A Keck Survey for Gravitationally-Lensed Lyman-alpha Emitters in the Redshift Range 8.5 < z < 10.4: New Constraints on the Contribution of Low Luminosity Sources to Cosmic Reionization

0.95 MB·English

by  Daniel P. Stark

Draftversion February5,2008
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
A KECK SURVEY FOR GRAVITATIONALLY-LENSED LYMAN α EMITTERS IN THE REDSHIFT RANGE
8.5<Z<10.4: NEW CONSTRAINTS ON THE CONTRIBUTION OF LOW LUMINOSITY SOURCES TO
COSMIC REIONIZATION
Daniel P. Stark1, Richard S. Ellis1, Johan Richard 1, Jean-Paul Kneib 1,2, Graham P. Smith 1,3, Michael R.
Santos 4
Draft versionFebruary 5, 2008
ABSTRACT
We discussnewobservationalconstraintsontheabundanceoffainthighredshiftLyman-αemitters
secured from a deep Keck near-infrared spectroscopic survey which utilizes the strong magnification
7
provided by lensing galaxy clusters at intermediate redshift (z = 0.2 0.5). In each of 9 clusters we
0 −
have undertaken a systematic ‘blind’ search for line emission with NIRSPEC in the J-band within
0
2 carefully-selected regions which offer very high magnifications (> 10-50) for background sources
×
with redshifts z 10. The high magnification enables the detec∼tion of emission at unprecedented
n flux limits (1041 ≃- 1042 erg s−1), much fainter than those of conventional narrow band imaging and
a
other spectroscopic searches. As the comoving volumes probed are small, our survey is designed to
J
address the important question of whether intrinsically feeble star forming galaxies could provide the
0
dominant ionizing flux at z 10. Our survey has yielded six promising (> 5σ) candidate Lyman-α
1 ∼
emitters which lie between z = 8.7 and z = 10.2. We carefully discuss the validity of our detections

in the light of earlier, apparently false, claims. We then focus on the likelihood that the detected
1
v line is Lyman-α from a distant lensed-source. All but one of the candidates remain undetected in
9 deep HST optical images and lower redshift line interpretations can be excluded, with reasonable
7 assumptions, through the non-detection of secondary emission in further spectroscopy undertaken
2 with LRIS and NIRSPEC. We demonstrate that rigorous confirmation of the Lyman-α hypothesis
1 for all of our candidates will be a challenge with current facilities. Nonetheless, as a result of our
0 tests, we argue that at least two of our candidates are likely to be at z 10. Given the small survey
7 volume, this suggests there is a large abundance of low luminosity sta≃r-forming sources occupying
0 the era when the reionization process is thought, from independent evidence, to be ongoing. While
/
h the predicted reionization photon budget depends upon a large number of physical assumptions, our
p first glimpse at the z 10 Universe suggests that low luminosity star-forming galaxies contribute a
≃
- significant proportion of the UV photons necessary for cosmic reionization.
o
Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies:
r
t high-redshift — gravitationallensing
s
a
:
v 1. INTRODUCTION in this important, but poorly understood, era.
