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

TL;DR

This paper reports the first results from the LAGER survey, detecting z~7 galaxies and analyzing their Lyα luminosity function to infer the state of cosmic reionization at that epoch.

Contribution

It presents the largest narrowband survey for z~7 galaxies, identifying 23 Lyα emitter candidates and analyzing their luminosity function to study reionization.

Findings

Detection of 23 Lyα emitter candidates at z=6.9.

Evidence for a fourfold reduction in Lyα luminosity density from z=5.7 to 6.9.

Indication of a neutral hydrogen fraction of 0.4-0.6 at z~7.

Abstract

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 $z \sim$ 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_{Ly\alpha} \sim $ 10$^{43.4\pm0.2}$ erg s$^{-1}$). The number density at $L_{Ly\alpha}\sim$ 10$^{43.4\pm0.2}$ erg s$^{-1}$ is little changed from z= 6.6, while at fainter $L_{Ly\alpha}$ it is substantially reduced. Overall, we see a fourfold reduction in Ly$\alpha$ 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 $\sim 3$ suppression of Ly$\alpha$ by radiative transfer through the $z \sim$ 7 intergalactic medium (IGM). It indicates an IGM neutral fraction $x_{HI}$ $\sim$ 0.4--0.6 (assuming Ly$\alpha$ velocity offsets of 100-200 km s$^{-1}$). The changing shape of the Ly$\alpha$ luminosity function between $z\lesssim 6.6$ and $z=6.9$ supports the hypothesis of ionized bubbles in a patchy reionization at $z\sim$ 7.

Tables

Table 1. Table 1 Ly α 𝛼 \alpha  luminosity function at z ∼ similar-to 𝑧 absent z\sim 7 from LAGER-COSMOS field.

log10(Li)	N $\pm$ $\Delta$ N‡	$⟨f_{\text{c}omp.}⟩$	log${}_{10}{}^{}\Phi \left(L_i\right)$
(ΔL=0.125)	[L${}_i{}^{}\pm \frac{\Delta L}{2}]$		[($d{\text{log}}_{10}L$)-1 Mpc-3]
42.71	6${}_{-2.4}{}^{}{}_{}{}^{+3.6}$	0.21	-3.74${}_{-0.22}{}^{}{}_{}{}^{+0.20}$
42.84	7${}_{-2.6}{}^{}{}_{}{}^{+3.8}$	0.44	-4.00${}_{-0.20}{}^{}{}_{}{}^{+0.19}$
42.96	4${}_{-1.9}{}^{}{}_{}{}^{+3.2}$	0.56	-4.34${}_{-0.28}{}^{}{}_{}{}^{+0.25}$
43.09	2${}_{-1.3}{}^{}{}_{}{}^{+2.7}$	0.80	-4.80${}_{-0.45}{}^{}{}_{}{}^{+0.37}$
43.21		0.82	-5.10
43.33	1${}_{-0.8}{}^{}{}_{}{}^{+2.3}$	0.85	-5.13${}_{-0.77}{}^{}{}_{}{}^{+0.52}$
43.46	2${}_{-1.3}{}^{}{}_{}{}^{+2.7}$	0.88	-4.84${}_{-0.45}{}^{}{}_{}{}^{+0.37}$
43.59	1${}_{-0.8}{}^{}{}_{}{}^{+2.3}$	0.90	-5.15${}_{-0.77}{}^{}{}_{}{}^{+0.52}$

Table 2. Table 2 Best-fit Schechter Parameters and Ly α 𝛼 \alpha  Luminosity Densities at z ∼ similar-to 𝑧 absent z\sim 5.7, 6.6, 6.9 and 7.3.

