First Spectroscopic Confirmations of z ∼ 7.0 Lyα Emitting Galaxies in the LAGER Survey

In the past two decades, narrowband imaging surveys have been proven to be a highly efficient approach for finding high-redshift Lyα emitting galaxies (LAEs). Many LAEs from z ∼ 2 to ∼6.6 have been selected via narrowband imaging and spectroscopically identified (e.g., Hu & McMahon 1996; Rhoads et al. 2003, 2004; Dawson et al. 2004, 2007; Hu et al. 2004, 2010; Ouchi et al. 2005, 2008, 2010; Tapken et al. 2006; Westra et al. 2006; Wang et al. 2009; Shibuya et al. 2014, 2017; Zheng et al. 2016).

As Lyα photons can be resonantly scattered by the neutral hydrogen in the intergalactic medium (IGM) in the early universe, Lyα emitters provide a powerful probe of cosmic reionization. Comparison of Lyα luminosity functions at z ∼ 5.7 and 6.5 (Malhotra & Rhoads 2004; Hu et al. 2010; Ouchi et al. 2010; Kashikawa et al. 2011; Matthee et al. 2015) suggests a mostly ionized IGM at z ∼ 6.5 (x_{H i} < 0.3). However, at z ≳ 7, when the dominant phase of cosmic reionization took place, only a few candidate LAEs have been selected from various narrowband surveys (at redshifts of ∼6.9–7.0, 7.3, 7.7, and 8.8; Iye et al. 2006; Ota et al. 2008, 2012; Hibon et al. 2010, 2011, 2012; Tilvi et al. 2010; Krug et al. 2012; Ota & Iye 2012; Shibuya et al. 2012; Konno et al. 2014; Matthee et al. 2014). More importantly, only three of them have been spectroscopically confirmed (two at z ∼ 6.9, one at z ∼ 7.3; Iye et al. 2006; Rhoads et al. 2012; Shibuya et al. 2012), demonstrating the significant challenge in the search for LAEs at z ≳ 7. The challenge is at least partly due to IGM attenuation of Lyα line flux at z ≳ 7 (e.g., Tilvi et al. 2010; Shibuya et al. 2012; Konno et al. 2014; Matthee et al. 2014; Zheng et al. 2017). Much larger samples of z ≳ 7 LAEs, particularly spectroscopically confirmed ones, are essential to probe the physics of reionization and galaxy formation/evolution in the early universe.

Lyman-Alpha Galaxies in the Epoch of Reionization (LAGER) is an ongoing large narrowband survey for LAEs at z ∼ 6.9, using the Dark Energy Camera (DECam; with a field of view (FOV) ∼ 3 deg²) on the NOAO-CTIO 4 m Blanco telescope with an optimally designed custom narrowband filter NB964 (FWHM ∼ 90 Å).

In the first LAGER field (COSMOS), we have selected 23 LAE candidates at z ∼ 6.9 (Zheng et al. 2017). The Lyα luminosity function shows a rapid faint-end evolution from z ∼ 6.9 to z ∼ 6.6 and 5.7, suggesting an IGM neutral hydrogen fraction of x_{H i} = 0.4–0.6 at z ∼ 6.9. More strikingly, the LF shows a clear excess in the bright end, including four of the brightest candidates (with L_Lyα ∼ 10^43.4 erg s⁻¹; Zheng et al. 2017). Such bright-end LF excess suggests that reionization could be patchy, and those four luminous candidates are located in ionized bubbles that reduce the IGM attenuation of Lyα lines.

In this Letter, we present spectroscopic follow-up observations of 12 of the candidate z ∼ 6.9 LAEs. In Section 2, we present the observations and data reduction. The spectra and identifications are given in Section 3, followed by a discussion in Section 4. Throughout this work, we adopt a flat ΛCDM cosmology with Ω_m = 0.3, Ω_Λ = 0.7, and H₀ = 70 km s⁻¹ Mpc⁻¹.

2.1. Spectroscopic Observations

With the narrowband filter NB964 (λ_c = 9642 Å, FWHM ∼ 90 Å) and the DECam mounted on the CTIO Blanco 4 m telescope, we have obtained 34 hr NB exposure in COSMOS field, reaching a 3σ limiting AB magnitude of 25.6 m (2'' diameter aperture). Candidate z ∼ 6.9 LAEs were selected with flux excess between NB964 and deep z-band images (z − NB964 ≥ 1.0). Foreground contaminations are efficiently excluded using deep, bluer broadband images. The final sample includes 23 candidates (for details, see Zheng et al. 2017). All of our candidates have L_Lyα ≥ 4.5 × 10⁴² erg s⁻¹ and rest-frame EW_0,Lyα ≥ 10 Å.

