SUPPLEMENTARY INFORMATION doi:10.1038/nature12657 • R P(z>[zsample − 2])dz ≥ 0.5
• zbest >zsample − 2 1Sample Selection • R P(z>[zsample − 2])dz ≥ 0.5 Our sample of high-redshift galaxies in the GOODS-North field 2 • χ• < 60 − was selected using an updated version of the criteria presented in pre- zbest >zsample 2 1Sample15,22 Selection vious papers; the full sample will be published in Finkelstein et al. 2 Our sample of high-redshift galaxies in the GOODS-North fieldThese are• χ very< similar60 to the criteria used in our previous publications, in prep.was selected These papers using can an updated be consulted version for of more the criteria details, presented but here we in pre-and they have been shown to produce samples which match up very 15,22 15 brieflyvious recap papers; our process.the full sample will be published in Finkelstein et al.wellThese with available are very similarspectroscopic to the criteria redshifts used at z< in our7. previous publications, 28 Thein prep. optical These imaging papers comes canfrom be consulted the GOODS for more survey, details,and webut used here we Theand selectedthey have sources been shownwere visually to produce inspected samples to reject which artifacts match such up very 15 the v2.0briefly ACS recap imaging, our process. consisting of mosaics in the F435W, F606W, as diffractionwell with spikesavailable and spectroscopic oversplit regions redshifts of bright at z< galaxies.7. Addition- 28 F775WThe and optical F850LP imaging filters. comes The near-infraredfrom the GOODS data survey, comes fromand we the usedally, theThe colors selected of galaxy sources candidates were visually were inspected compared to to reject the artifactsexpected such CANDELSthe v2.0 survey, ACS imaging, and we used consisting the CANDELS of mosaics team’s in the early F435W, data prod- F606W,colorsas diffractionof M, L and spikes T-dwarf and stars, oversplit and regionsany sources of bright with galaxies. star-like colors Addition- uctsF775W (v0.1) in and the F850LP F105W, filters.F125WThe and F160W near-infrared filters. data The comes CANDELS from thewhichally, were the also colors unresolved of galaxy were candidates rejected from were the compared sample. to Finally, the expected the surveyCANDELS obtained survey, data atand two we depths, used denotedthe CANDELS as “WIDE” team’s and early “DEEP”. data prod-opticalcolors bands of M, were L andalso T-dwarf inspected stars, to ensure and any that sources they visually with star-like appeared colors Theucts imaging (v0.1) used in thehere F105W, consists F125W of the full and depth F160W in the filters. Northeast The CANDELS WIDE to containwhich were no significant also unresolved (>1.5σ were) flux rejected (in practice, from the sources sample. with Finally, sig- the region, and about half of the full depth of the DEEP region. The 5σ survey obtained data at two depths,′′ denoted as “WIDE” and “DEEP”.nificantoptical optical bands flux were would also have inspected already to beenensure rejected that they by visually our selection appeared limitingThe magnitudes,imaging used measured here consists in 0.4 of the-diameter full depth apertures, in the Northeast for the ACS WIDEcriteria).to contain Our final no significant galaxy samples (>1.5 consistσ) flux of (in 175 practice, candidate sources galaxies with at sig- bandsregion, are: 28.1, and about 28.3, 27.8 half ofand the 27.7 full mag, depth respectively of the DEEP (all region. magnitudes The 5σz ≈ 6, 85 at z ≈ 7 and 25 at z ≈ 8. 29 ′′ nificant optical flux would have already been rejected by our selection are quotedlimiting in magnitudes, the AB system measured). For in the 0.4 three-diameter WFC3 apertures, bands, the for exist- the ACS criteria). Our final galaxy samples consist of 175 candidate galaxies at σ ing DEEPbandsare: 5 depths 28.1, 28.3, are 27.9, 27.8 27.9 and and27.7 27.7 mag, mag, respectively while for (all the magnitudes WIDE 2z Spectroscopic≈ 6, 85 at z ≈ 7 and Followup 25 at z ≈ Sample8. region, the depths are 27.4, 27.4 and29 27.3 mag, respectively. Addition- are quoted in the AB system ). For the three WFC3 bands, the exist- From our parent sample of candidate galaxies, we selected those ally, we add to ourσ analysis new, extremely deep, optical data obtained ing DEEP 5 depths are 27.9, 27.9 and 27.7 mag, while for the WIDEfor spectroscopic2 Spectroscopic followup with Followup MOSFIRE Sample via two criteria: 1) appar- withregion, ACS in the parallel depths to are the 27.4, CANDELS 27.4 and observations. 27.3 mag, respectively. These data Addition- were ent F160WFrom magnitude, our parent and sample 2) maximizing of candidateR galaxies,P(7.0 WWW.NATURE.COM/NATURE | 1 1 RESEARCH SUPPLEMENTARY INFORMATION the objectthe object spectrum, spectrum, by measuring by measuring the standard the standard deviation deviation in the in signal- the signal-usedused on our on primaryour primary observations. observations. to-noiseto-noise of pixels of pixels in regions in regions clear clear of sky of skyemission emission lines, lines, and and scaled scaled the the WeWe derived derived the fluxthe flux calibration calibration array array by taking by taking a A0V a A0V Kurucz Kurucz39 39 errorerror spectrum spectrum so that so that this thisequaled equaled unity. unity. To determine To determine the significance the significancemodelmodel spectrum, spectrum, and andscaling scaling it to it match to match the integrated the integrated 2MASS 2MASS magni- magni- of theof eightthe eight extracted extracted lines, lines, we fit we a fit Gaussian a Gaussian function function using using the MP-the MP- J HJ H K K F3 F3 tudetude for this for this star, star, interpolating interpolating among among the the, ,, and, and2MASS2MASS mag- mag- FITFIT IDL IDL package package, finding, finding that that only only one one object object had had a line a line detected detectednitudesnitudes to obtain to obtain the magnitudethe magnitude appropriate appropriate for our for ourspectral spectral range range at at at >at5σ>significance.5σ significance. This This object, object, called called z8 GND z8 GND52965296 in our in catalog,our catalog,1.051.05µm(µmm(1.05mµm,AB1.05µm,AB= 8.08).= 8.08). We Wethen then created created a calibration a calibration array array is measuredis measured to have to have an emission an emission line line at λ at=λ 1.0343= 1.0343µmµ withm with a signifi- a signifi-by dividingby dividing this thisscaled scaled model model spectrum spectrum by our by observedour observed spectrum, spectrum, in- in- cancecance of 7.8 ofσ 7.8(Figureσ (Figure 1). Assuming 1). Assuming this this line line is Ly isα Lyplacesα places this this object objectterpolatingterpolating over over intrinsic intrinsic stellar stellar absorption absorption features features common common to both to both at zat=z7.5078= 7.5078± 0.0004,± 0.0004, making making this this the highestthe highest redshift redshift galaxy galaxy that thatspectra.spectra. The The final final array array was was then then multiplied multiplied by our by ourobject object spectrum spectrum has beenhas been spectroscopically spectroscopically confirmed confirmed via Ly viaα Lytoα date.to date. There There has beenhas been(both(both normalized normalized by their by their respective respective exposure exposure times), times), which which both both flux flux a publisheda published spectroscopic spectroscopic confirmation confirmation of the of gamma the gamma ray burstray burst (GRB) (GRB)calibratedcalibrated our our object object spectrum, spectrum, and and corrected corrected for telluricfor telluric absorption absorption 090423090423 at z at=z8.2,= 8.2, confirmed confirmed via continuum via continuum spectroscopy spectroscopy of the of Ly- the Ly-features.features. Nominally, Nominally, this this procedure procedure also also corrects corrects for slit for losses,slit losses, but but manman break break33,3433,34, though, though due due to its to very its very nature nature this this object object cannot cannot be re- be re-onlyonly in the in casethe case when when the seeingthe seeing during during both both the standardthe standard and and object object observed.observed. Additionally, Additionally, although although a spectroscopic a spectroscopic redshift redshift of z of=8z =8.56 .56observationsobservations was was the same.the same. In our In ourcase, case, the seeingthe seeing was was moderately moderately ′′ ′′ for afor galaxy a galaxy has hasbeen been claimed, claimed,35 subsequent35 subsequent observations observations have have shown showndifferent;different; the median the median seeing seeing during during the mask the mask observations observations was was 0.65 0.65, , ′′ ′′ thisthis to be to spurious. be spurious.36 The36 The properties properties of z8 ofGND z8 GND52965296 are summarized are summarizedwhilewhile it was it was 0.85 0.85duringduring the standardthe standard observations. observations. Thus Thus an addi- an addi- in Tablein Table S1. S1. We Wenote note that that the uncertaintythe uncertainty on the on redshiftthe redshift denotes denotes the thetionaltional aperture aperture correction correction of 1.22 of 1.22 was was applied applied to account to account for the for seeing the seeing uncertaintyuncertainty on centroiding on centroiding the line. the line. However, However, as seen as seen at lower at lower redshift, redshift,differences.differences. −1 −1 LyαLyisα frequentlyis frequently detected detected at 200–800 at 200–800 km s km sredwardredward of the of systemic the systemic We measuredWe measured the linethe line flux flux of our of ourdetected detected emission emission line line by again by again 37,3837,38 redshiftredshift, thus, thus the systemicthe systemic redshift redshift for this for this system system may may be a be few a fewfittingfitting a Gaussian a Gaussian with with MPFIT, MPFIT, only only now now to the to calibratedthe calibrated spectrum. spectrum. −1 −1 −18 −18 −1 −1−2 −2 hundredhundred km s km slower.lower. We measuredWe measured a line a line flux flux of 2.64 of 2.64× 10× 10 erg serg scm cm. We. hadWe had ex- ex- −18 −18 −1 −1−2 −2 TheThe MOSFIRE MOSFIRE data data reduction reduction pipeline pipeline provides provides a nominal a nominal estimate estimatepectedpected to achieve to achieve a 5σ alimiting 5σ limiting line line flux flux of 2.1 of× 2.110× 10 erg serg scm cm −18 −18 −1 −1 for thefor central the central row row for each for each objects objects spectrum, spectrum, accounting accounting for differing for differingin 5.5in hr 5.5 (scaled hr (scaled from from our ourinitial initial expectation expectation of 2.0 of× 2.010× 10 erg serg s −2 −2 verticalvertical positions positions in the in slit. the slit. To ensure To ensure that that our extractedour extracted emission emission line linecm cmin 6in hr). 