$z$	Area	Volume	$L$ Fitted Range	log10[$L_{Ly\alpha }^{\ast }$]	log10[${\Phi }^{\ast }$]	log10[${\rho }_{Ly\alpha }$]†	log10[${\rho }_{Ly\alpha }^{\text{Bright}}$]‡	Reference
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
	deg2	[cMpc3]		[erg s-1]	[Mpc-3]	[erg s-1 Mpc-3]	[erg s-1 Mpc-3]
				Two free parameters [$L_{Ly\alpha }^{\ast }$, ${\Phi }^{\ast }$] with fixed $\alpha$ = -2.5
5.7	13.8	1.16$\times$107	42.4–44.0	43.24$\pm$0.05	-4.09${}_{-0.11}{}^{}{}_{}{}^{+0.13}$	39.54$\pm$0.01	38.24	Ouchi et al. (2008) + Konno et al. (2017)
6.6	21.2	1.91$\times$107	42.4–44.0	43.25${}_{-0.05}{}^{}{}_{}{}^{+0.06}$	-4.32$\pm$0.13	39.33$\pm$0.02	38.05	Ouchi et al. (2010) + Konno et al. (2017)
6.9	2.0	1.26$\times$106	42.65–43.25	42.77${}_{-0.16}{}^{}{}_{}{}^{+0.32}$	-3.77${}_{-0.76}{}^{}{}_{}{}^{+0.54}$	∗38.92${}_{-0.04}{}^{}{}_{}{}^{+0.03}$	∗37.90	This study
			42.65–43.65	43.74${}_{-0.38}{}^{}{}_{}{}^{+0.55}$	-5.76${}_{-0.81}{}^{}{}_{}{}^{+0.72}$	38.77$\pm$0.05	37.82	This study
7.3	0.45	2.5$\times$105	42.4–43.0	42.77${}_{-0.34}{}^{}{}_{}{}^{+1.23}$	-4.09${}_{-1.91}{}^{}{}_{}{}^{+1.09}$	38.55${}_{-0.07}{}^{}{}_{}{}^{+0.08}$	∗∗—	Konno et al. (2014)

Equations

$$\Phi (L)dL=\sum _{L_i\in [L-\Delta L/2,L+\Delta L/2]}\frac{1}{V_{max}{f}_{comp}(N{B}_i)}.$$

$$\Phi (L)dL={\Phi }^{\ast }{\left(\frac{L}{L^{\ast }}\right)}^{\alpha }\exp \left(-\frac{L}{L^{\ast }}\right)d\left(\frac{L}{L^{\ast }}\right).$$

$$T_{z=6.9}^{\prime }=\frac{T_{Ly\alpha ,z=6.9}^{\text{IGM}}}{T_{Ly\alpha ,z=5.7}^{\text{IGM}}}=\frac{{\rho }_{z=6.9}^{Ly\alpha }/{\rho }_{z=5.7}^{Ly\alpha }}{{\rho }_{z=6.9}^{\text{UV}}/{\rho }_{z=5.7}^{\text{UV}}}.$$

Full text

1. Introduction

In the last decade, much progress has been made in narrowing down the epoch of cosmological reionization to be between $z\sim 6$ (Ly$\alpha$ saturation in z $\sim$ 6 quasar, Fan et al., 2006) and $z\sim 9$ (Polarization of CMB photons, Planck results XIII, 2015). In between, quasars, gamma ray bursts (GRBs), and galaxies at $z\gtrsim$ 6 are important to constrain the nature of reionization. However, the rarity of high-$z$ quasars (with only one known at $z\gtrsim$ 7, Bolton et al., 2011), and of high-$z$ GRBs with rapid followup, limit their application as reionization probes. In contrast, hundreds of galaxies at $z\gtrsim$ 6 have been reported from ground-based narrowband Ly$\alpha$ line searches (e.g., Ouchi et al., 2010), from space-based broadband Lyman break searches (e.g., Bouwens et al., 2015; Finkelstein et al., 2015), and from newly taken space-based grism spectroscopic surveys (Tilvi et al., 2016; Bagley et al., 2017). Their luminosities, number densities, clustering, and ionizing powers are essential to probe the epoch of reionization (EoR).

Because Ly$\alpha$ photons are resonantly scattered by neutral hydrogen, Ly$\alpha$ emitters (LAEs) provide a sensitive, practical, and powerful tool to determine the epoch, duration, and inhomogeneities of reionization. With a sample of LAEs in EoR, the easiest Ly$\alpha$ reionization test is the luminosity function (LF) test. Rhoads & Malhotra (2001) first applied this test at $z$ = 5.7, then at 6.5 (Malhotra & Rhoads, 2004; Stern et al., 2005). Subsequent surveys have confirmed and refined the $z$ = 6.5–6.6 neutral fraction measurement to $x_{HI}$ $\lesssim$ 0.3 (Ouchi et al., 2010; Kashikawa et al., 2011), and have found a significant neutral fraction increase from z = 6.6 to z = 7.3 (Konno et al., 2014). Other implementations of Ly$\alpha$ reionization tests include the Ly$\alpha$ visibility test (Jensen et al., 2013; Schenker et al., 2014; Faisst et al., 2014), i.e., the Ly$\alpha$ fraction in LBGs in EoR, the volume test (Malhotra & Rhoads, 2006, Rhoads & Malhotra 2017, in prep.) , i.e., each Ly$\alpha$ galaxy is taken as evidence for a certain volume of ionized gas, and the clustering test, e.g., the signature of a patchy partially-ionized IGM is sought by looking for excess spatial correlations in the Ly$\alpha$ galaxy distribution (Furlanetto et al., 2006; McQuinn et al., 2007; Jensen et al., 2013).