We carried out spectroscopic follow-up for 12 of 23 candidate LAEs using the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the 6.5 m Magellan I Baade Telescope on 2017 February 6–8 and March 21–22. We used the IMACS f/2 camera (with an FOV of 27' diameter) with 300-line red-blazed grism. IMACS masks with 1'' slit width were designed to cover candidate LAEs, foreground fillers, 6 late-type standard stars from Ultra-VISTA catalog (Muzzin et al. 2013) for on-mask flux calibration, and at least 11 alignment stars. Two masks were observed during the run, with each mask covering six candidate LAEs (LAE-4, 6, 8, 13, 17, 18 on Mask 1, and LAE-1, 2, 3, 11, 21, 23 on Mask 2). We ultimately obtained a 390 minute IMACS exposure for Mask 1 and a 340 minute exposure for Mask 2 (Table 1). The seeing was 05–08 during the February run and 08–12 in the March run; however, the February run (Mask 2) was affected by significant moonlight pollution (with the moon 81%–95% illuminated and 65°–38° from the field on 2017 February 6–8).

Table 1.  Summary of IMACS Spectroscopic Observations

Mask ID	Observation Data	ta (s)	Number of Exposures	ttotalb (s)	Seeing
Mask 1	2017 Mar 21	600	1	600	12
Mask 1	2017 Mar 21–22	1200	19	22,800	08–12
Mask 2	2017 Feb 6–8	1200	17	20,400	05–08

Notes.

^aExposure time per frame. ^bTotal exposure time.

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2.2. Data Reduction and Analysis

We used the COSMOS2¹³ software package for IMACS data reduction. COSMOS2 does bias subtraction, flat fielding using quartz lamp exposures, wavelength solution and calibration using arc lamp exposures, and sky subtraction. Finally, it extracts a two-dimensional spectrum for each slit. After standard data reduction steps, we remove cosmic rays using L.A.Cosmic (van Dokkum 2001) and stacked spectra from individual frames using a weighted-stack method to maximize the signal-to-noise ratio (S/N) of the co-added spectra. The S/N of an exposure is inversely proportional to the rms background variation. Thus, it is reasonable to build a wavelength-dependent weighting map for an exposure based on this background variation as measured from source-free pixels along the slit direction at each wavelength:

$$\begin{eqnarray}&&{w}_{i}(\lambda )=\displaystyle \frac{1}{{\sigma }_{i}^{2}},\end{eqnarray} \tag{ 1 }$$

where w_i(λ) is the weight at λ in the ith exposure, and σ_i is the rms background variation at λ. Finally, we obtained a two-dimensional stacked spectrum for each slit. We then extracted one-dimensional spectra from stacked two-dimensional spectra using the IRAF task APALL with an extraction window of 12 along the spectral traces. We flux-calibrated the spectrum of each candidate LAE using the spectra of the six stars included on the slit mask, using the stars' known magnitudes and colors to estimate their intrinsic spectral energy distributions.

Among the 12 candidates, emission lines at expected wavelengths are detected in six sources. The lines are visible in both halves of the data set stacked separately, showing that they are not artificial features. We do not detect a continuum from any of them. We present the 2D and 1D spectra of the six sources in Figure 1. Except for LAE-18 (in which we identified two lines), these lines are the only ones we detected in the spectra. We fitted a Gaussian profile to every detected line and measured line fluxes using the Pyspeckit Python module (Ginsburg & Mirocha 2011). The uncertainties in the line fluxes are estimated through Monte Carlo simulations. We added random noise based on the variation along the slit direction to the 2D spectra, re-extracted the 1D spectra, and re-measured the line fluxes. We ran the simulations 10,000 times and fitted a Gaussian to the flux distribution to derive the uncertainty of the flux measurement. The same technique was applied to obtain the uncertainties of other line parameters. We also performed weighted skewness (Kashikawa et al. 2006) measurements for those emission lines. Weighted skewness >3 is considered a robust indication that a line is asymmetric (Kashikawa et al. 2006). Table 2 summarizes the spectroscopic properties of our candidate LAEs.

