A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at Z = 7.730 Using Keck/Mosfire

Draft version May 5, 2015 Preprint typeset using LATEX style emulateapj v. 5/2/11

A SPECTROSCOPIC REDSHIFT MEASUREMENT FOR A LUMINOUS LYMAN BREAK GALAXY AT Z = 7.730 USING KECK/MOSFIRE

P. A. Oesch1,2, P. G. van Dokkum2, G. D. Illingworth3, R. J. Bouwens4, I. Momcheva2, B. Holden3, G. W. Roberts-Borsani4,5, R. Smit6, M. Franx4, I. Labbe´4, V. Gonzalez´ 7, D. Magee3 Draft version May 5, 2015

ABSTRACT We present a spectroscopic redshift measurement of a very bright Lyman break galaxy at z = 7.7302 ± 0.0006 using Keck/MOSFIRE. The source was pre-selected photometrically in the EGS field as a robust z ∼ 8 candidate with H = 25.0 mag based on optical non-detections and a very red Spitzer/IRAC [3.6]−[4.5] broad-band color driven by high equivalent width [O III]+Hβ line emission. The Lyα line is reliably detected at 6.1σ and shows an asymmetric profile as expected for a galaxy embedded in a relatively neutral inter-galactic medium near the Planck peak of cosmic reionization. ˚ +90 The line has a rest-frame equivalent width of EW0 = 21 ± 4 A and is extended with VFWHM = 360−70 km s−1. The source is perhaps the brightest and most massive z ∼ 8 Lyman break galaxy in the 9.9±0.2 full CANDELS and BoRG/HIPPIES surveys, having assembled already 10 M of stars at only 650 Myr after the Big Bang. The spectroscopic redshift measurement sets a new redshift record for galaxies. This enables reliable constraints on the stellar mass, star-formation rate, formation epoch, as well as combined [O III]+Hβ line equivalent widths. The redshift confirms that the IRAC +180 ˚ [4.5] photometry is very likely dominated by line emission with EW0([O III]+Hβ)= 720−150 A. This detection thus adds to the evidence that extreme rest-frame optical emission lines are a ubiquitous feature of early galaxies promising very efficient spectroscopic follow-up in the future with infrared spectroscopy using JWST and, later, ELTs. Subject headings: galaxies: high-redshift — galaxies: formation — galaxies: evolution — dark ages, reionization, first stars