Redshift $z\gtrsim 7$ is the frontier in Ly$\alpha$ and re-ionization studies. Many searches for LAEs at z = 6.9–7.0 (Iye et al., 2006; Ota et al., 2008, 2010; Hibon et al., 2011, 2012), z = 7.2–7.3 (Shibuya et al., 2012; Konno et al., 2014), and z = 7.7 (Hibon et al., 2010; Tilvi et al., 2010; Krug et al., 2012) have yielded $\sim$ two dozen narrowband-selected candidates, but only 3 of these have been spectroscopically confirmed so far (Iye et al., 2006; Rhoads et al., 2012; Shibuya et al., 2012). In comparison, hundreds of LAEs have been found at lower redshift, at $z=6.6$ (e.g., Hu et al., 2010; Ouchi et al., 2010; Kashikawa et al., 2011; Matthee et al., 2015), and a large fraction of them have been spectroscopically confirmed (i.e., Kashikawa et al., 2011). The comoving volumes of the searches for $z\gtrsim$ 7 LAEs are all less than 6$\times 10^{5}$ cMpc3, which are more likely to be affected by cosmic variance. In fact cosmic variance is more important for the observability of LAEs in patchy reionization. Further more, some of the surveys with 200Å wide filters in the far-red inevitably (e.g., Ota et al., 2008, 2010) include one or two weak OH emission lines, limiting their sensitivity. Thus, there is an urgent need for a systematic survey at z $\sim 7$ with sufficient volume and depth to determine if the Ly$\alpha$ line is attenuated due to neutral IGM in a statistically significant way.

In this Letter, we report the discovery of a population of candidate $z\sim$ 7 Ly$\alpha$ emitters in the first field (COSMOS) of the Lyman-Alpha Galaxies in the Epoch of Reionization (LAGER) survey. With LAGER-COSMOS, we have the largest sample to date of candidate $z\sim$ 7 LAEs. In § 2, we describe the observation and data reduction of LAGER, then introduce the candidates selection method. In § 3, we present the candidates and their Ly$\alpha$ LF at $z\sim$ 7. In § 4, we discuss the implications of these discoveries for EoR at $z\sim$ 7.

2. Survey Description and Data

2.1. LAGER Survey

The LAGER survey is the largest narrowband survey yet for LAEs at $z\gtrsim$ 7. It is currently ongoing, using the Dark Energy Camera (DECam, with FOV $\sim$ 3 deg2) on the NOAO-CTIO 4m Blanco telescope together with an optimally designed custom narrowband filter NB964111Please see the filter information at NOAO website: http://www.ctio.noao.edu/noao/content/Properties-N964-filter (Central wavelength $\sim$ 9642Å, FWHM $\sim$ 90Å). LAGER is designed to select more than a hundred z $\sim$ 7 LAEs over an area of 12 deg2 in 4 fields ($8\times 10^{6}$ $\hbox{cMpc}^{3}$). Currently, LAGER has collected 47 hrs NB964 narrowband imaging in 3 fields (CDF-S, COSMOS, and DLS-f5) taken during 9 nights in 2015 December, 2016 February, March and November.

The deepest NB964 imaging is done in COSMOS, where we have obtained 34 hours’ NB964 exposure in a 3 deg2 field. The exposure time per NB964 frame is 900s. Consecutive exposures were dithered by $\sim 100^{\prime\prime}$ so that chip gaps and bad pixels do not lead to blank areas in the final stacks. We also took $\sim$ 0.5–1 hrs z and Y bands exposure per field to exclude possible transients. There are deep archival DECam and Subaru broadband images in COSMOS.

2.2. Data Reduction and Analysis

We downloaded the reduced and calibrated DECam resampled images from the NOAO Science Archive, which were processed through the NOAO Community Pipeline (version 3.7.0, Valdes et al., 2014). The individual frames were stacked following the weighting method in Annis et al. (2014) with SWarp (version 2.38.0, Bertin, 2010). The seeing of the final stacked LAGER-COSMOS narrowband image is $0.93^{\prime\prime}$.