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Table 2.  Spectroscopical Properties of the Six LAEs

Name	R.A.	Decl.	Redshifta	fLyαb	fLyαc	EW0,photc	EW0,specd	Skewnesse	M1500f	FWHMintg	S/N
LAE-1	10:02:06.0	+2:06:46.1	6.936	3.86 ± 0.27	7.79	115 ± 14	57 ± 6	7.0 ± 2.0	−21.3 ± 0.2	${251}_{-31}^{+30}$	14.3
LAE-2	10:03:10.5	+2:12:30.8	6.922	2.36 ± 0.29	5.86	>40	>16	−6.0 ± 3.5	>−23.6	${134}_{-65}^{+48}$	8.1
LAE-3	10:01:53.5	+2:04:59.6	6.931	2.89 ± 0.28	5.11	73 ± 9	41 ± 6	6.1 ± 3.00	−21.2 ± 0.2	${364}_{-46}^{+45}$	10.3
LAE-14	9:58:45.2	+2:31:29.2	6.924	2.10 ± 0.24	1.48	>134	>190	0.8 ± 1.8	>−20.4	${195}_{-36}^{+33}$	8.6
LAE-17	9:59:21.7	+2:14:53.2	6.885	1.26 ± 0.15	1.20	24 ± 6	25 ± 4	0.2 ± 1.9	−21.7 ± 0.1	<155	8.4
LAE-18	9:59:59.8	+2:29:06.5	6.925	0.85 ± 0.19	1.21	>36	>25	1.0 ± 4.7	>− 21.5	<173	4.5

Notes.

^aThe redshift is determined from the Lyα emission line fitted with Gaussian profile. ^bSpectroscopically measured Lyα line flux (10⁻¹⁷ erg s⁻¹ cm⁻²). ^cPhotometrically measured Lyα line flux and rest-frame EW₀ using NB964 and Ultra-VISTA Y-band (DR3) photometry (Zheng et al. 2017; McCracken et al. 2012), in units of 10⁻¹⁷ erg s⁻¹ cm⁻² and Å. Lower limits to EW₀ are obtained using 2σ upper limits to Y-band magnitudes. ^dRest-frame Lyα line EW₀ derived using spectroscopical line flux and Y-band based continuum flux (Å). ^eWeighted skewness, as defined in Kashikawa et al. (2006). ^fUV magnitude, measured using Ultra-VISTA J-band (DR3) photometry (McCracken et al. 2012). ^gLine width corrected for the instrumental broadening, km s⁻¹.

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The significant emission lines in LAE-1 and LAE-3 are clearly asymmetric with red wings, typical of high-redshift Lyα emission lines. The four other lines are narrower and statistically consistent with symmetry, though the non-detections of line asymmetry in these sources might be due to low line S/Ns or an adjacent sky line (in LAE-2). The intrinsic line widths of the six sources span ∼100–300 km s⁻¹, while the instrumental spectral resolution is ∼184 km s⁻¹. Although line profiles alone can only robustly confirm LAE-1 and LAE-3 as LAEs, we identify all six lines as Lyα at z ∼ 6.9, and low-z foreground emission lines can be ruled out as follows.

[O iii] λ4959 can be ruled out because the stronger [O iii] λ5007 lines are not seen. If the detected lines are [O iii] λ5007, the lower limits to the [O iii] λ5007/Hβ ratio of most sources would be implausibly large (>10). For LAE-17, if the observed line was [O iii] λ5007, the lower limit of [O iii] λ5007/[O ii]λ3727 (>11.45) would require a rather low metallicity of 12 + log(O/H) < 7 (Nagao et al. 2006). However, LAE-17 has a red color (z − J > 1) inconsistent with a young and low-metallicity galaxy at z ∼ 0.9.

Hβ is disfavored as the upper limits to the [O iii] λ5007/Hβ line ratio (<0.08–0.5) correspond to metallicities of 12 + log[O/H] ≥ 8.9 (Nagao et al. 2006). Galaxies at z ∼ 1.0 with such high metallicity should be very massive and bright and should have been detected in the deep broadband images. For LAE-17, we lack the spectral coverage to measure [O iii] λ5007 if the detected line were Hβ, but the [O ii]λ3727/Hβ line ratio (<0.15) would again require very low metallicity.

If the detected lines are Hα, then Hβ and/or [O iii] should be detected for each candidate LAE. Finally, the [O ii]λ3727 doublet can also be ruled out. For LAE-2, LAE-14, LAE-17, and LAE-18, the detected lines are too narrow to be [O ii]λ3727 (see Figure 2). If the broader and asymmetric lines in LAE-1 and LAE-3 were [O ii]λ3727, they will have improbably large [O ii] equivalent widths (>3000 Å in the rest frame, derived with NB964 magnitudes and 3σ upper limits to the g-band magnitude). [O ii] emitters usually have small line EW₀s. A sample of 1300 [O ii] emitters at z ∼ 1.5 have an average line EW₀ of ∼45 Å, and the largest line EW₀ < 1000 Å (Ly et al. 2012). Furthermore, according to an [O ii] luminosity function (Comparat et al. 2015), we expect ∼400 [O ii] emitters in our field as bright as LAE-1 and LAE-3. Therefore, even if [O ii] emitters with EW₀ > 3000 Å do exist and have a fraction of 1/1300, we only expect 0.3 of them in the LAGER COSMOS field.