1. INTRODUCTION spectroscopic surveys being unsuccessful or only result- The spectroscopic confirmation and characterization of ing in uncertain candidate lines (e.g. Treu et al. 2013; galaxy candidates within the cosmic reionization epoch Jiang et al. 2013a; Tilvi et al. 2014; Caruana et al. 2014; has been a major challenge for observational extragalac- Faisst et al. 2014; Vanzella et al. 2014; Schenker et al. tic astronomy for the last few years. Recently, large 2014). samples of several hundred galaxy candidates have been Current studies at z > 6 thus rely on photometric identified at z ∼ 7 − 11 thanks to the exceptional near- samples, with selection criteria and photometric redshifts infrared sensitivity of the WFC3/IR camera onboard that are somewhat uncertain due to the difficulty of es- the Hubble Space Telescope (HST ; e.g., Bouwens et al. tablishing reliable priors of potential contaminant pop- 2011, 2015; Schenker et al. 2013; McLure et al. 2013; ulations at lower redshift. Spectroscopic follow-up is Oesch et al. 2012, 2014; Finkelstein et al. 2014). How- therefore particularly important for the very rare galaxies ever, despite this unprecedented target sample, very lit- at the bright end of the UV luminosity and mass func- tle progress has been made in spectroscopically confirm- tions where any contamination has a very large impact. ing galaxies in the cosmic reionization epoch. Currently, The low success rate of spectroscopic follow-up sur- only a handful of normal galaxies have reliably measured veys is likely caused by a decreased fraction of galaxies redshifts at z > 7 (see, e.g., Vanzella et al. 2011; Pen- showing strong Lyα emission due to an increased neutral arXiv:1502.05399v2 [astro-ph.GA] 3 May 2015 tericci et al. 2011; Ono et al. 2012; Schenker et al. 2012; fraction in the inter-galactic medium (IGM) at z > 6 Shibuya et al. 2012; Finkelstein et al. 2013), with most (Stark et al. 2010, 2011; Treu et al. 2013; Schenker et al. 2012, 2014; Pentericci et al. 2014). While Lyα is the primary spectral feature for spectroscopic confirmation of 1 Yale Center for Astronomy and Astrophysics, Physics De- high-redshift candidates, new surveys targeting the Ly- partment, New Haven, CT 06520, USA; [email protected] 2 Department of Astronomy, Yale University, New Haven, CT man continuum break are underway using the WFC3/IR 06520, USA grism on the HST , or alternatively, weak UV lines may 3 UCO/Lick Observatory, University of California, Santa also be detectable from the ground (Stark et al. 2014). Cruz, CA 95064, USA Two recent successful Lyα detections of Lyman break 4 Leiden Observatory, Leiden University, NL-2300 RA Leiden, The Netherlands selected galaxies at z > 7.2 were published in Ono et al. 5 Department of Physics and Astronomy, University College (2012, z = 7.213) and Finkelstein et al.(2013, z = 7.508). London, Gower Street, London WC1E 6BT, UK Both these sources are relatively bright with H = 25.2, 6 Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK and 25.6 mag and both show a significant flux excess 7 University of California, Riverside, 900 University Ave, in their IRAC photometry, which is consistent with ex- Riverside, CA 92507, USA tremely strong [O III] line emission at 4.1−4.3 µm. Such 2 Oesch et al.

F606W F814W F105W F125W F160W [3.6] [4.5] We exploit the availability of deep Spitzer/IRAC photometry over the HST CANDELS-Wide fields to system- atically search for bright galaxies with IRAC colors of [3.6]−[4.5]> 0.5 mag in addition to a Lyα break (i.e., a 23 non-detection at < 1 µm), characteristic for z > 7 galax- 2 EGS-zs8-1 zbest=7.7 (χ = 1.7) ies. This resulted in two candidates with H < 25.1 mag 2 24 zlow=1.8 (χ = 34.2) in the EGS field (see Roberts-Borsani et al., in prep.). Fortuitously, these two sources are < 60 from each other 25 and can be targeted in a single MOSFIRE mask. Stamps and SED fits for one of these sources (EGS-