The zero-magnitudes of the DECam images were calibrated to the Subaru Suprime-Cam broadband magnitudes of the stars in the public COSMOS/UltraVISTA $K_{S}$-selected catalog (Muzzin et al., 2013). We estimated the image depth by measuring the root mean square (rms) of the background in blank places where detected ($>3\sigma$) signals were masked out. In a 2" diameter aperture, the 3$\sigma$ limiting AB magnitudes of the DECam images are [$u$, $g$, $r$, $i$, $z$, $Y$, NB964]3σ = [26.2, 27.4, 26.9, 26.6, 26.1, 24.3, 25.6]. Deep Subaru Suprime-Cam broad and narrow band images are available in the central 2-deg2 area. We downloaded the raw images from the archival server SMOKA (Baba et al., 2002), and produced our own broad and narrow band stacked images (follow-up work of Jiang et al., 2013). Their 3$\sigma$ limiting magnitudes within a 2" diameter aperture are [$B$, $g^{\prime}$, $V$, $r^{\prime}$, $i^{\prime}$, $z^{\prime}$, NB711, NB816, NB921]3σ $\sim$ [27.9, 27.6, 27.2, 27.4, 27.3, 25.9, 26.0, 26.1, 26.2]. These Subaru Suprime-Cam images are deeper and have better seeing than the DECam images on average.

We calculated the narrowband completeness via Monte Carlo simulations with the BALROG software (Suchyta et al., 2016). The completeness fraction is defined as the SExtractor recovery percentage of the randomly distributed artificial sources in NB964 image as a function of narrowband magnitude. For point sources, the narrowband NB964 aperture magnitudes corresponding to the 80%, 50% and 30% completeness fractions are 24.2, 25.0, and 25.3, respectively. For pseudo LAEs, we choose fake sources similar to that used in Konno et al. (2017), which have a Sérsic profile with the Sérsic index of $n$ = 1.5, and the half-light radius of $r_{e}$ $\sim$ 0.9 kpc (0.17 arcsec at $z=$ 6.9). The narrowband NB964 aperture magnitudes corresponding to the 80%, 50% and 30% completeness fractions are 24.3, 24.7, and 25.0 for pesudo LAEs, respectively.

2.3. Selection Criteria for $z\sim$ 7 LAEs

Since Subaru broadband images are deeper and have better seeings than the DECam broadband images, we only apply selection criteria for z $\sim$ 7 LAEs in the central 2-deg2 area covered by both Suprime-Cam and DECam. The selection criteria for $z\sim$ 7 LAEs include narrowband NB964 with $\geq$5-$\sigma$ detection (NB$964$ $\leq$ NB$964_{5\sigma}=25.07$), narrowband NB964 excess over broadband z (z - NB964 $\geq$ 1.0) 222As NB964 lies to the edge of z band transmission curve, very red continuum sources may mimic emission line galaxies. We examine the z-NB vs z-Y plot for bright NB detected sources, and find red sources with z-Y $\sim$ 1.0 mag have z-NB $\sim$ 0.4 mag, thus such effect is not important., and non-detections in both DECam $ugri$ and Suprime-Cam $Bg^{\prime}Vr^{\prime}i^{\prime}$, N711, N816, N921 bands. We specially exclude broadband detections in the 1″.08 (4 pixel) diameter aperture to exclude marginal signals in these bands. 156 sources passed the selection criteria, but a large fraction of them are fake sources (bleed trails, diffraction spikes, etc) after our visual check on NB964 image. With our team’s tripartite visual check on both NB964 and broadband images, 27 candidate z $\sim$ 7 galaxies are selected. No transient was identified among them using available broadband images.

3. Results

3.1. Candidate LAEs at $z\sim$ 7 in LAGER-COSMOS

We estimate the Ly$\alpha$ line fluxes and EWs of the candidates using NB and broadband photometry333UltraVISTA Y band (McCracken et al., 2012, DR3) is used to directly estimate the UV continuum of LAGER LAEs. For those without UltraVISTA Y band coverage, we use DECam z band and follow the Appendix of Zheng et al. (2016) to estimate the Ly$\alpha$ line fluxes and EWs. For broadband non-detections, $2\sigma$ upper limits are chosen. We assume a flat UV continuum of $f_{v}\propto v^{0}$ and the IGM absorption from Madau (1995) in the above calculations. For candidates with $z$ or $Y$ band detection, such assumption holds within statistical uncertainty. . We further exclude 4 out of the 27 candidates with estimated line EW $<$ 10 Å.

In total, 23 candidate z $\sim$ 7 LAEs, which have Ly$\alpha$ line fluxes in the range of 0.8–$7.8\times 10^{-17}{\rm\,erg\,cm^{-2}\,s^{-1}}$ and rest-frame equivalent widths EWR $\geq$ 10Å, are selected. The corresponding observed Ly$\alpha$ line luminosities are 5.4–$43.4\times 10^{42}$ erg s*-1*. Their positions and their multi-band thumbnail images are plotted in Figure 1.