Xi After the epoch of recombination at z 1000, neutral Recently, important constraints have emerged on the
≃ extent of star formation activity that occurred in the
r hydrogen permeated the universe. The earliest observ-
a able structure dates back to the beginning of this era at latter part of this era. The WMAP temperature-
polarization cross correlation signal on large angular
the surface of last scattering traced by the cosmic mi-
scales (Spergel et al. 2006) implies scattering of mi-
crowave background (CMB). The next most distant ob-
crowave photons by free electrons from ionizing sources
servable represent the populations of galaxies, gamma-
at z 10 12, suggesting an early period of effi-
ray bursts, and quasars at z 6.2 7.0 (Hu et al. 2002;
≃ −
≃ − cient star formation. Meanwhile, Hubble Space Tele-
Iye et al.2006;Totani et al.2006;Kashikawa et al.2006;
scope (HST) and Spitzer observations have uncovered
Fan et al. 2006; Bouwens & Illingworth 2006). The in-
a population of very massive (M 1010 - 1011
terval between z 1000 and z 6.5 contains many stellar
∼
≃ ≃ M ) galaxies at z 6 with well-established (>100 Myr)
landmarkevents: the formationofthe firststars,the as- ⊙ ≃
stellar populations (Egami et al. 2005; Eyles et al. 2005;
semblyoffirstgalaxies,thegrowthofthefirstsupermas-
Mobasher et al. 2005; Yan et al. 2005; Stark et al. 2006;
siveblackholesthatpowerquasars,andthe reionization
Eyles et al. 2006; Yan et al. 2006). The assembly of
of neutral hydrogen in the intergalactic medium. One
such massive galaxies by these redshifts requires signif-
of the primary driving forces in observationalcosmology
icant star formation at yet earlier times. Observations
todayistoilluminatethegrowthofthesefirststructures
of z-dropouts and J-dropouts with HST indicate a de-
1Department of Astrophysics, California Institute of Technol- cline in the star formation activity of luminous Lyman-
ogy,MS105-24,Pasadena, CA91125; [email protected] break galaxies out to z 7 10 (Bouwens et al. 2005;
2ObservatoireAstronomiquedeMarseille-Provence,Traversedu Bouwens & Illingworth 2≃006)−; if there is significant star
Siphon-BP8,13376MarseilleCedex12,France
formationactivityinthisredshiftinterval,theseobserva-
3School of Physics and Astronomy, University of Birmingham,
tions indicate it likely occurred in lower-luminosity sys-
Edgbaston, Birmingham,B152TT,UK
4SpaceTelescopeScienceInstitute,3700SanMartinDrive,Bal- tems.
timore,MD21218 Despite these indications of early activity, current fa-
2 Stark et al.
cilities are poorly-equipped to undertake conventional (Santos et al. 2004) to which the reader is referred.
searches for star-forming sources beyond z 7. Even A plan of the paper follows. We contrast our lensing
≃
with HST and 8 meter class telescopes, continuum searchtechniquewithother,moreconventionalprobesof
(‘drop-out’)andnarrow-bandimagingLyman-αsearches early star forming sources in 2. The NIRSPEC obser-
§
probe only the most luminous, rare, sources at these vationsandreductionmethodsarepresentedin 3. Here
§
redshifts; the contribution to the reionization flux from wedescribethecandidateselectioncriteriaandderivethe
these sources maywell be small. While narrowbandsur- survey sensitivity function. A set of 6 promising candi-
veys have had significant success at detecting Lyman- date z 10 Lyman-αemitters are presentedin 4. Here
≃ §
α emitters at z 6 (Hu et al. 1998, 2002, 2004; wediscussindetailboththerealityofthelinedetections
≃
Malhotra & Rhoads 2004; Hu et al. 2005; Stern et al. and the likelihood that the line identifications represent
2005;Kashikawa et al.2006;Shimasaku et al.2006)only highredshift Lyman-α. Noting the likelihood that a sig-
one Lyman-α emitter has been convincingly detected at nificant fraction of our sources may lie at z 10, we in-
≃
z 7 (Iye et al. 2006). At z 9, narrowband sur- fertheabundanceofhighredshiftlowluminositysources
vey≃s for relatively luminous Lyma≃n-α emitters (>1042.5 andcomparethesewithotherconstraintsin 5. In 6we
§ §
erg s−1) have yet to find any sources (Willis & C∼ourbin discuss whether the abundance so derived might repre-
2005; Cuby et al. 2006). While a few candidate Lyman- sent a significant fraction of that necessary to complete
break galaxies beyond z 7 have been suggested from cosmic reionization. We present our conclusions in 7.
photometric data (Bouw≃ens et al. 2005; Labb´e et al. Throughoutthepaper,wehaveassumedaflatuniv§erse
2006; Richard et al. 2006; Bouwens & Illingworth 2006; and(Ωm,ΩΛ)=(0.27,0.73)followingtheresultspresented
Henry et al. 2007), none has yet been spectroscopically in the initial WMAP data release (Spergel et al. 2003).