AB 26 1

m zs8-1) are shown in Figure1. The F606W, F814W, zphot=7.7±0.3 F125W and F160W images come from the CANDELS 27 0.5 survey (Grogin et al. 2011), while the IRAC images are from the SEDS survey (Ashby et al. 2013). Also shown normalized p(z) 28 0 0 2.5 5 7.5 10 12.5 are WFC3/IR F105W observations that are fortuitously Redshift available over this source as a result of a separate follow- 29 0.5 1.0 2.0 4.0 6.0 up program (GO:13792, PI: Bouwens). Despite its mod- observed wavelength [µm] est depth, this Y105 image still provides a highly improved Fig. 1.— Top – Images showing a 500×500 region around our photometric redshift measurement by constraining the primary target galaxy EGS-zs8-1 in the HST and Spitzer/IRAC spectral break at 1 µm. filters. These are, from left to right, V606, I814, Y105, J125, H160, As seen from the figure, the source EGS-zs8-1 is only and 3.6 µm and 4.5 µm. Bottom – The spectral energy distri- detected at > 1 µm in the WFC3/IR imaging as well as in bution of EGS-zs8-1 based on fits to the HST+Spitzer+K-band photometry. Downward pointing vectors represent 2 σ upper lim- both IRAC 3.6 and 4.5 µm bands. The [3.6]−[4.5] color of its in non-detection bands. A significant flux excess in the IRAC this source is measured to be 0.53±0.09, i.e., at the edge 4.5 µm band is evident. Together with the strong spectral break, of our IRAC color selection window ([3.6]−[4.5]> 0.5). this constrains the photometric redshift to zphot = 7.7 ± 0.3, in excellent agreement with the spectroscopic measurement as shown later. The best fit low-redshift solution at z ∼ 1.8 is shown as a 3. OBSERVATIONS gray line for completeness. However, this SED has a likelihood of < 10−7 and is ruled out by the photometry. 3.1. MOSFIRE Spectroscopy We use the Multi-Object Spectrometer for Infra-Red strong lines are characteristic of early star-forming galax- Exploration (MOSFIRE; McLean et al. 2012) on the ies, as evidenced by a clear increase in broad-band flux Keck 1 telescope for Y-band spectroscopy of our pri- excess with redshift (e.g., Schaerer & de Barros 2009; mary z ∼ 8 targets in the search for their Lyα emission Labbéet al. 2013; Stark et al. 2013; Gonzálezet al. lines. MOSFIRE offers efficient multiplex observations 2014; Smit et al. 2014). Such excesses can be used to over a field of view of ∼ 60 × 30 at a spectral resolution select relatively clean samples of star-forming galaxies at of R ∼ 3000 (R = 3500 and R = 2850 for a 000. 7 or 000. 9 z ∼ 6.6 − 6.9 as well as z ∼ 8 (e.g. Smit et al. 2015, slit, respectively). Roberts-Borsani et al., 2015, in prep.). Data over the EGS field were taken during three nights, In this Letter we present a successful spectroscopic red- 2014 April 18, 23, and 25. While the first night was es- shift measurement at z ∼ 8 using Keck/MOSFIRE of one sentially lost due to bad seeing and clouds, the remain- of the brightest Lyman Break galaxies (LBGs) at that ing nights had better conditions with a median seeing of epoch. This galaxy was pre-selected as a high-priority 100 and only few cirrus clouds during the last night. A target because of a very red [3.6]-[4.5] color, likely caused dome shutter break problem also led to some vignetting by strong [O III] emission. Our target selection is sum- during the last 30 minutes of the April 25 night before marized in Section2, while Section3 outlines the spec- we stopped observations early. In total, we obtained 2.0 troscopic observations, and our results are presented in hours of good quality Y-band spectroscopy during April Section4. 23 (Night 1), and 2.0 hours on April 25 (Night 2). Throughout this paper, we adopt ΩM = 0.3, ΩΛ = −1 −1 Data were taken with 180 s exposures and AB dither 0.7,H0 = 70 kms Mpc , i.e. h = 0.7, consistent offsets along the slit with ±100 and ±1.200, respectively. with the measurements from Planck (Planck Collabora- In night 2, we also increased the slit width from 000. 7 (as tion et al. 2015). Magnitudes are given in the AB system. used in Night 1) to 000. 9 in anticipation of the slightly worse seeing forecast. During these nights we observed 2. TARGET SELECTION two masks with a total of eight z ∼ 7 − 8 candidate We briefly summarize our selection of a robust z ∼ 8 galaxies, in addition to lower redshift fillers. LBG sample over the CANDELS fields using extreme IRAC photometry. For more details see Roberts-Borsani 3.2. Data Reduction et al. (2015, in prep.). The data were reduced using a modified version of the The selection builds on Smit et al.(2015), who iden- 8 tify a sample of z ∼ 6.8 galaxies based on strong public MOSFIRE reduction code DRP . This pipeline produces 2D sky-subtracted, rectified, and wavelength- [O III]λλ4959, 5007 plus Hβ emission lines resulting in very blue [3.6]−[4.5] IRAC colors. As these lines shift calibrated data for each slitlet with a spatial resolution 00 ˚ into the IRAC 4.5 µm band, galaxies at z ∼ 7 to z ∼ 9 of 0. 1799 per pixel and a dispersion of 1.086 A per pixel. exhibit red [3.6]−[4.5] IRAC colors (see also Stark et al. 2013; Labbéet al. 2013; Bowler et al. 2014). 8 https://code.google.com/p/mosfire/ Spectroscopic Redshift for a Luminous z = 7.73 Galaxy 3