The 4 brightest of them, with L(Ly$\alpha$) = 19.3–43.4 $\times$ 1042 erg s*-1*, are the most luminous candidate LAEs known at $z\gtrsim$ 7, thanks to the large survey volume of LAGER. We have confirmed 3 of the brightest LAEs at $z\sim$ 7 (Hu et al., 2017) with our recent Magellan/IMACS spectroscopic follow-up observations.

One of the candidates, J09:59:50.99+02:12:19.1, was previously selected and spectroscopically confirmed as a $z=6.944$ LAE with Magellan IMACS narrowband imaging and spectroscopy over a much smaller field (Rhoads et al., 2012). It is well recovered with our selection using new DECam narrowband imaging.

3.2. Ly$\alpha$ Luminosity Function at $z\sim$ 7

We use the $V/V_{\textrm{max}}$ method to calculate the Ly$\alpha$ LF at $z=$ 6.9. The formula below is adopted:

[TABLE]

Here Vmax is simply a constant, the maximum survey volume for LAEs (Vmax = 1.26$\times$106 cMpc3 calculated from sky coverage and NB964 central wavelength/FWHM), and $f_{comp}(NB_{i})$ is the detection completeness for sources with narrowband magnitude m($\textrm{NB}_{i}$) interpolated in the narrowband detection completeness for fake LAEs (see §2.2).

The Ly$\alpha$ LF of LAEs at $z\sim$ 7 in the LAGER-COSMOS field is listed in Table 1, and plotted in Figure 2 together with LAE LFs at $z\sim$ 5.7 – 7.3 from literature. The LF shows dramatic evolution not only in normalization, but also in the shape.

The relatively fainter end of our LF (42.65 $\leq$ log${}_{10}L_{Ly\alpha}$ $\leq$ 43.25) at $z\sim$ 7 can be fitted with a a single Schechter function in the form of

[TABLE]

Because our survey does not go far below $L^{*}$, we fix $\alpha=$ -2.5 in our fitting, which is suggested from recent studies on Ly$\alpha$ LFs at $z=5.7$ and $z=6.6$ by Konno et al. (2017). Konno et al. (2017) report the largest narrowband surveys to date for LAEs at $z=5.7$ and $z=6.6$ taken with Subaru HSC on the 14–21 deg2 sky, and their best-fit Schechter function analyses also include the smaller but much deeper narrowband surveys taken with Subaru Suprime-Cam (Ouchi et al., 2008, 2010) at the corresponding redshifts. Since the Ly$\alpha$ LFs by Konno et al. (2017) are derived from the largest LAE samples at $z=5.7$ and $z=6.6$ with the widest luminosity range, and we have consistent selection and analysis methods, we choose their Ly$\alpha$ LFs for comparison in the following sections. At $z>7$, we choose the Ly$\alpha$ LF at $z=7.3$ by Konno et al. (2014) for comparison. The best-fit Schechter function parameters of these LFs are listed in Table 2, and plotted in the upper panel of Figure 3. We find a large drop in $\Phi$ of Ly$\alpha$ LF, compared with that at $z=$ 5.7 and $z=$ 6.6. In the relatively fainter end, our Ly$\alpha$ LF at $z\sim$ 7 is in agreement within the 1$\sigma$ measurement uncertainties of that at $z=$ 7.3 .

More strikingly, we see a clear bump, i.e., significant excess to the Schechter function in the bright end of our $z\sim$ 7 LF (log${}_{10}L_{Ly\alpha}$ $\gtrsim$ 43.25). It demonstrates that, while the space density of faint LAEs drops tremendously from $z=$ 5.7 and 6.6 to $z=$ 6.9, that of the luminous ones shows no significantly change. Similarly but much less prominently, the LF evolution between $z=$ 5.7 and $z=$ 6.6 may also be differential, with a significant decline at the faint end but not clear evolution at the bright end (Matthee et al., 2015; Santos et al., 2016; Konno et al., 2017). In Fig. 2 we also plot a scaled-down and truncated version (red solid line at high luminosity end) of Ly$\alpha$ LF at $z=5.7$ from Konno et al. (2017), normalized by the UV luminosity density evolution between $z=5.7$ and $z=6.9$444Using $\rho^{UV}_{z=6.9}/\rho^{UV}_{z=5.7}$ interpolated from Finkelstein et al. (2015)., which appears well consistent with our LF at the bright end within statistical uncertainties.