6 hr). Given Given our ourmeasured measured line line flux, flux, and and signal-to-noise signal-to-noise re- re- in thein spectrum the spectrum of z8 ofGND z8 GND52965296 is in is the in correct the correct spatial spatial position, position, we weportedported above above of 7.8, of 7.8, this this would would imply imply a 5σ alimiting 5σ limiting line line flux flux of 1.7 of× 1.7 × −18 −18 −1 −1−2 −2 usedu threesed three sources sources in our in maskour mask with with well-detected well-detected continuum; continuum; one onewas was10 10erg serg scm cm. While. While this thismay may be the be case, the case, there there is an is additional an additional a star,a star, while while the other the other two two were werez ∼z1∼ galaxies1 galaxies placed placed in the in mask the mask as assystematicsystematic uncertainty uncertainty in our in ourflux flux calibration, calibration, as the as counts the counts varied varied by by fillers.fillers. We found We found that thatall three all three sources sources had hadcentroids centroids∼4–5∼4–5 pixels pixels below below∼15%∼15% in the in four the four individual individual observations observations of the of standard the standard star. star. Taking Taking the pipelinethe pipeline estimate, estimate, with with a mean a mean offset offset of 4.7 of pixels. 4.7 pixels. Examining Examining the thethisthis into into account, account, our measured our measured line line flux flux is 2.64 is 2.64± 0.34± 0.34 (photometric) (photometric)± ± −18 −18 −1 −1−2 −2 emissionemission line line in the in 2Dthe spectrum2D spectrum of z8 ofGND z8 GND5296,5296, we find we find that that this this0.400.40 (systematic) (systematic)×10×10 erg serg scm cm. Accounting. Accounting for the for system- the system- linel alsoine also has ahas centroid a centroid offset offset from from the pipeline the pipeline estimate estimate by 4.7 by pixels. 4.7 pixels.aticatic uncertainty, uncertainty, our measuredour measured line line flux flux is consistent is consistent with with that that expected expected Thus,Thus, we conclude we conclude that that the observedthe observed emission emission line line is at is the at expectedthe expectedfor afor signal-to-noise a signal-to-noise=7.8= detection7.8 detection at ∼ at1.2∼σ1.2. Theσ. The flux flux calibration calibration does does positionposition for the for high-redshiftthe high-redshift galaxy galaxy we intended we intended to observe, to observe, and and we wenot havenot have an impact an impact on our on primaryour primary science science results, results, but we but will we will use thisuse this useuse this this offset offset position position as the as extractionthe extraction center. center. As shown As shown in Figure in Figurecalibratedcalibrated line line flux flux below below when when discussing discussing the Ly theα Lyequivalentα equivalent width. width. 2, there2, there are noare other no other sources sources in the in slit,the slit, though though there there are twoare two galax- galax- ′′ ′′ ′′ ′′ ies locatedies located 2.3 2.3andand 3.2 3.2southwestsouthwest of our of ourtarget. target. The The closer closer galaxy galaxy ′′ ′′ ′′ ′′ 44 Line Line Identification Identification wouldwould lie 1.1 lie 1.1off theoff slit the center, slit center, and andwould would be offset be offset by 1.9 by 1.9alongalong the the AlthoughAlthough our photometricour photometric redshift redshift favors favors Lyα Lyasα theas identification the identification slit fromslit from our object,our object, which which corresponds corresponds to ∼ to10∼ pixels10 pixels in the in 2D the spec- 2D spec- for ourfor ourdetected detected emission emission line line in the in spectrum the spectrum of z8 ofGND z8 GND5296,5296, here here trum.trum. Any Any emission emission from from these these objects objects which which happened happened to fall to fallin the in the we examinewe examine the alternatives. the alternatives. Other Other plausible plausible alternatives alternatives to Ly toα Ly(i.e.,α (i.e., slit wouldslit would thus thus be clearly be clearly separated separated from from our ourobserved observed emission emission line. line. lineslines that that have have been been observed observed to be to reasonably be reasonably strong strong at high-redshift) at high-redshift) In SectionIn Section S4.3, S4.3, we find we find that that both both of these of these nearby nearby galaxies galaxies have have spec- spec- are [OareII [O] λλII]3726,3729,λλ 3726,3729, Hβλ H4861,βλ4861, [O III [O] λIII4959,] λ4959, [O III [O] λIII5007,] λ5007, and and troscopictroscopic redshifts redshifts of 0.39, of 0.39, which which would would not place not place any anyknown known emission emission HαλH6563αλ6563 (other (other lines lines are possible are possible if the if object the object is an is AGN, an AGN, but this but thisis is lineline near near 1.0343 1.0343µm.µm. not likelynot likely due due to the to lack the lack of X-ray of X-ray or long-wavelength or long-wavelength detections; detections; see see In orderIn order to examine to examine the possibility the possibility of a of false a false positive positive detection, detection, we we below).below). Of these Of these alternatives, alternatives, Hβ Handβ and [O III [O] canIII] can be ruled be ruled out, out, as if as if examinedexamined the signal-to-noise the signal-to-noise spectrum, spectrum, smoothed smoothed by the by velocity the velocity width width our detectedour detected line line was was one oneof these of these three three lines, lines, the remaining the remaining two two lines lines of ourof spectrum,our spectrum, and andscaled scaled it such it such that that the value the value at the at peak the peak of our of de-our de- shouldshould be observed be observed as well. as well. Specifically, Specifically, were were our observedour observed line line [O III [O] III] tectedtected line line is equal is equal to the to integrated the integrated signal-to-noise signal-to-noise of the of line the line of 7.8, of 7.8, λ5007,λ5007, we would we would expect expect to see to [OseeIII [O] λIII4959] λ4959 at 10243.5 at 10243.5A,˚ whichA,˚ which is is as illustratedas illustrated in Figure in Figure S1. S1.We searchedWe searched the entire the entire spectrum spectrum for appar- for appar- a regiona region clear clear of sky of skyemission. emission. We Wesimulated simulated a [O aIII [O] λIII4959] λ4959 line line at at entlyently significant significant negative negative features; features; these these would would be due be dueto noise, to noise, and andthe the thisthis position position in our in spectrum,our spectrum, with with a line a line strength strength 2.98 2.98× less× less than than that that lacklack of such of such features features provides provides greater greater confidence confidence that that our ourobservation observation 40 40 of theof [O theIII [O] λIII5007] λ5007 line, line,andand found found that that such such a line a line would would have have been been representsrepresents a true a true emission emission line line from from the objectthe object z8 GND z8 GND5296,5296, while while detecteddetected at 4.1 atσ 4.1, thusσ, thus we rule we rule out out[O III [O] λIII5007] λ5007 as the as identificationthe identification the lackthe lack of positive of positive features features other other than than our identifiedour identified Lyα Lylineα line provides provides of ourof ourdetected detected line. line. Additionally, Additionally, we can we can rule rule out bothout both [O III [O] lines,III] lines, furtherfurther confidence confidence that that the line the line is in is fact in fact Lyα Ly. α. as wellas well as H asα, H asα, they as they are not are locatednot located near near strong strong continuum continuum breaks. breaks. 