5 4 pixel binned S/N Night 1 1.05 1.055 1.06 1.065 1.07 1.075 Mask Layout 4 3 2 EGS-zs8-1 1 binned S/N 0 −1 0.2 Night 2 −2 EGS-zs8-1 1.0555 1.06 1.065 1.07

(2 hrs) ] Observed Wavelength [µNightm] 2

4 Å /Å] / 2 3 2 0.1 2 Night 1 N 1

(2 hrs) binned S/N 0 erg/s/cm

erg/s/cm 0 −1 4 arcsec 17 E −2 − 1.055 1.06 1.065 1.07 Observed Wavelength [µm] −0.1 z=7.7302±0.0006 Fig. 2.— Left – Mask layout of the two nights of MOSFIRE Flux [10 −17 2 Y-band observations of our primary target. These two nights pro- f(Ly) = 1.7±0.3 10 erg/s/cm vide two completely independent measurements of this galaxy at −0.2 two different orientations as well as two different positions along 1.05 1.055 1.06 1.065 1.07 1.075 different slitlets. This also allows us to exclude the possibility of Observed Wavelength [µm] contamination in the final stacked spectrum from the two faint 00 neighboring galaxies present within 2 of the primary galaxy along Fig. 3.— MOSFIRE spectra of EGS-zs8-1. The full 2D spectrum the slits. Right – The signal-to-noise ratio around the detected after 2-by-2 binning is shown in the top panel, while the optimally emission line in the two independent 1D spectra of the two nights, extracted 1D spectrum is shown on the bottom. The 1D spec- averaged over a 4 pixel width (∼ 4 A).˚ A line is clearly detected at trum was smoothed by a 3 pixel (∼ 3 A)˚ moving average filter for > 4σ independently in both 2 hr spectra from each night. We also clarity. The gray shaded area represents the 1σ flux uncertainty, checked the unrectified frames to ensure that the positive flux in while the dark red line shows the best-fit model. The line is quite the spectrum indeed originated from the expected position of the extended in the wavelength direction and shows clear asymmetry galaxy along the spectrum. with the expected shape typical for high-redshift Lyα lines. The spectroscopic redshift measurement is zspec = 7.7302 ± 0.0006 in excellent agreement with the previously determined photometric Each of our mask contains one slitlet placed on a star redshift. Other line characteristics are summarized in Table1. for monitoring the sky transparency and seeing conditions of each exposure. We use this star to track the mask drift across the detector (see, e.g., Kriek et al. 2014), TABLE 1 which we find to be ±1.5 pixels (±000. 27) and ±1 pixel Measurements of Galaxy EGS-zs8-1 (±000. 18) during night 1 and 2, respectively. We separately reduce different batches of the data (of 30-45 min Target duration) to limit any S/N reduction caused by this drift, R.A. (J2000) 14:20:34.89 before shifting and stacking the data. Dec (J2000) 53:00:15.4 The masks for the two nights have different orienta- H160 25.03±0.05 tions (Fig2). The two independent data sets of the pri- MUV −22.06 ± 0.05 mary target thus add to the robustness of any detection. Emission Line After creating the 2D spectra for the different masks, we applied the appropriate relative shift of the two 2D zspec 7.7302±0.0006 frames before stacking the observations of the two nights f(Lyα) 1.7±0.3×10−17 erg s−1cm−2 to our final 2D spectrum. L(Lyα) 1.2±0.2×1043 erg s−1 a ˚ Similarly, 1D spectra were extracted separately for EW0(Lyα) 21±4 A ˚ each mask using an optimal extraction based on a pro- Sw 15±6 A b ˚ file determined by the slit star. The extracted 1D spec- FWHM 13±3 A V b 360+90 km s−1 tra were corrected for Galactic extinction and for telluric FWHM −70 absorption using nearby A0 stars observed in the same Physical Parametersc night at similar airmass. The uncertainty in our optimally extracted 1D spectra was determined empirically log Mgal/M 9.9±0.2 from empty rows in the full, rectified 2D spectra of the log age/yr 8.0±0.5 −1 mask. log SFR/(M yr ) 1.9 ± 0.2 log SSFR −8.0 ± 0.4 The absolute flux calibration was obtained from the AUV 1.6 mag slit stars by comparison of the spectra with the 3D-HST UV slope β −1.7±0.1 photometric catalogs (Skelton et al. 2014). An additional a small correction was applied to account for the extension Not corrected for IGM absorption. b Derived from truncated Gaussian fit, corrected for of individual sources in the slit mask by integrating the instrumental broadening, but not for IGM absorp- seeing-matched HST images over the slit and comparing tion. with the slit loss of stellar sources. c Based on SED fits (see Sect 5; Oesch et al. 2014).