4. Discussion

4.1. The Evolution of Ly$\alpha$ LF in EoR

Cosmological reionization was well under way by $z\sim 9$ (Planck results XIII, 2015), and ended by $z\sim 6$ (Fan et al., 2006). Ly$\alpha$ galaxies at $z\gtrsim$ 6, e.g., from LAE surveys at $z$ = 5.7 (Ouchi et al., 2008; Santos et al., 2016; Konno et al., 2017), at $z$ = 6.6 (Ouchi et al., 2010; Matthee et al., 2015; Konno et al., 2017), at $z$ = 6.9 (our LAGER-COSMOS sample in this work and Ota et al., 2017), and at $z$ = 7.3 (Konno et al., 2014), are unique samples useful to probe both galaxy evolution and reionization.

In this work, we detect for the first time a significant bump at the bright end of Ly$\alpha$ LF at $z\gtrsim$ 7. Previous surveys for LAEs at $z\gtrsim$ 7 failed to reveal it as they covered much smaller volumes.

The existence of such bright end bump is consistent with the scenario proposed by Haiman & Cen (2005), that bright LAEs are less attenuated by a neutral IGM than faint LAEs, as the larger Strömgren sphere surrounding luminous LAEs alleviates the neutral IGM absorption (also see Santos et al., 2016; Konno et al., 2017). Therefore, the evolution in the bright end of the LF better reflects the intrinsic evolution of LAEs, while the faint end is controlled by the evolution of both galaxies and the IGM.

The bottom panel of Figure 3 shows the evolution of luminosity densities both for $Ly\alpha$ and UV photons. Compared to LBGs (yellow shaded region), an accelerated evolution of LAE $Ly\alpha$ densities from z = 6.6 (green diamonds) to z = 7.3 (purple cross), as reported by Konno et al. (2014), is clearly seen in Figure 3. Since UV photons detected in LBGs are insensitive to the neutral hydrogen in EoR, Konno et al. (2014) concluded that such rapid evolution in LAE LFs can be attributed to a large neutral IGM fraction at $z\sim$ 7.3. Our LAGER survey shows rapid evolution from $z=$ 6.6 to 6.9, and a smaller difference between $z=$ 6.9 and 7.3 (see Figure 3). It is possible that the evolution in the neutral fraction accelerated after $z=6.9$, though a smooth, monotonic neutral fraction evolution at $6.6\leq z\leq 7.3$ also fits the LAGER results and Konno et al. (2014) measurements within their uncertainties. Note that the cosmic time from $z\sim$ 6.6 to 6.9 and from $z\sim$ 6.9 to 7.3 are approximately equal, which is about 50 Myr.

Furthermore, compared to the evolution of Ly$\alpha$ densities $\rho_{Ly\alpha}$ between redshifts of 5.7 and 6.9, the evolution in the bright end only $\rho_{Ly\alpha}^{\textrm{Bright}}$ follows a much smoother trend, somehow similar to that of the UV densities of LBGs (bottom panel of Figure 3). This demonstrates that either the intrinsic LAE LF evolves moderately between z=5.7 and 6.9, or IGM attenuation plays a role even at the highest luminosity bin. Consequently, the dramatic and rapid evolution between $z=$ 6.6 and 6.9 in the faint end can be mainly attributed to the change of neutral IGM fraction. This evolution trend is in agreement with the Ly$\alpha$ fraction test which shows the fraction of LBGs with visible Ly$\alpha$ lines drops significantly at $z\gtrsim$ 7 (e.g., Schenker et al., 2014).

4.2. Neutral IGM fraction at $z\sim$ 7 with LAGER

Following Ouchi et al. (2010) and Konno et al. (2014), we compare the Ly$\alpha$ luminosity densities at $z=$ 6.9 (in EoR) and at $z=$ 5.7 (when reionization is completed) to estimate the effective IGM transmission factor $T^{\textrm{IGM}}_{Ly\alpha,z}$. Assuming the stellar population, interstellar medium (ISM), and dust are similar at $z$ = 6.9 and $z$ = 5.7, we obtain

[TABLE]

Here $\rho^{\textrm{UV}}$ and $\rho^{\textrm{Ly}\alpha}$ are UV and Ly$\alpha$ luminosity densities, respectively. The ratio of UV luminosity densities $\rho^{\textrm{UV}}_{z=6.9}$/$\rho^{\textrm{UV}}_{z=5.7}$ = 0.63$\pm$0.09 is obtained from Finkelstein et al. (2015). We obtain the ratio of observed Ly$\alpha$ luminosity densities (Col.-7 in Table 2) $\rho^{Ly\alpha}_{z=6.9}$/$\rho^{Ly\alpha}_{z=5.7}$ = 0.24$\pm$0.03. Thus we estimate $T^{\prime}_{z=6.9}$ = 0.38$\pm$0.07 555Noe the error of $T^{\prime}_{z=6.9}$ is likely underestimated since we ignore the uncertainties from the faint-end slope of the Ly$\alpha$ LFs. (c.f., $T^{\prime}_{z=7.3}$ = 0.29 from Konno et al. 2014 and $T^{\prime}_{z=6.6}$ = 0.70$\pm$0.15 from Konno et al. 2017).