3.23.2 Flux Flux Calibration CalibrationWeWe flux flux calibrated calibrated the the spectrum spectrum of of As seenAs seen in Figure in Figure 3, we 3, havewe have detected detected a large a large photometric photometric break break at at z8 GNDz8 GND52965296 using using observations observations of the of standard the standard star starHIP HIP 56157, 56157, with with λ ∼λ1∼µm.1µm. We Weinterpret interpret it as it the as Lymanthe Lyman break, break, but itbut could it could also also be be a spectrala spectral type type of A0V, of A0V, which which we observed we observed in a in single a single long long slit directly slit directly the Balmerthe Balmer break break at 3646 at 3646A˚ dueA˚ due to a to combination a combination of the of high-orderthe high-order beforebefore our our science science observations observations during during our our first first night night of observing. of observing. BalmerBalmer series series transitions, transitions, or the or 4000 the 4000A˚ breakA˚ break due dueto metal to metal absorption absorption WeWe obtained obtained four four spectra spectra of this of this star star with with an ABAB an ABAB dither dither pattern, pattern, lineslines common common in older in older stellar stellar populations. populations. Were Were this this the case, the case, then then the the withwith each each exposure exposure consisting consisting of 10 of 2s 10 co-additions, 2s co-additions, to guard to guard against against detecteddetected line line would would be [O beII [O]. II]. persistencepersistence and and non-linearity. non-linearity. These These observations observations were were reduced reduced in in As [OII]As [OII] is a doublet,is a doublet, we examine we examine the spectrum the spectrum for signs for signs of the of sec- the sec- the samethe same manner manner as our as ourmasks masks described described above, above, and and extracted extracted into into a a ondond line. line. The The ratio ratio of the ofλ the3726/λ3726/λ3729λ3729 line linestrength strength varies varies from from∼0.5–∼0.5– one-dimensionalone-dimensional spectrum spectrum with with the samethe same size size extraction extraction box box as that as that 41 41 1.5inH1.5inHII regions,II regions, with with a typical a typical ratio ratio of order of order unity. unity.If ourIf detectedour detected F3 F3 http://www.physics.wisc.edu/http://www.physics.wisc.edu/∼craigm/idl/fitting.html∼craigm/idl/fitting.html lineline was was the red the side red side of the of doublet the doublet (at 3729 (at 3729A˚ rest),A˚ rest), we should we should detect detect 2 2 2 | WWW.NATURE.COM/NATURE SUPPLEMENTARY INFORMATION RESEARCH thethe bluer bluer line line at at 10334 10334A˚ A(which˚ (which is is a clean a clean region) region) at at>10>10σ,σ and, and no no 2.8,2.8, 3.2 3.2±±2.12.1 and and 3.9 3.9±±2.62.6 for for the the simulations simulations where where the the input input asym- asym- lineline is seen.is seen. If theIf the detected detected line line is theis the bluer bluer side side of of the the doublet, doublet, then then we we metrymetry value value was was 2.0, 2.0, and and 2.8 2.8±±4.7,4.7, 3.2 3.2±±2.02.0 and and 2.3 2.3±±2.12.1 where where the the wouldwould expect expect to tosee see the the redder redder line line at at10351 10351A.˚ A.˚ This This would would be be directly directly inputinput asymmetry asymmetry value value was was 1.5. 1.5. Although Although each each of of these these simulations simulations underunder the the sky sky line line just just to to the the red red of of our our detected detected line, line, which which hampers hampers resultsresults in in a mean a mean asymmetry asymmetry value value greater greater than than unity, unity, the the very very large large ourour ability ability to to discern discern its its presence. presence. However, However, given given the the width width of of our our uncertaintiesuncertainties imply imply that that our our spectra spectra are are not not of of high high enough enough signal-to- signal-to- detecteddetected line, line, if thereif there was was a second a second line line under under the the sky sky line, line, we we would would noisenoise to to detect detect a moderate a moderate amount amount of of asymmetry asymmetry were were it presentit present in in the the expectexpect to to see see excess excess flux flux just just to to the the red red side side of of the the sky sky residual residual (i.e., (i.e., the the detecteddetected emission emission line. line. Deeper Deeper spectra spectra with with higher higher spectral spectral resolution resolution truetrue line line would would be be broader broader than than the the sky sky residual), residual), in in between between the the two two maymay make make this this possible, possible, but but given given the the presence presence of of sky sky emission emission lines lines skysky lines. lines. As As shown shown in in Figure Figure S2, S2, for for line line ratios ratios of of unity unity or or less, less, the the aroundaround our our detected detected object, object, it may it may yet yet prove prove difficult. difficult. We We thus thus conclude conclude observedobserved spectrum spectrum can can rule rule out out the the presence presence of of the the redder redder [O [OII]II line.] line. If If thatthat we we cannot cannot rule rule out out moderate moderate asymmetry asymmetry in in our our detected detected emission emission thetheλ3726/λ3726/λ3729λ3729 ratio ratio is ishigh; high; close close to to 1.5, 1.5, then then it becomesit becomes harder harder to to line.line. rulerule out out the the presence presence of of this this line. line. However, However, there there should should still still be be excess excess 4.24.2 Equivalent Equivalent Width WidthPhotometricPhotometric surveys surveys for for Ly Lyα emittingα emitting galax- galax- fluxflux over over what what is isobserved observed on on either either side side of of the the sky sky line line residual residual – in– in iesies at at high high redshift redshift using using narrowband narrowband filters filters frequently frequently use use the the equiv- equiv- particular,particular, on on the the red red side side of of the the sky sky line, line, we we would would have have expected expected to to alentalent width width of of the the line line as as a a method method to to remove remove [O [OII]II] emitting emitting ∼∼ 47,4847,48 seesee emission emission line line flux flux at at the the 2σ2σlevel.level. Given Given the the lack lack of of detectable detectable “contaminants”.“contaminants”. TheThe dividing dividing line line used used is iscommonly commonly 20 20A˚ A in˚ in the the fluxflux in in this this region, region, we we conclude conclude that that the the line line is isunlikely unlikely to to be be [O [OII].II]. restrest frame frame of of Ly Lyα.α As. As we we do do not not detect detect the the continuum continuum in in our our spectrum, spectrum, However,However, given given the the unknown unknown strength strength of of any any potential potential 3729 3729A˚ A line,˚ line, wewe must must use use the the photometry photometry to toderive derive the the continuum continuum level level near near the the de- de- inin the the following following we we examine examine further further evidence evidence to to differentiate differentiate between between tectedtected emission emission line. line. We We use use the the best-fitting best-fitting model model from from our our SED SED fitting fitting α LyLyα andand [O [OII].II]. (see(see the the next next subsection), subsection), to to derive derive the the continuum continuum flux flux density density just just red- red- 4.14.1 Line Line Asymmetry AsymmetryAnotherAnother feature feature which which could could confirm confirm the the Ly Lyα α wardward of of Ly Lyα α(at(at a rest-frame a rest-frame wavelength wavelength of of 1225 1225A),˚ A),˚ which which we we find find to to −20−20 −1−1 −2−2 −1−1 naturenature of of this this line line would would be be any any measured measured asymmetry. asymmetry. Ly Lyα αatat high high bebe 4.15 4.15××1010 ergerg s s cmcm A˚ A˚ . The. The EW EW is is then then defined defined as as the the 1,31,3 redshiftredshift is frequentlyis frequently observed observed to to be be asymmetric, asymmetric, thoughthough it has it has been been ratioratio of of the the line line flux flux to to the the continuum continuum level, level, which which we we find find to to be be 64 64±± observedobserved to to be be symmetric symmetric as as well. well.42 42It isIt isassumed assumed that that the the asymmetry asymmetry 8 (photometric)8 (photometric)±±1010 (systematic) (systematic)A.˚ A.˚ If Ifthe the line line is isLy Lyα αat atz z==7.51,7.51, isis caused caused by by absorption absorption of of the the blue blue half half of of the the line line by by neutral neutral hydro- hydro- thisthis would would correspond correspond to to a rest-frame a rest-frame EW EW==7.57.5A,˚ A,˚ while while for for [O [OII]II at] at gengen in in the the IGM. IGM. However, However, a few a few lines lines of of evidence evidence imply imply that that internal internal z =z =1.78,1.78, the the rest-frame rest-frame EW EW would would be be 23 23A.˚ A.