminous LAEs (L Lyα > 10 43.3 erg s −1 ). Interestingly, the bump is absent in CDFS, in which only one luminous LAE is detected. Meanwhile, the faint end luminosity functions from the two fields well agree with each other. The 6 luminous LAEs in COSMOS coincide with 2 LAE-overdense regions, while such regions are not seen in CDFS. The bright-end luminosity function bump could be attributed to ionized bubbles in a patchy reionization. It appears associated with cosmic overdensities, thus supports an inside-out reionization topology at z ∼ 7.0, i.e., the high density peaks were ionized earlier compared to the voids. An average neutral hydrogen fraction of x HI ∼ 0.2 -0.4 is derived at z ∼ 7.0 based on the cosmic evolution of the Lyα luminosity function.
We present a new measurement of the Lyα luminosity function (LF) at redshift z = 6.9, finding moderate evolution from z = 5.7 that is consistent with a fully or largely ionized z ∼ 7 intergalactic medium. Our result is based on four fields of the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. Our survey volume of 6.1 × 106 Mpc3 is double that of the next largest z ∼ 7 survey. We combine two new LAGER fields (WIDE12 and GAMA15A) with two previously reported LAGER fields (COSMOS and CDFS). In the new fields, we identify N = 95 new z = 6.9 Lyα emitter (LAEs) candidates, characterize our survey’s completeness and reliability, and compute Lyα LFs. The best-fit Schechter LF parameters for all four LAGER fields are in good general agreement. Two fields (COSMOS and WIDE12) show evidence for a bright-end excess above the Schechter function fit. We find that the Lyα luminosity density declines at the same rate as the UV continuum LF from z = 5.7 to 6.9. This is consistent with an intergalactic medium that was fully ionized as early as redshift z ∼ 7 or with a volume-averaged neutral hydrogen fraction of x H I < 0.33 at 1σ.
Narrowband imaging is a highly successful approach for finding large numbers of high redshift Lyα emitting galaxies (LAEs) up to z ∼ 6.6. However, at z 7 there are as yet only 3 narrowband selected LAEs with spectroscopic confirmations (two at z ∼ 6.9 − 7.0, one at z ∼ 7.3), which hinders extensive studies on cosmic reionization and galaxy evolution at this key epoch. We have selected 23 candidate z ∼ 6.9 LAEs in COSMOS field with the large area narrowband survey LAGER (Lyman-Alpha Galaxies at the End of Reionization). In this work we present spectroscopic followup observations of 12 candidates using IMACS on Magellan. For 9 of these, the observations are sufficiently deep to detect the expected lines. Lyα emission lines are identified in six sources (yielding a success rate of 2/3), including 3 luminous LAEs with Lyα luminosities of L Lyα ∼ 10 43.5 erg s −1 , the highest among known spectroscopically confirmed galaxies at 7.0. This triples the sample size of spectroscopically confirmed narrowband selected LAEs at z 7, and confirms the bright end bump in the Lyα luminosity function we previously derived based on the photometric sample, supporting a patchy reionization scenario. Two luminous LAEs appear physically linked with projected distance of 1.1 pMpc and velocity difference of ∼ 170 km s −1 . They likely sit in a common ionized bubble produced by themselves or with close neighbors, which reduces the IGM attenuation of Lyα. A tentative narrow N vλ1240 line is seen in one source, hinting at activity of a central massive black hole with metal rich line emitting gas.
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