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In LAE-18, we detect a second emission line at 9819.6 ± 1.1 Å (see Figure 3). We identify it as N v λ1239 line at the same redshift of Lyα (z = 6.925),¹⁴ with a 3σ upper limit of ∼130 km s⁻¹ to the potential velocity shift between Lyα and N v. We fit the line with a Gaussian and derive a flux of 1.13 ± 0.34 × 10⁻¹⁷ erg s⁻¹ cm⁻², corresponding to an N v/Lyα line ratio of 1.33 ± 0.50. Note Hamann et al. (2017) presents an extreme red quasar population that exhibits similarly large N v/Lyα ratios at z ∼ 2–3.4. Tilvi et al. (2016) also show a tentative N v line with an N v/Lyα line ratio of 0.85 ± 0.25 in a z = 7.512 LBG.

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The Lyα identifications of the six lines are further strengthened by non-detections in the deep broadband images bluer than the z-band (Zheng et al. 2017). We find no g, r, i detections either using the recently released ultra-deep HSC images (Aihara et al. 2017).

We identified neither lines nor continuum from the spectra of the remaining six sources. For the three non-detections on Mask 2 (LAE-11, 21, 23), the sky background is elevated by bright moonlight. The expected line S/Ns are therefore rather low (∼2, based on photometric line fluxes, and assuming 50% slit loss, which is typical for our spectroscopically confirmed LAEs—cf. Table 2). Deeper spectroscopy is therefore still required to clarify whether these sources are in fact z ∼ 6.9 LAEs.

For the three sources on Mask 1 (LAE-4, 6, 8), we expect to detect Lyα lines with an S/N of ∼4–9.¹⁵ In particular, LAE-4 is one luminous LAE candidate, among the four we identified. These sources are therefore unlikely LAEs, and they could be faint transients instead.

We obtained IMACS spectra for 12 candidate LAEs and spectroscopically confirmed six sources at z ∼ 6.9, including three luminous LAEs with Lyα luminosities of ∼10^43.5 erg s⁻¹ and three fainter ones at ∼10^42.9 erg s⁻¹. The three luminous LAEs have the highest Lyα luminosities among known spectroscopically confirmed ≳7.0 galaxies (e.g., Iye et al. 2006; Ono et al. 2012; Rhoads et al. 2012; Stark et al. 2017). Considering the expected S/Ns are insufficient for three candidates on mask, we achieved a success rate of 67% (6/9). Including the z = 6.944 LAE J095950.99+021219.1 (which was independently identified both in LAGER and by Hibon et al. 2011, and spectroscopically confirmed by Rhoads et al. 2012), we now have a spectroscopic sample of seven LAEs at z ∼ 6.9 in COSMOS field. This demonstrates for the first time that narrowband imaging is a highly efficient tool to select LAEs at z ∼ 7.0, as at lower redshifts. The high success rate validates the direct use of the photometric sample to derive the Lyα luminosity function (Zheng et al. 2017).

4.1. The Luminous LAEs and Ionized Bubbles

4.2. The Tentative Detection of N_V Emission Line

We thank the anonymous referee for useful comments that helped to improve the manuscript. We acknowledge financial support from the National Science Foundation of China (grant Nos. 11233002 and 11421303) and the National Program for Support of Top-notch Young Professionals for covering the cost of the NB964 narrowband filter. J.X.W. thanks the National Basic Research Program of China (973 program, grant No. 2015CB857005) and CAS Frontier Science Key Research Program QYCDJ-SSW-SLH006 for support. Z.Y.Z. acknowledges support by the China-Chile Joint Research Fund (CCJRF No. 1503) and the CAS Pioneer Hundred Talents Program (C). The work of S.M., J.E.R., and A.G. on this project is supported in part by US National Science Foundation grant AST-1518057. 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).

This research uses data obtained partly through the Telescope Access Program (TAP), which is funded by the National Astronomical Observatories, Chinese Academy of Sciences, and the Special Fund for Astronomy from the Ministry of Finance. We thank the scientists and telescope operators at Magellan telescope for their help.

Facility: Magellan:Baade (IMACS). -