˚ An An emission emission line line of of this this processesprocesses in in the the galaxy galaxy may may dominate dominate the the observed observed line line profile. profile. First, First, smallsmall EW EW would would have have a negligible a negligible impact impact on on the the integrated integrated F105W F105W LyLyα linesα lines at atz ∼z ∼2–32–3 have have been been observed observed to tobe be asymmetric, asymmetric, at atan an epoch epoch magnitude,magnitude, and and it doesit does not not provide provide further further evidence evidence excluding excluding the the pos- pos- wherewhere the the IGM IGM absorption absorption is muchis much less. less. Second, Second, also also at atz ∼z ∼2–3,2–3, where where sibilitysibility of of [O [OII],II although], although it doesit does support support our our primary primary conclusion conclusion that that thethe systemic systemic redshift redshift can can be be measured measured via via rest-frame rest-frame optical optical nebular nebular thethe equivalent equivalent width width distribution distribution at atz>z>7 has7 has been been drastically drastically reduced. reduced. −1−1 lines,lines, Ly Lyα αisis seen seen to to reside reside∼200-400∼200-400 km km s s toto the the red red of of the the sys- sys- 4.34.3 Grism Grism Spectroscopy Spectroscopy and and Lensing LensingMostMost of of the the GOODS-North GOODS-North 38,43–4538,43–45 temictemic redshift. redshift. ThisThis is islikely likely a result a result of of interstellar interstellar winds winds driven driven field,field, including including the the region region of of interest interest here, here, has has been been observed observed with withHSTHST byby intense intense star-formation, star-formation, as as Ly Lyα αphotonsphotons will will preferentially preferentially escape escape WFC3WFC3 infrared infrared slitless slitless grism grism spectroscopy spectroscopy (Weiner (Weiner et et al. al. in in prepara- prepara- afterafter they they have have gained gained some some net net redshift, redshift, and and are are thus thus no no longer longer reso- reso- tion),tion), covering covering the the 1.1–1.65 1.1–1.65µmµm spectral spectral range. range. This This range range does does not not in- in- nantlynantly scattered. scattered. This This enables enables them them to to pass pass through through neutral neutral hydrogen hydrogen cludeclude the the line line at at1.0343 1.0343µmµm that that we we observe observe with with MOSFIRE, MOSFIRE, but but if thatif that bothboth within within the the galaxy galaxy as as well well as as in inthe the IGM. IGM. Simulations Simulations of of galaxies galaxies at at lineline was was [O [OII]II at] atz =1z =1.78.78oror [O [OIIIIII] at] atz =1z =1.07.07, other, other emission emission lines lines ∼ −1−1 z>z>8 show8 show that that with with a wind a wind velocity velocity of of∼ a fewa few hundred hundred km km s s , not, not (namely,(namely, [O [OIIIIII]+H]+Hβ orβ or Hα H+[Nα+[NII],II respectively)], respectively) would would fall fall within within the the onlyonly can can Ly Lyα αemissionemission be be detectable detectable from from a mostly-neutral a mostly-neutral epoch, epoch, but but grismgrism spectral spectral range. range. These These are are not not observed, observed, to toan an approximate approximate 3σ 3fluxσ flux 46 46 −17−17 −1−1 −2−2 it canit can be be observed observed with with a symmetric a symmetric profile. profile.TheThe large large inferred inferred SFR SFR limitlimit of of3 ×3 ×1010 ergerg s s cmcm , neither, neither in in the the spectrum spectrum of of the the faint faint ofof our our object object is isconsistent consistent with with this this scenario, scenario, as as it isit isvery very likely likely driv- driv- galaxygalaxy z8 z8GNDGND5296,5296, nor nor in inthe the two two galaxies galaxies that that fall fall a few a few arcseconds arcseconds inging a strong a strong wind wind in in the the interstellar interstellar medium. medium. Ly Lyα αisis also also symmetric symmetric awayaway to to the the southwest, southwest, near near (but (but not not on) on) the the MOSFIRE MOSFIRE slit slit (see (see Sec- Sec- 42 42 inin another another bright bright (m (UVmUV=25.75)=25.75) galaxy galaxy at atz=6.944, z=6.944,perhapsperhaps indicat- indicat- tiontion S3). S3). The The closer closer (northeastern) (northeastern) of of these these two two galaxies galaxies has has a secure a secure inging that that strong strong star-formation star-formation driven driven winds winds are are common common in in these these very very KeckKeck DEIMOS DEIMOS spectroscopic spectroscopic redshift redshiftz =0z =0.387.387(Stern(Stern et etal. al. in in prepa- prepa- luminousluminous objects. objects. ration),ration), which which would would not not place place any any strong strong emission emission lines lines at at1.0343 1.0343µm.µm. TheThe asymmetry asymmetry of of our our observed observed line line is isdifficult difficult to to measure, measure, given given TheThe second second (southwestern) (southwestern) galaxy galaxy has has no no ground-based ground-based spectroscopy spectroscopy thethe night night sky sky line line residual residual to to the the red-side red-side of of our our line. line. We We measure measure toto our our knowledge. knowledge. Spectral Spectral templates templates cross-correlated cross-correlated with with the the WFC3 WFC3 thethe asymmetry asymmetry of of our our emission emission line line by by fitting fitting an an asymmetric asymmetric Gaussian Gaussian grismgrism spectrum spectrum of of this this southwestern southwestern galaxy galaxy yield yield a possible a possible redshift redshift ±± functionfunction to to the the line line profile, profile, where where the theσ valuesσ values on on the the blue blue and and red red side side z z=0=0.39.39 0.010.01, largely, largely due due to to a feature a feature that that would would correspond correspond to to ofof line line center center are are allowed allowed to to be be different. different. We We then then quantify quantify the the asym- asym- thethe [S [SIIIIII] λ]9069λ9069A˚ A emission˚ emission line line at atthat that redshift. redshift. While While quite quite tentative, tentative, metrymetry as as the the ratio ratio of ofσredσred/σ/σblueblue, measuring, measuring this this ratio ratio to to be be 1.2 1.2±±1.4,1.4, thisthis is alsois also consistent consistent with with the the secure secure and and accurate accurate Keck Keck redshift redshift for for the the thusthus the the measured measured asymmetry asymmetry is isof of no no significance. significance. As As a further a further test test of of northeasternnortheastern galaxy galaxy that that is aboutis about 1 arcsec 1 arcsec away, away, suggesting suggesting that that the the two two ourour ability ability to to measure measure any any asymmetry asymmetry in in the the detected detected emission emission line, line, we we maymay be be a physical a physical pair. pair. In In any any case, case, there there is isno no evidence evidence to to favor favor (and (and ranran a series a series of of simulations, simulations, placing placing mock mock emission emission lines lines with with the the same same severalseveral reasons reasons to todiscount) discount) the the possibility possibility that that the the MOSFIRE MOSFIRE emission emission integratedintegrated line line flux flux as as our our measured measured line, line, but but with with a known a known value value of of lineline is isdue due to to contamination contamination from from a nearby a nearby foreground foreground galaxy. galaxy. asymmetry,asymmetry, in in our our one-dimensional one-dimensional spectra. spectra. We We investigated investigated asymme- asymme- ThisThis nearby nearby pair pair of of galaxies galaxies is isunlikely unlikely to toact act as as a significant a significant grav- grav- trytry values values of of both both 2.0 2.0 and and 1.5, 1.5, and and we we placed placed these these mock mock lines lines at at three three itationalitational lens. lens. At At the the spectroscopic spectroscopic redshift redshift of ofz =z =0.390.39 for for both both galax- galax- locations:locations: 11082.6, 11082.6, 10119.4 10119.4 and and 10250.0 10250.0A.˚ A.˚ The The first first two two locations locations ies,ies, we we measure measure stellar stellar masses masses from from SED SED fitting fitting for for the the NE NE galaxy galaxy of of 7 7 7 7 correspondcorrespond to toregions regions 7.4 7.4A˚ A blueward˚ blueward of of a skyline a skyline with with a similar a similar ampli- ampli- 5.85.8××1010M⊙M,⊙ and, and for for the the SW SW galaxy galaxy of of 1.7 1.7××1010M⊙M.⊙ To. To deter- deter- tudetude to to the the skyline skyline 7.4 7.4A˚ A redward˚ redward of of our our detected detected emission emission line; line; the the first first minemine whether whether these these could could plausibly plausibly magnify magnify our ourz =z =7.517.51 galaxy, galaxy, we we ofof these these two two has has a positive a positive sky-subtraction sky-subtraction residual, residual, while while the the second second computecompute their their Einstein Einstein radius, radius, assuming assuming a lens a lens redshift redshift of ofz z==0.39,0.39, hashas a negative a negative residual. residual. The The third third wavelength wavelength is isa region a region with with no no sky sky andand a source a source redshift redshift of ofz z==7.51.7.51. For For this this calculation, calculation, we we require require emissionemission lines. lines. In In each each of of these these six six simulations, simulations, the the measured measured asym- asym- thethe total total mass mass of of the the galaxies, galaxies, including including dark dark matter, matter, which which we we con- con- metrymetry was was consistent consistent with with unity unity (i.e., (i.e., a symmetric a symmetric line) line) at at∼1∼σ1.