4. RESULTS 4.1. A Lyα Emission Line at z = 7.730 Out of the eight z ∼ 7 − 8 galaxy candidates, we de- The spectra of our target source EGS-zs8-1 (see Ta- tected a signiﬁcant emission line (at > 5σ) for only one ble1 for summary of properties) revealed a signiﬁcant source (EGS-zs8-1). This line is discussed in detail be- emission line at the expected slit position in both masks low. independently (right panels Fig2). The full 4 hr stacked 4 Oesch et al.

2D and 1D spectra are shown in Figure3, showing a line −23 with a clear asymmetric profile, as expected for a Lyα −22.5 Previous LBG+LAEs line at high redshift (z & 3). Furthermore, it lies at the EGS-zs8-1 expected wavelength based on our photometric redshift −22 estimate zphot = 7.7 ± 0.3. We therefore interpret this line as Lyα (other possibilities are discussed in section −21.5 UV 4.3). UV −21 M We fit the line using a Markov Chain Monte Carlo M M* (UV LF) (MCMC) approach based on the emcee python library −20.5 (Foreman-Mackey et al. 2013). Our model is based on a truncated Gaussian profile to account for the IGM ab- −20 sorption and includes the appropriate instrumental res- −19.5 olution. The model also includes the uncertainty on the background continuum level. The MCMC output pro- −19 6 6.5 7 7.5 8 vides full posterior PDFs and uncertainties for the red- SpectroscopicRedshift Redshift shift, line flux, significance, and line width. The line corresponds to a redshift of zLyα = 7.7302 ± ]