Converting the Ly$\alpha$ emission line transmission factor to neutral IGM fraction $x_{HI}$ is model dependent (e.g., Santos, 2004; McQuinn et al., 2007; Dijkstra et al., 2007). An important factor is the shift of the Ly$\alpha$ line with respect to the systemic velocity, which is widely observed and may be explained by Ly$\alpha$ radiative transfer in a galactic wind or outflow. The Ly$\alpha$ velocity shift measurements of $z\sim$ 6–7 galaxies are very limited and are mostly around 100–200 km s*-1* (Stark et al. 2015, 2017; Pentericci et al. 2016, but see two UV luminous galaxies with 400-500 km s*-1* reported by Willott et al. 2015). With the analytic model of Santos (2004), assuming a Ly$\alpha$ velocity shift of 0 and 360 km s*-1*, the value of $T^{\prime}_{z=6.9}$ = 0.38 corresponds to $x_{HI}$ $\sim$ 0.0 and 0.6, respectively.

By comparing our observed Ly$\alpha$ LF at $z=6.9$ to that predicted with the radiative transfer simulations by McQuinn et al. (2007, their Fig. 4), we obtain $x_{HI}$ $\sim$ 0.40–0.60 at $z$ = 6.9. Similar results are obtained in the analytic studies considering ionization bubbles (Furlanetto et al., 2006; Dijkstra et al., 2007). We should note that these studies predicted a suppression of the luminosity function that is rather uniform across a wide range of luminosities (e.g., Sec. 4 of McQuinn et al., 2007). This suggests that the true distribution of ionized region sizes may differ appreciably from those used in literature (Furlanetto et al., 2006; Dijkstra et al., 2007; McQuinn et al., 2007). We conclude that the neutral hydrogen fraction is $x_{\textrm{HI}}$ $\sim$ 0.4–0.6 at $z=6.9$, where both the uncertainties in the IGM transmission factor calculation and the theoretical model predictions are considered. The LBG Ly$\alpha$ fraction test by Schenker et al. (2014) yields a similar neutral hydrogen fraction ($x_{\textrm{HI}}=$ 0.39${}^{+0.08}_{-0.09}$) at $z\sim$ 7. Note smaller $x_{\textrm{HI}}=$ 0.3$\pm$0.2 at $z=$ 6.6 is obtained by Konno et al. (2017) with Subaru HSC and Suprime-Cam surveys.

4.3. Ionized Bubbles at $z\sim$ 7

The bump at the bright end of the Ly$\alpha$ LF is an indicator of large enough ($>$ 1 pMpc radius) ionized bubbles, where the Hubble flow can bring Ly$\alpha$ photons out of resonance, thus leading to different evolution of Ly$\alpha$ LF at the bright end and the faint end. The uneven distribution of these LAEs may indicate ionized bubbles in a patchy reionization at $z\sim$ 7. Larger LAGER samples in future will more definitely establish whether the degree of clustering in Figure 1 requires patchy reionization.

The origin of the ionized bubbles could be extrinsic or intrinsic, or both. From Malhotra & Rhoads (2006), which gave the proper radius of a Strömgren sphere and the Gunn-Peterson effect optical depth of a LAE with outflow velocity $\Delta V$ in EoR, we know that even the brightest LAE can not produce a large enough ionized bubble to effectively reduce the optical depth to $\tau\sim$ 1. We thus would expect extrinsic contribution from additional satellite galaxies associated with the luminous LAEs. From Figure 1, we do see some such fainter nearby sources (e.g., the projected separations between LAE-1, 3 and 11 are $\lesssim$ 3.4 arcmin, which corresponds to 1pMpc at $z=6.93$). Further deeper NB imaging would enable us to probe the bubbles by detecting more fainter galaxies associated with the luminous LAEs.