σ The. The servativelyservatively assume assume is is10 10××thethe stellar stellar mass mass (cf. (cf. compare compare to to samples samples of of measuredmeasured asymmetry asymmetry values values and and associated associated uncertainties uncertainties were were 3.2 3.2±± massivemassive galaxies galaxies in in strong strong lensing lensing surveys surveys that that find find stellar-mass stellar-mass frac- frac- 3 3 WWW.NATURE.COM/NATURE | 3 RESEARCH SUPPLEMENTARY INFORMATION 49 53 tionstions of 50-100% of 50-100% within within the the Einstein Einstein radius radius49). For). For the the NE NE galaxy, galaxy, we weCharlot.Charlot.53 In theseIn these models, models, we we assumed assumed a Salpeter a Salpeter initial initial mass mass func- func- ′′ ′′ findfind an an Einstein Einstein radius radius of 0.05of 0.05, while, while for for the the SE SE galaxy galaxy we we find find an antion,tion, and and varied varied the the stellar stellar population population age, age, metallicity, metallicity, dust dust content content ′′ ′′ EinsteinEinstein radius radius of of0.03 0.03. The. The separation separation between between these these sources sources and andandand star-formation star-formation history. history. There There is mounting is mounting evidence evidence that that a dust a dust ′′ ′′ ′′ ′′ z8 z8GNDGND52965296 is ∼ is2∼.32.3andand 3.2 3.2, respectively., respectively. Additionally, Additionally, even even the theattenuationattenuation law, law,A(λA)(/Eλ)/E(B(B− −V )V, similar), similar to to that that derived derived for for the the largestlargest Einstein Einstein radius radius from from the the strong-lensing strong-lensing galaxies galaxies of Sloan of Sloan Lens LensSmallSmall Magellanic Magellanic Cloud Cloud (SMC) (SMC) better better reproduces reproduces the the UV-optical UV-optical col- col- 49 ′′ ′′ ACSACS Survey Survey49 wouldwould reach reach only only 1.3 1.3at zat=z 0.39= 0.39 (for (for a lensing a lensing galaxy galaxyorsors and and IR/UV IR/UV ratios ratios for for young, young, presumably presumably lower-metallicity lower-metallicity galaxies galaxies 11 11 54–56 withwith stellar stellar mass mass>10>10M⊙M;⊙ more; more than than 100 100 times times that that of the of thez =z0.39=0.39at highat high redshifts, redshifts,54–56 comparedcompared to theto the dust dust attenuation attenuation law law for for local local 57 galaxiesgalaxies here). here). We We thus thus conclude conclude that that strong strong gravitational gravitational lensing lensing is not is notUV-luminousUV-luminous starbursts starbursts57 thatthat is more is more commonly commonly used used (see (see discussion discussion 55 affectingaffecting the the inferred inferred luminosity. luminosity. in Tilviin Tilvi et al. et55 al.). This). This is perhaps is perhaps unsurprising unsurprising as the as the SMC SMC is frequently is frequently 4.44.4 Spectral Spectral Energy Energy Distribution Distribution Fitting FittingIn theIn the above above subsections, subsections,pointedpointed to asto asa local a local analog analog for for high-redshift high-redshift galaxies. galaxies. We We thus thus use use 58 58 wewe have have attempted attempted to discern to discern between between the the [O [OII] andII] and Lyα Lyidentificationα identificationthethe SMC SMC dust-attenuation dust-attenuation curve curve derived derived by by Pei Peito modelto model the the effects effects ofof the the detected detected emission emission line line by by looking looking at theat the line line properties properties itself. itself.ofof dust dust on on our our model model spectra. spectra. Additionally, Additionally, recent recent evidence evidence implies implies However,However, the the strongest strongest evidence evidence either either way way can can likely likely be behad had by bylook- look-thatthat high-redshift high-redshift galaxies galaxies likely likely have have a rising a rising star-formation star-formation history history on on 54,59–6154,59–61 inging at the at the full full photometric photometric SED. SED. Although Although an emissionan emission line line near near a spec- a spec-average,average, thusthus we we allow allow both both exponentially exponentially rising rising and and declining declining traltral break break can can be indicativebe indicative of both of both Lyα Lyorα [Oor [OII],II the], the stellar stellar populations populationsstar-formationstar-formation histories. histories. The The stellar stellar mass mass is found is found as the as the normalization normalization whichwhich would would create create these these signatures signatures would would be drasticallybe drastically different. different. We Webetweenbetween the the observed observed fluxes fluxes and and the the best-fit best-fit model. model. We We include include nebular nebular utilizedutilized the the same sameHSTHSTphotometryphotometry that that went went into into the the photometric photometric red- red-emissionemission lines lines using using the the emission emission line line ratios ratios published published by by Inoue Inoue et al. et al. (see also Salmon et al., in prep).62 62 The best-fitting model is found via shiftshift fitting, fitting, only only now now we we also also added added in Spitzer in Spitzer/IRAC/IRAC data data at 3.6 at 3.6 and and(see2 also Salmon et al., in prep). The best-fitting model is found via 2 χ 4.54.5µm.µm. We We utilized utilized new new IRAC IRAC data data from from the theSpitzerSpitzerVeryVery Deep Deep Sur- Sur-χ minimization,minimization, and and the the uncertainties uncertainties on on the the best-fitting best-fitting parameters parameters veyvey of theof the HST/CANDELS HST/CANDELS fields fields (S-CANDELS; (S-CANDELS; PI PIFazio), Fazio), which which is isareare found found via via Monte Monte Carlo Carlo simulations, simulations, varying varying the the observed observed fluxes fluxes by by aCycle8Spitzer/IRAC program to cover the CANDELS wide fieldsanan amount amount proportional proportional to their to their photometric photometric errors. errors. This This procedure procedure is is aCycle8Spitzer2 /IRAC program to cover the CANDELS wide fields (0.2 deg2 ) with a total integration time∼ of ∼50 hr in both IRAC bandssimilarsimilar to that to that used used in our in our previous previous work, work, to which to which we we refer refer the the reader reader (0.2 deg ) with a total integration time of 50 hr in both IRAC bands 15,2215,22 at 3.6at 3.6 and and 4.5 4.5µm.µm. S-CANDELS S-CANDELS data data acquisition acquisition in thein the CANDELS CANDELSforfor more more details. details. GOODS-NGOODS-N field field was was completed completed over over the the course course of twoof two visits, visits, during during WeWe perform perform two two fits; fits; first first fixing fixing the the redshift redshift to z to =z 7.51= 7.51 should should 20122012 January January and and 2012 2012 July. July. The The data data were were reduced reduced to mosaicto mosaic form formourour detected detected line line be Lybeα Ly, andα, and secondly secondly fixing fixingz =z 1.78,= 1.78, if the if the line line were were followingfollowing procedures procedures identical identical to those to those described described for for the the coextensive, coextensive,[O[OII].II We]. We note note that that in the in the high-redshift high-redshift fit, fit, we we exclude exclude the theY -bandY -band pho- pho- 50 50 widerwider but but shallower shallowerSpitzerSpitzerExtendedExtended Deep Deep Survey. Survey.AtAt the the position positiontometry,tometry, as the as the highly-star-forming highly-star-forming nature nature of this of this object object implies implies that that it it of z8of z8GNDGND5296,5296, the the exact exact integration integration times times are are 47.2 47.2 and and 57.8 57.8 hr inhr inlikelylikely has has strong strong intrinsic intrinsic Lyα Lyemission,α emission, which which will will be beincluded included in the in the thethe 3.6 3.6 and and 4.5 4.5µmµ bands,m bands, respectively. respectively. A rmsA rms image image was was produced producedmodels.models. However, However, given given the the weak weak Lyα Lyfluxα flux observed, observed, the the emission emission is is forfor each each band band by by taking taking the the inverse inverse of theof the square square root root of theof the coverage coveragelikelylikely being being attenuated attenuated by by gas gas somewhere somewhere along along the the line-of-sight; line-of-sight; this this map,map, and and scaling scaling it so it thatso that the the mean mean value value was was equal equal to the to the mean mean of theof theeffecteffect is not is not included included in the in the modeling. modeling. As As briefly briefly discussed discussed in the in the main main pixel-to-pixelpixel-to-pixel fluctuations fluctuations in empty in empty regions regions of theof the image. image. text,text, the the observed observed photometry photometry of thisof this source source is much is much more more consistent consistent AsAs shown shown in Figurein Figure S3, S3, z8 z8GNDGND52965296 is clearly is clearly detected detected in bothin bothwithwith a redshift a redshift of 7.51,of 7.51, and and thus thus a line a line identification identification of Lyofα Ly(reducedα (reduced 2 χ2[z =7.51] = 2 χ2[z =1.78] = bands,bands, but but due due to tothe the large large beam beam of ofSpitzerSpitzer/IRAC,/IRAC, simple simple aperture apertureχr[zr =7.51] = 0.80.8 and andχr[zr =1.78] = 14.7).14.7). This This is primarily is primarily photometry will result in inaccurate fluxes due to contamination fromduedue to two to two wavelength wavelength regimes, regimes, highlighted highlighted by by the the right right panel panel of Fig-of Fig- photometry will result in inaccurate fluxes due to contamination from 2 2 nearbynearby neighbors. neighbors. We We therefore therefore fit fitand and subtracted subtracted nearby nearby sources sources in inureure 3, which 3, which shows shows the the values values of χ of χforfor each each band band and and redshift. redshift. First, First, ′′ ′′ ′′ ′′ a 19a 19× 19× 19regionregion around around z8 z8GNDGND52965296 in eachin each of ofthe the IRAC IRAC im- im-z8 z8GNDGND52965296 is completely is completely undetected undetected in thein the optical, optical, even even in thein the ul- ul- ages.ages. Positions, Positions, magnitudes, magnitudes, and and radial radial profiles profiles of of the the sources sources in intradeeptradeep F814W F814W band. band. As As can can be beseen seen in Figure in Figure 3, the 3, the lack lack of a of signif- a signif- this region were derived by running Source Extractor on the highericanticant detection detection in the in the optical optical strongly strongly favors favors the the high-redshift high-redshift solution, solution, this region were derived by running Source Extractor on the higher 2 2 2 2 2 2 ∆χ χ =1 78 − χ =7 51 = resolutionresolutionHSTHSTF160W-bandF160W-band images. images. Each Each source source found, found, including includingwithwith∆χ (χz(=1z.78 .