43 1 0.0006, with a total luminosity of LLyα = 1.2±0.2×10 − z~8 Galaxies erg s−1, and a total detection significance of 6.1σ. This 0.5 mag CANDELS+BoRG 2 is somewhat lower, but consistent with a simple estimate − (Bouwens et al. 2014) of 7.2σ detection significance from integrating the 1D 0.4 extracted pixel flux over the full extent of the line (i.e., not accounting for background continuum offsets). 0.3 Note that the redshift of the line is determined from our model of a truncated Gaussian profile and is thus 0.2 EGS−zs8−1 corrected for instrumental resolution and the asymmetry arising from the IGM absorption. The peak of the 0.1 observed line (λ = 10616 A)˚ thus lies ∼ 2.5 A˚ to the red Surface Density [arcmin LBG candidates of the actual determined redshift. 0 24.5 25 25.5 26 26.5 27 27.5 28 Given the brightness of the target galaxy, the de- H AB mag tected line corresponds to a rest-frame equivalent width 160 EW = 21 ± 4 A.˚ This is lower than the Lyα emitter Fig. 4.— Top – UV absolute magnitudes of spectroscopically 0 confirmed Lyman break galaxies and Lyα emitters in the cosmic criterion EW0 > 25 A˚ set in recent analyses that use the reionization epoch, at z > 6. Our target, EGS-zs8-1 (red square), Lyα fraction among LBGs to constrain the reionization represents the highest-redshift source and is the brightest galax- process (e.g. Stark et al. 2011; Treu et al. 2013). ies currently confirmed. For reference, the gray dashed line shows the evolution of the characteristic magnitude M∗ of the UV LF (Bouwens et al. 2015). The galaxies shown as black squares are as- 4.2. Line Properties sembled from a compilation from Jiang et al.(2013b); Finkelstein et al.(2013); Shibuya et al.(2012); Ono et al.(2012), and Vanzella Different quantities of the detected line are tabulated et al.(2011). Bottom – Surface density of the full sample of z ∼ 8 in Table1. In particular, we compute the weighted galaxies in the combined CANDELS and BoRG/HIPPIES fields skewness parameter, Sw (Kashikawa et al. 2006) find- (Bouwens et al. 2015, gray histogram). EGS-zs8-1 is the brightest ˚ ˚ and also one of the most massive sources at these redshifts. Note ing Sw = 15±6 A. This puts the line above the 3 A limit that all three z ∼ 8 candidates with H ∼ 25.0 mag (∼ 0.5 mag found for emission lines at lower redshift (see also section brighter than the rest) are identified in the CANDELS/WIDE sur- 4.3). vey area where ancillary Y105 imaging is generally not available. The full-width-at-half-maximum (FWHM) of the line Our spectroscopic confirmation is thus especially valuable. ˚ is quite broad with FWHM = 13 ± 3 A, corresponding i.e., with a ratio of [O II]λ3726 / [O II]λ3729 > 2, and +90 to a velocity width of V = 360 km s−1. Our −1 FWHM −70 with a velocity dispersion of σv & 100 km s . galaxy thus lies at the high end of the observed line width If the observed line is an [O II]λλ3726, 3729 doublet, distribution for z ∼ 5.7 − 6.6 Lyα emitters (e.g. Ouchi the redshift of this galaxy would be zOII = 1.85. This et al. 2010), but is consistent with previous z > 7 Lyα is very close to the best low redshift SED fit shown in lines (Ono et al. 2012). Figure1. However, that SED requires a strong spectral break caused by an old stellar population, for which no 4.3. Caveats emission line would be expected. Additionally, the low- While the identification of the detected asymmetric redshift solution can not explain the extremely red IRAC emission line as Lyα is in excellent agreement with the color (used to select this galaxy), and predicts significant expectation from the photometric redshift, we can not detections in the ACS/F814W band, as well as in the rule out other potential identifications. As pointed out ground-based WIRDS K-band image (Bielby et al. 2012). in the previous section, Kashikawa et al.(2006) find that No such detections are present, however, resulting in a likelihood for such an SED of L < 10−7 (see also Fig1). weighted asymmetries Sw > 3 A˚ are not seen in lower redshift lines, but almost exclusively in Lyα of high- Thus all the evidence points to this line being Lyα at redshift galaxies. However, at the resolution of our spec- z = 7.73. tra, the observed asymmetry is also consistent with an [O II] line doublet in a high electron density environment, 5. DISCUSSION Spectroscopic Redshift for a Luminous z = 7.73 Galaxy 5