The intrinsic explanation is that these luminous LAEs may be physically unusual objects, perhaps of a type not commonly found at lower redshift. For example, they could represent an AGN population that dominates over the ordinary star forming galaxies at lg${}_{10}L_{Ly\alpha}$ $>$ 43.3 (e.g., Matsuoka et al., 2016). Alternatively, they could be star forming galaxies where metallicities and/or dust abundances are low enough to enable considerably larger Ly$\alpha$ production or escape than is commonly seen at $z<$ 6 (e.g., Stark et al., 2015, 2017; Mainali et al., 2017). At $z\sim$ 7, Bowler et al. (2014) find an excess of luminous LBGs where there should be an exponential cutoff at the bright-end of their UV LFs. NIR and FIR spectroscopic follow-up of these luminous LBGs show very large Ly$\alpha$ velocity offsets ($>$300 km s*-1*, Willott et al., 2015; Stark et al., 2017), which are significantly larger than the predicted velocity offsets for galaxies in EoR by Choudhury et al. (2015). Besides that, unusually strong carbon lines reported in two other LBGs at $z\sim$ 7 (CIV$\lambda$1548Å in A1703-zd6 at $z$ = 7.045 and C iii] 1909Å in EGS-zs8-1 at $z$ = 7.73) indicate unusual harder and more intense UV radiation than that of $z\sim$ 3 LBGs (Stark et al., 2015, 2017). In addition, a strong He ii line has been reported in the brightest Ly$\alpha$ emitter at $z$ = 6.6 (CR7), suggesting it could be powered by either Pop. III stars (Sobral et al., 2015) or perhaps accretion on to a direct collapse black hole (Pallottini et al., 2015).

Future deeper NB964 imaging and IR (NIR and/or FIR) spectroscopic observation will help us to determine the nature of these luminous LAEs, and probe the ionized bubbles in a patchy reionization at $z\sim$ 7.

5. Conclusion

In this letter, we report the first results of our LAGER project, the discovery of 23 (22 new) candidate LAEs at $z\sim$ 7. This is the largest sample to date of candidate LAEs at $z\gtrsim$ 7. Further more, thanks to the large survey volume of LAGER, we find 4 most luminous candidate LAEs at $z\sim$ 7 with L(Ly$\alpha$) = 19–43$\times$1042 erg s*-1*. Compared to previous Ly$\alpha$  LFs at $z\gtrsim$ 6, the Ly$\alpha$ LF of LAGER LAEs at $z\sim$ 7 shows different evolution at the faint-end and at the bright-end, which indicates a large neutral hydrogen fraction $x_{\textrm{HI}}$ $\sim$ 0.4–0.6 and the existence of ionized bubbles at $z\sim$ 7. Our findings support the patchy reionization scenario at $z\sim$ 7.

We thank the anonymous referee for careful and helpful comments which improve the manuscript. We acknowledge financial support from National Science Foundation of China (grants No. 11233002 & 11421303) and Chinese Top-notch Young Talents Program for covering the cost of the NB964 narrowband filter. J.X.W. thanks support from National Basic Research Program of China (973 program, grant No. 2015CB857005), and CAS Frontier Science Key Research Program QYCDJ-SSW-SLH006. Z.Y.Z acknowledges supports by the China-Chile Joint Research Fund (CCJRF No. 1503) and the CAS Pioneer Hundred Talents Program (C). L.I. is in part supported by CONICYT-Chile grants Basal- CATA PFB-06/2007, 3140542 and Conicyt-PIA-ACT 1417. C.J. acknowledges support by Shanghai Municipal Natural Science Foundation (15ZR1446600) We thank Materion company for the manufacture of the NB964 filter, which made the LAGER project possible. We greatly appreciate the kind support from staffs at NOAO/CTIO to make our observations successful. Based on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory (NOAO PID: 016A-0386, PI: Malhotra, and CNTAC PIDs: 2015B-0603 and 2016A-0610, PI: Infante), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. Based in part on data collected at the Subaru Telescope and obtained from the Subaru-Mitaka-Okayama-Kiso Archive System (SMOKA), which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan. This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. Funding for the DES Projects has been provided by the DOE and NSF (USA), MISE (Spain), STFC (UK), HEFCE (UK). NCSA (UIUC), KICP (U. Chicago), CCAPP (Ohio State), MIFPA (Texas A&M), CNPQ, FAPERJ, FINEP (Brazil), MINECO (Spain), DFG (Germany) and the collaborating institutions in the Dark Energy Survey, which are Argonne Lab, UC Santa Cruz, University of Cambridge, CIEMAT-Madrid, University of Chicago, University College London, DES-Brazil Consortium, University of Edinburgh, ETH Zurich, Fermilab, University of Illinois, ICE (IEEC-CSIC), IFAE Barcelona, Lawrence Berkeley Lab, LMU Munchen and the associated Excellence Cluster Universe, University of Michigan, NOAO, University of Nottingham, Ohio State University, University of Pennsylvania, University of Portsmouth, SLAC National Lab, Stanford University, University of Sussex, and Texas A&M University. Facilities: $Blanco$ (DECam), $Subaru$ (Suprime-Cam)