− χz=7z.51). =) 2.0,2.0, 17.6 17.6 and and 3.4 3.4 for for the the F606W, F606W, z8 z8GNDGND5296,5296, was was modeled modeled on on the the IRAC IRAC images images with with the the galaxy- galaxy-F814WF814W and and F850LP F850LP bands, bands, respectively respectively (the (the F775W F775W band band is less is less dis- dis- 51 fittingfitting software software package package GALFIT GALFIT51 (v3.0)(v3.0) in a in manner a manner similar similar to our to ourcerning,cerning, as asit has it has a formal a formal 1.3 1.3σ detection;σ detection; due due to theto the non-detections non-detections 52 previousprevious work. work.52 FigureFigure S3 S3 illustrates illustrates the the process. process. GALFIT GALFIT requires requires a ain thein the surrounding surrounding bands bands and and in the in the stack stack of all of all optical optical bands, bands, as well as well point-spread function (PSF), which was constructed using stars in theas theas the non-detection non-detection in this in this band band in a in smaller a smaller circular circular aperture, aperture, we we at- at- point-spread function (PSF), which was constructed using stars in′′ the large IRAC mosaics. The FWHMs of the IRAC PSFs were 1.9′′ . Thetributetribute this this to random to random noise). noise). Second, Second, the the IRAC IRAC bands bands also also strongly strongly large IRAC mosaics. The FWHMs of the IRAC PSFs were 1.9 . The 2 2 3 6 ± ∆χ = extractedextracted AB AB magnitudes magnitudes of z8of z8GNDGND52965296 are arem3m.6 =. 25= .2538.38± 0.090.09favorfavor the the high-redshift high-redshift solution, solution, with with∆χ = 19.119.1 and and 17.4 17.4 for for the the 3.6 3.6 4 5 ± µ z = andandm4m.5 .== 24. 2440.40± 0.070..07 We. We note note that that these these photometric photometric errors errorsandand 4.5 4.5µm bands,m bands, respectively. respectively. This This is understandable is understandable as atasz at= 7.5,7.5, includeinclude the the uncertainty uncertainty due due to deblending, to deblending, which which we we verified verified by by vary- vary-[O[OIII]III is] located is located in the in the 4.5 4.5µmµ band,m band, and and a strong a strong emission emission line line could could inging the the neighbor neighbor fluxes fluxes within within their their 1σ 1uncertainties,σ uncertainties, and and noted noted that thatcreatecreate the the observed observed color. color. At Atz =z 1.78,= 1.78, there there is no is nosuch such strong strong emission emission it changedit changed the the flux flux of theof the galaxy galaxy of interestof interest by by≤ 9%≤ 9% for for the the 3.6 3.6µmµmlineline in this in this band, band, thus thus the the models models struggle struggle to fit to the fit the observed observed color. color. As As band,band, and and≤5%≤5% for for the the 4.5 4.5µmµ band;m band; both both at or at lessor less than than the the quoted quotedwewe discussed discussed in thein the main main text, text, the the inferred inferred [O [OIII]III EW] EW can can be be used used to to photometricphotometric uncertainties. uncertainties. As As expected expected from from inspecting inspecting the the image, image, the thediagnosediagnose the the metallicity metallicity of this of this galaxy. galaxy. We We quote quote the the [O [OIII]III EW] EW as that as that 4.54.5µmµ fluxm flux is much is much brighter, brighter, which which we we will will comment comment on on below. below. We Wefromfrom the the best-fitting best-fitting model, model, with with the the quoted quoted 68% 68% and and 95% 95% confidence confidence alsoalso included included constraints constraints during during SED SED fitting fitting at 5.8 at 5.8 and and 8.0 8.0µm,µm, using usingrangesranges coming coming from from the the Monte Monte Carlo Carlo simulations simulations (Table (Table S3). S3). imagesimages from from the the GOODS GOODSSpitzerSpitzersurvey.survey. There There was was no nosignificant significant flux flux TheThe SFR SFR quoted quoted in Tablein Table S3 S3 is a is time a time averaged averaged SFR. SFR. For For models models at theat the position position of z8of z8GNDGND52965296 (as (as expected expected for for a source a source at high at high red- red-wherewhere the the stellar stellar population population age age is older is older than than 100 100 Myr, Myr, we we integrate integrate the the shift),shift), thus thus during during the the SED SED fitting, fitting, these these fluxes fluxes were were set set to zero,to zero, and andstar-formationstar-formation history history over over the the past past 100 100 Myr Myr to determine to determine the the SFR. SFR. For For σ thethe flux flux errors errors were were set set to the to the 1σ 1limitlimit of theof the images, images, which which are are AB AByoungeryounger populations, populations, we we simply simply divide divide the the stellar stellar mass mass by by the the stellar stellar µ µ magnitudesmagnitudes of 23.485of 23.485 for for 5.8 5.8µm andm and 23.355 23.355 for for 8.0 8.0µm.m. These These limits limitspopulationpopulation age. age. This This time-averaged time-averaged SFR SFR is extremely is extremely high high for for our ourz =z = −1 −1 areare 1-2 1-2 mag mag brighter brighter than than both both of ourof our best-fit best-fit models. models. The The photometry photometry7.517.51 fit, fit, with with a 68% a 68% confidence confidence range range from from 320 320 – 1040 – 1040 M⊙ Myr⊙ yr (best-(best- −1 −1 of z8of z8GNDGND52925292 is listed is listed in Table in Table S2. S2. fit =fit330= 330 M⊙ Myr⊙ yr). Given). Given the the observed observed photometry, photometry, this this is plausible, is plausible, WeW comparede compared the the 12 12 photometric photometric points points of ofour our SED SED to toa suite a suiteas theas the bright bright rest-frame rest-frame UV UV coupled coupled with with strong strong inferred inferred [O [OIII]III emis-] emis- of stellarof stellar population population models, models, using using the the updated updated models models of Bruzualof Bruzual & &sionsion drives drives the the fit tofit a to young a young age age (3 Myr(3 Myr in this in this case), case), when when the the SFR SFR 4 4 4 | WWW.NATURE.COM/NATURE SUPPLEMENTARY INFORMATION RESEARCH willwill be bevery very high. high. However, However, any any SED-fitting-based SED-fitting-based SFR SFR for for very veryyoungyoungtingting prefers prefers strong strong Lyα Lyemission.α emission. The The best-fit best-fit model model has has a Ly aα Lylineα line −17−17 −1−1 −2−2 agesages will will be beextremely extremely sensitive sensitive to very to very short short timescale timescale variations variations in the in thefluxflux of 4.2of 4.2× 10× 10 ergerg s s cmcm, or, aor factor a factor of ∼ of15∼15 greater greater than than our our SFRSFR that that are are extremely extremely difficult difficult to constrain, to constrain, thus thus the the inferred inferred SFR SFR has hasobservedobserved line line flux flux (likewise, (likewise, the the best-fit best-fit model model rest-frame rest-frame Lyα LyEWα EW is is a largea large uncertainty. uncertainty. Although Although a young a young age age is necessary is necessary to reproduceto reproduce120120A).˚ A).˚ This This is certainly is certainly due due to theto the way way we we treat treat Lyα Lyinα ourin our model- model- 13,1370 ,70 thethe inferred inferred [O [OIII]III EW,] EW, metallicity metallicity will will also also have have a strong a strong effect effect on oning,ing, where where we we follow follow our our previous previous work work andand assume assume that that half half of of thethe [O [OIII]III EW,] EW, and and we we only only coarsely coarsely sample sample the the metallicity. metallicity. To To see seethethe line line is subject is subject to tothe the IGM IGM optical optical depth depth at 1215 at 1215A.˚ A.˚ This This is anal-is anal- whatwhat constraints constraints we we can can place place on on the the SFR SFR without without requiring requiring assump- assump-ogousogous to ato Gaussian a Gaussian line line symmetric symmetric about about the the resonance resonance wavelength wavelength tionstions on on the the [O [OIII]III line,] line, we we performed performed another another fit to fit the to the data, data, excluding excludingofof Lyα Ly.