In this Letter we used Keck/MOSFIRE to spectro- However, it is completely consistent with the average scopically confirm the redshift of one of the brightest EW0([O III]+Hβ) found for z ∼ 7 − 8 galaxies in previ- z ∼ 8 galaxies identified by Bouwens et al.(2015) over ous work (e.g. Labbéet al. 2013; Smit et al. 2014, 2015), the five CANDELS fields. Interestingly, this source is where such strong lines were found to be ubiquitous (see ∼ 0.5 mag brighter than any source identified in the also Laporte et al. 2014). wide-area BoRG and HIPPIES surveys (e.g. Trenti et al. The fact that these strong lines are seen in a significant 2011; Yan et al. 2011; Bradley et al. 2012; Schmidt et al. fraction of the z ∼ 7 − 8 galaxies is at odds with the 2014). interpretation of extremely young galaxy ages of < 10 As shown in Fig.4, with zspec = 7.730 and an abso- Myr (e.g. Finkelstein et al. 2013). Nevertheless, very lute magnitude MUV = −22.06 ± 0.05 the source EGS- stochastic star-formation may explain some of the line zs8-1 is currently the most distant and brightest spec- strength. Instead, it is possible that an evolution in the troscopically confirmed galaxy (apart from a gamma ray ionization properties of early galaxy populations may be burst at z = 8.2; Tanvir et al. 2009; Salvaterra et al. causing stronger emission lines with more extreme line 2009). EGS-zs8-1 also populates the brightest bin of ratios [O III]/Hβ as is observed at z ∼ 3 − 4 (see e.g. the recent Bouwens et al.(2015) z ∼ 8 UV luminosity Holden et al. 2014). We will discuss models that will function (LF), which makes it an unusually rare object. provide greater insights into these strong emission line A spectroscopic confirmation of its high redshift is thus sources in a future paper. particularly valuable for proving the existence of bright Our confirmation of a source with extremely strong H = 25.0 mag galaxies at z ∼ 8 and for validating the rest-frame optical emission lines at zspec = 7.730 together bright end LF constraints. with two very similar sources at zspec = 7.213 and 7.508 An SED fit at the spectroscopic redshift of the source (Ono et al. 2012; Finkelstein et al. 2013) provides fur- reveals a relatively high stellar mass log M/M = 9.9 ± ther support for the likelihood of ubiquitous strong rest- −1 0.2, a star-formation rate of log SFR/(M yr ) = 1.9 ± frame optical lines as evidenced in the IRAC photometry 0.2, and a relatively young, but not extreme age of of z ∼ 7 − 8 galaxies. This has important consequences log age/yr = 8.0±0.5 based on an apparent Balmer break for future observations with JWST, which promises ex- between the WFC3/IR and the Spitzer photometry (see tremely efficient spectroscopic follow-up of such strong also Table1). The corresponding formation redshift of line emitters with NIRspec out to the highest redshifts this galaxy thus lies at zf = 8.8. For details on our SED of currently known galaxies. fitting see, e.g., Oesch et al.(2014). Interestingly, the source has a UV continuum slope of The authors thank the referee, Eros Vanzella, for very β = −1.7±0.1 (measured from the SED fit) and is con- helpful feedback to improve this paper. This work was sistent with considerable dust extinction, E(B−V)= 0.15 supported by NASA grant NAG5-7697 and NASA grants mag. The detection of a significant Lyα emission line is HST-GO-11563.01, HST-GO-13792. RS acknowledges not inconsistent, however, given the complexities of line the support of the Leverhulme Trust. The authors wish formation in such young galaxies. to recognize and acknowledge the very significant cul- These observations also allow us to reliably constrain tural role and reverence that the summit of Mauna Kea the equivalent widths of the [O III]+Hβ emission lines has always had within the indigenous Hawaiian commu- in this galaxy based on its IRAC colors. At the spec- nity. We are most fortunate to have the opportunity troscopic redshift of the source, these lines are shifted in to conduct observations from this mountain. This work the 4.5 µm channel resulting in a color of [3.6]−[4.5]= is in part based on data obtained with the Hubble Space 0.53±0.09 mag. This is consistent with a combined rest- +180 Telescope operated by AURA, Inc. for NASA under con- frame equivalent width of EW0([O III]+Hβ)= 720−150 tract NAS5-26555, and with the Spitzer Space Telescope, A.˚ operated by the Jet Propulsion Laboratory, California Such a high equivalent width is in some tension Institute of Technology under NASA contract 1407. with the inferred stellar population age of ∼ 100 Myr. Facilities: Keck:I (MOSFIRE), HST (ACS, WFC3), Spitzer (IRAC)

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