α However,. However, as asdiscussed discussed above, above, this this is rarely is rarely the the case; case; in fact,in fact, thethe IRAC IRAC 4.5 4.5µmµ band.m band. In thisIn this fit, fit, the the time-averaged time-averaged SFR SFR ranges ranges from fromLyαLyisα typicallyis typically observed observed to be to beredward redward of theof the systemic systemic redshift redshift due due to to −1−1 −1−1 120120 – 530 – 530 M⊙ Myr⊙ yr (best-fit(best-fit= =260260 M⊙ Myr⊙ yr).). Thus, Thus, even even without withoutradiativeradiative transfer transfer effects. effects. However, However, in thesein these cases, cases, all all of ofthe the line line flux flux allowingallowing the the [O [OIII]III emission] emission to influence to influence our our fit, fit, this this galaxy galaxy still still has has an anbluewardblueward of of1216 1216A˚ A as˚ aswell well as asmany many of ofthe the photons photons redward redward of ofres- res- extremelyextremely high high time-averaged time-averaged SFR. SFR. onanceonance (due (due to theto the damping damping wing) wing) are are scattered scattered by by neutral neutral hydrogen. hydrogen. AsAs one one final final check, check, we we calculate calculate the the SFR SFR using using the the UV-luminosity UV-luminosityThus,Thus, the the observed observed line line flux flux may may be be severely severely attenuated attenuated from from the the in- in- 27 27 46,7146,71 to SFRto SFR conversion conversion published published by by Kennicutt Kennicutt et al., et al.,whichwhich provides provides a atrinsictrinsic line line flux. flux. AsAs our our stellar stellar population population model model implies implies significant significant −1−1 SFRSFR 68% 68% confidence confidence range range of of 50–90 50–90 M⊙ Myr⊙ yr, significantly, significantly lower lowerstar-formation,star-formation, it is it not is not surprising surprising that that this this galaxy galaxy may may indeed indeed have have a a thanthan the the range range derived derived from from our our SED SED modeling. modeling. However, However, this this UV-to- UV-to-veryvery strong strong line line flux. flux. The The factor factor of ∼ of15∼15 difference difference between between the the best-fit best-fit SFRSFR conversion conversion assumes assumes constant constant star-formation star-formation over over the the previous previous 100 100lineline flux flux and and our our observed observed Lyα Lylineα line flux flux could could further further imply imply that that the the 27 27 Myr,Myr,whereaswhereas our our analysis analysis favors favors substantially substantially younger younger stellar stellar popu- popu-LyαLyfluxα flux of ofthis this galaxy galaxy is being is being severely severely attenuated, attenuated, perhaps perhaps due due to ato a lations.lations. Therefore, Therefore, this this conversion conversion will will significantly significantly underestimate underestimate the therisingrising neutral neutral fraction fraction in the in the IGM IGM (see (see the the following following section). section). SFRSFR in such in such galaxies galaxies (and (and caution caution should should be beused used when when interpreting interpreting the the4.54.5 [O [OIII]III Emission] EmissionThoughThough typically typically the the small small variations variations of galaxyof galaxy SFRsSFRs inferred inferred from from the the UV UV luminosity luminosity that that do do not not correct correct for for possibly possiblySEDsSEDs with with changing changing metallicity metallicity makes makes conclusions conclusions on on the the metallicity metallicity 9, 9, lowlow ages ages63).63). In Inthe the main main text, text, we we thus thus assume assume the the fiducial fiducial SFR SFR of ofdifficultdifficult from from photometry photometry alone, alone, the the strong strong inferred inferred [O [OIII]III emission] emission in in −1−1 330330 M⊙ Myr⊙ yr, with, with the the caveat caveat here here that that given given uncertainties uncertainties in model-in model-ourour object object makes makes at least at least moderate moderate conclusions conclusions possible. possible. In Inthe the main main inging the the [O [OIII]III emission,] emission, it may it may be be slightly slightly lower. lower. In theIn the main main text, text, we wetexttext we we discussed discussed how how the the strong strong [O [OIII]III emission] emission can can be be used used to con-to con- discussdiscuss the the implications implications of suchof such a high a high SFR, SFR, assuming assuming that that it is it due is due to tostrainstrain the the metallicity metallicity of ofthis this galaxy. galaxy. Figure Figure S4 S4 shows shows how how the the [O [OIII]III] fuelingfueling via via gas gas accretion accretion from from the the IGM. IGM. Alternatively, Alternatively, this this high high SFR SFREWEW varies varies with with age age as aas function a function of stellarof stellar population population metallicity. metallicity. Un- Un- couldcould be be due due to ato merger-induced a merger-induced starburst, starburst, which which would would be be detected detectedfortunately,fortunately, as weas we are are limited limited to the to the metallicity metallicity grid grid of ourof our chosen chosen stel- stel- at itsat its peak peak SFR SFR with with a ∼ a ∼1010− 20%− 20%probabilityprobability64.64 This. This galaxy galaxy does doeslarlar population population models models (which (which are are not not unlike unlike most most available available models), models), appearappear to haveto have a faint a faint companion, companion, though though a clumpy a clumpy morphology morphology is not is notwewe cannot cannot make make a precise a precise determination determination of theof the limits limits of theof the metallicity metallicity necessarilynecessarily indicative indicative of anof anongoing ongoing merger merger65. 65. in thisin this galaxy. galaxy. However, However, as asshown shown in Figurein Figure S4, S4, we we can can make make a few a few AsAs noted noted in thein the main main text, text, the the best-fit best-fit model model for for the the low-redshift low-redshiftconclusions.conclusions. First, First, models models with with solar solar metallicity metallicity cannot cannot come come within within a a solutionsolution has has zero zero [O [OII]II emission] emission line line flux, flux, inconsistent inconsistent with with the the spec- spec-factorfactor of threeof three of creatingof creating such such high high [O [OIII]III emission,] emission, thus thus even even one one of of troscopictroscopic detection detection of our of our emission emission line, line, providing providing further further evidence evidence for forthethe highest-star-forming highest-star-forming galaxies galaxies in the in the distant distant universe universe cannot cannot enrich enrich ∼ ∼ ∼ ∼ ourour high-redshift high-redshift solution. solution. To To see see if we if we could could reconcile reconcile the the photomet- photomet-to toSolarSolar metallicity metallicity by byz z 7.5.7.5. Secondly, Secondly, even even with with a continuous a continuous ricric non-detection non-detection at < at <1 µ1mµ withm with the the detectable detectable emission emission line line flux flux if ifstar-formationstar-formation history, history, models models with withZ Z= =0.020.02Z⊙Zare⊙ are still still excluded excluded ≫≫ thethe line line were were [O [OII],II we], we tried tried fitting fitting this this galaxy galaxy with with two two populations populationsat at 95%95% confidence. confidence. Models Models with with 20 20 or or40% 40% Solar Solar metallicity metallicity can can – one– one maximally maximally old old (formed (formed at z at =z =20),20), and and one one with with an an age age and andreproducereproduce our our inferred inferred [O [OIII]III EW,] EW, though though with with relatively relatively young young ages, ages, star-formationstar-formation history history which which was was allowed allowed to vary.to vary. Even Even including including the theconsistentconsistent with with the the results results from from our our SED SED fitting. fitting. Additionally, Additionally, we we also also emissionemission line line flux flux as asa constraint, a constraint, this this fit fitstill still preferred preferred a completely a completelyhavehave constraints constraints on on the the stellar stellar metallicity metallicity from from our our SED SED fitting, fitting, as weas we passivelypassively evolving evolving model model with with minimal minimal line line emission. emission. foundfound that that all all of ourof our 1000 1000 Monte Monte Carlo Carlo simulations simulations preferred preferred a metal- a metal- Z⊙Z⊙ [O[OII]II emission] emission at z at=z 1.78= 1.78 could could be beconsistent consistent with with a passive a passive pop- pop-licitylicity of eitherof either 0.2 0.2 or 0.4or 0.4 . The. The conservative conservative conclusion conclusion from from these these 0.020.02<< Z/Z Z/Z⊙<⊙<1.01.0 ulationulation if the if the galaxy galaxy hosted hosted an an active active galactic galactic nucleus nucleus (AGN). (AGN). This This is istwotwo results results is that is that at veryat very high high confidence, confidence, and and ′′ ′′ << Z/Z Z/Z⊙ <⊙ < unlikelyunlikely as thereas there is no is noChandraChandraX-rayX-ray source source within within 30 30.66 .The66 TheChan-Chan-givengiven the the metallicity metallicity spacing spacing of ofour our model model grid, grid, 0.2 0.2 0.40.4 42 42 −1−1 dradraimagingimaging reaches reaches Lx L=x 10= 10ergerg s s at zat=z 1.78,= 1.78, sufficient sufficient to detect to detectis inis ingood good agreement agreement with with our our measurements. measurements. Further Further nebular nebular mod- mod- weakweak AGNs. AGNs. To To see see if an if an obscured obscured AGN AGN interpretation interpretation matches matches the theelingeling may may yield yield a more a more precise precise lower lower limit limit for for the the metallicity metallicity in thisin this availableavailable data, data, we we examined examined the the Spitzer/MIPS Spitzer/MIPS 24 24µm,µm,HerschelHerschel/PACS/PACSsystem,system, but but the the best best results results will will come come from from rest-frame rest-frame optical optical nebular nebular 100100 and and 160 160µm,µm,HerschelHerschel/SPIRE/SPIRE 250, 250, 350 350 and and 500 500µmµ andm and the the JVLA JVLAlineline spectroscopy spectroscopy with with the theJamesJames Webb Webb Space Space Telescope Telescope. . 1.41.4 GHz GHz data data67–6967–69. We. We found found no no counterpart counterpart to z8 to z8GNDGND52965296 at any at any of of thesethese wavelengths. wavelengths. To To examine examine the the constraining constraining power power of ofthese these data, data,55 EW EW Evolution Evolution wewe compared compared the the spectral spectral energy energy distribution distribution of ofthe the low-redshift low-redshift ob- ob- HereHere we we examine examine the the implications implications of ourof our lone lone emission emission line line detec- detec- scuredscured AGN AGN Mrk Mrk 231, 231, redshifted redshifted to z to =z =1.78,1.78, to tothe the available available data. data.tion.tion. We We performed performed a simulation a simulation to predict to predict the the number number of galaxiesof galaxies we we SuchSuch a galaxy a galaxy would would have have been been very very well well detected detected at all at all wavelengths. wavelengths.wouldwould expect expect to toobserve observe using using a fiducial a fiducial Lyα LyEWα EW distribution, distribution, with with However,However, the the observed observed WFC3 WFC3 and and IRAC IRAC fluxes fluxes for for z8 z8GNDGND52965296 are arethethe goal goal of of measuring measuring the the significance significance at whichat which we we could could rule rule out out a a muchmuch fainter fainter than than this this redshifted redshifted template. template. Scaling Scaling down down the the template templategivengiven distribution. distribution. For For these these simulations, simulations, we we included included all all high-redshift high-redshift byby a factor a factor of 40of× 40to× matchto match the the observed observedH-bandH-band flux flux renders renders the theλ>λ>candidatecandidate galaxies galaxies observed observed on on both both masks. masks. We We chose chose as ouras our EW EW dis- dis- 19 19 24 24µmµ datam data unable unable to constrainto constrain this this possibility. possibility. However, However, the the observed observedtributiontribution the the predicted predictedz =z 7= Ly7α LyEWα EW distribution distribution from from Stark Stark et al.et al. H H− 3−.63µ.m6µ colorm color is very is very inconsistent inconsistent with with such such a template, a template, as weas we ob- ob-TheyThey use use observations observations of ofthe the evolution evolution of ofthe the Lyα LyEWα EW distribution distribution at at serveserve this this color color to be to be blue, blue, while while an an obscured obscured AGN AGN would would have have a very a very3 5 5 WWW.NATURE.COM/NATURE | 5 RESEARCH SUPPLEMENTARY INFORMATION and then computing the corresponding Lyα line flux using the contin- 42. Rhoads, J. E., Hibon, P., Malhotra, S., Cooper, M. & Weiner, B. A Lyα uum flux of the given galaxy redward of the line. In each simulation Galaxy at Redshift z = 6.944 in the COSMOS Field. Astrophys. J. Let. for each observed candidate galaxy, we first drew a redshift from the 752, L28 (2012). and then computing the corresponding Lyα line flux using the contin- 42.43. Rhoads, McLinden, J. E., E. Hibon, M. et al. P.,First Malhotra, Spectroscopic S., Cooper, Measurements M. & Weiner, of B.[OIII] A Ly Emis-α galaxy’s redshift probability distribution function. If the correspond- uum flux of the given galaxy redward of the line. In each simulation Galaxysion from at Redshift Lyα Selected z = 6.944 Field in Galaxies the COSMOS at z ˜ 3.1.Field.Astrophys.Astrophys. J. 730 J. Let., 136 ing Lyα wavelength fell outside the MOSFIRE Y -band spectral range, for each observed candidate galaxy, we first drew a redshift from the 752(2011)., L28 (2012). or if it fell on a sky emission line (using an extracted sky spectrum to 43.44. McLinden, Finkelstein, E. M. S.et L. al.et al.FirstThe Spectroscopic HETDEX Pilot Measurements Survey. III. The of [O Low III] Metallici- Emis- galaxy’s redshift probability distribution function. If the correspond- denote the position and extent of emission lines), then the galaxy was sionties from of High-redshift Lyα Selected Ly Fieldα Galaxies. GalaxiesAstrophys. at z ˜ 3.1. 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Dec magF 160W Photo-z 68% C.L. λline zLyα SNRline (J2000) (J2000) (AB) (A)˚ z8 GND 5296 12:36:37.90 62:18:08.5 25.6 7.5 – 7.9 10342.6 7.508 7.8 Table S2: Measured Broadband Flux Densities of z8 GND 5296 F435W F606W F775W F814W F850LP F105W F125W F160W 3.6µm 4.5µm −5.4 ± 10.5 −5.0 ± 8.5 17.8 ± 13.9 0.0 ± 5.04 0.7 ± 16.0 102 ± 10 194 ± 12 218 ± 14 256 ± 25 631 ± 51 −32 −1 −2 −1 Table S2 | All fluxes are in nJy (10 erg s cm Hz ). While the measured signal-to-noise in the F775W band is 1.3, the lack of detections in all other optical bands (including the stacked optical image) as well as in a smaller circular aperture implies that this is due to random noise. Table S3: 68% Confidence Range of Physical Properties for z8 GND 5296 z Stellar Mass Age E(B-V) SFR (t < 10 Myr) EW ([O III]) −1 (M⊙) (Myr) (M⊙ yr )(A)˚ 9 7.51 0.9 – 1.2 × 10 1 – 3 0.12 – 0.18 320 – 1040 560 – 640 9 1.78 1.6 – 1.8 × 10 510 – 570 0.0 – 0.0 0 – 0 — Table S3 | The values given correspond to the 68% confidence range for the quoted parameters. The initial mass function (IMF) was assumed to be Salpeter; were it of a Chabrier form, the stellar masses and star-formation rates would be lower by a factor of 1.8. Figure S1 | Emission line signal-to-noise test. The results of a signal-to-noise test for the one-dimensional spectrum of z8 GND 5296 (each row represents a different region of the spectrum). We divided the object spectrum by the error spectrum, smoothed by the velocity width of our observed line, and normalized the result so that the value at the peak of the Lyα line equaled the measured integrated line signal-to-noise of 7.8. The horizontal lines denote the ±3σ points, and the gray filled spectrum denotes the (arbitrarily scaled) sky emission. Only the detected emission line has a |signal-to-noise| >3; the absence of negative fluctuations at this level, which would be due to noise, gives confidence in the real nature of this emission line. 8 WWW.NATURE.COM/NATURE | 7 RESEARCH SUPPLEMENTARY INFORMATION Figure S2 | [O II] Doublet. A zoomed in view of our source spectrum, overplotting hypothetical [O II] doublet lines for three values of the ratio between the line fluxes. Unless the [O II] 3729 A˚ line is substantially weaker than the 3726 A˚ line, we would have expected to see highly significant flux from the redder line. Even in the case where the redder line is 50% the strength of the bluer line, we should still have detected emission line flux redward of the sky line residual at the ∼2σ level. ′′ Figure S3 | IRAC photometry. 18.6 stamps of z8 GND 5296 in the IRAC 3.6 (top row) and 4.5 (bottom row) µm bands, highlighting the de-blending algorithm we used to perform our source photometry. The first column is the image, the second is the GALFIT source model of nearby sources, and the third is the model-subtracted image, which clearly shows a significant detection for z8 GND 5296 in both bands, with minimal residuals from other sources. This fitting was straightforward, as the neighbors are relatively faint, and are well fit by point-sources. When we performed photometry, z8 GND 5296 was included in the GALFIT model, thus the quoted magnitudes come from this point-source fitting method rather than a less accurate circular aperture. 9 8 | WWW.NATURE.COM/NATURE SUPPLEMENTARY INFORMATION RESEARCH Figure S4 | [O III] EW variation with age and metallicity. The change of the rest-frame [O III] λ5007 EW with stellar population age, for the considered values of metallicity. The solid lines represent a continuous star-formation history, while the dashed lines represent an instantaneous burst. The 95% (2σ) confidence range of our inferred [O III] EW, 520–640 A,˚ is denoted by the gray bar. At 95% confidence, we can restrict the gas-phase metallicity in this galaxy to be sub-solar yet >0.02Z⊙. These results are consistent with the stellar metallicity results from the SED fitting; which also prefer Z = 0.2–0.4 Z⊙. Figure S5 | EW test. The results of our Lyα EW evolution test, assuming that the Lyα EW distribution at z = 7 continues its upward evolution with redshift observed at z = 3 – 6. The dashed curve shows the expected number of detected galaxies in our MOSFIRE data accounting for only the spectral range observed. The solid line shows how this changes if we also assume that we will not detect lines which fall on a night sky emission line; these sky lines reduce the expected detected number by >50%. Even accounting for this, our simulations show that if the EW continues its evolution previously observed at z = 3–6 out to z = 7, we would have expected to detect Lyα at >5σ significance from 6 galaxies. The fact that we only detected one such source implies that the Lyα EW distribution has evolved at 2.5σ significance. 10 WWW.NATURE.COM/NATURE | 9