confirmed, due largely to the difficulties inherent to ob-
serving in the near-infrared. 2. GRAVITATIONALLENSINGANDSURVEYSTRATEGIES
-ACRITIQUE
Recent observations of the faint end slope of the star-
forming luminosity function at z 6 (Yan & Windhorst Strong gravitational lensing by intermediate redshift
2004; Bouwens et al. 2006) suppo≃rt the suggestion that galaxyclustersoffersthecapabilityofextendingthecur-
thebulkoftheintegratedstarformationathighredshift rent generation of high redshift galaxy surveys so that
may arise from very low luminosity sources, as expected lower luminosity sources can be brought into view. This
theoretically (Barkana & Loeb 2001; Wyithe & Loeb added boost in sensitivity is particularly effective at the
2006). However,forming stars in low mass halos is com- highest source redshifts (z > 7) where the bulk of star
plicated by a number of feedback effects, any of which formationactivity maybe ta∼king place inlowmass dark
couldsignificantly decreasethe star formationefficiency. matter halos (Barkana & Loeb 2001).
Clarifyingwhetherlow luminositysystems areabundant Lensing increases the apparent area of a background
at early times requires observations probing well below source while conserving the surface brightness; thus the
thesensitivitylimitsobtainablewithcurrentfacilitiesus- gain in sensitivity is the magnification factor, . Rich
M
ing conventional methods. clusters of galaxies at intermediate redshifts (e.g. z
≃
The study of lowerluminosity sources renderedvisible 0.2 0.5)amplifyhighredshiftsourcesbyfactorsoftyp-
bythestronggravitationallensinginducedbyforeground ical−ly 3 5 over areas of a few arcmin2. However,
M≃ −
clusters is the only current means to advance this quest boosts of 10 50 are possible near the critical line
M≃ −
before the next generation of telescopes is available. Al- - the theoretical locus of points in the image plane cor-
thoughatypicallensingclustermagnifiessourcesby 5- respondingtothecausticofinfinite magnification. Clus-
10over1arcmin2forsourcesatz >7,faintsignalscan×be ters are usually characterized by both an inner and an
boosted> 15ontheso-calledcriticalline-thelocation outer criticalline, the location of which varies as a func-
of the lensi×ng caustic in the image plane. Spectroscopy tion of source (background) redshift.
anddetailedstudies becomefeasibleatotherwiseimpos- The accurate location on the sky of these critical lines
sible (unlensed) limits. For several objects recently lo- can only be determined in the case of clusters for which
cated via this technique (Ellis et al. 2001; Kneib et al. detailed mass models are available. Such a mass model
2004; Santos et al. 2004; Egami et al. 2005), stellar con- is primarily constrained by the positions and redshifts
tinuumslopes,Lyman-αprofilesandstarformationrates of lensed features (Kneib et al. 1996). A mass model
have been determined for sources whose intrinsic flux is can be validated, for example, by securing spectrosopy
close to the faintest direct imaging limits so far reached, of lensed features whose redshifts were predicted by the
I 30 (Beckwith et al. 2006). Until the availability mass model using an ‘inversion’ technique (Ebbels et al.
AB
ofmo≃repowerfulgroundandspace-basedfacilities,scan- 1998). Currently,suchhighqualitymassmodelsareonly
ning the critical lines of massive clusters provides possi- availablefor 20clusters;howeverthisshouldincreasein
≃
bly the only reliable means of estimating the abundance the future with additional surveys (Ebeling et al. 2001).
of low luminosity star forming sources in the 7< z <12 However,thegaininsensitivityaffordedbystronglens-
intervaloverwhichcosmicreionizationisthoughttotake ing comes with a loss in survey area, compared to an
place. equivalentunlensed survey,by a factor of . This is an
M
This paper presents the results of a spectroscopic sur- important point as the area of high magnification (say
veyforstrongly-lensedLyman-αemitters at8.5<z<10.4 >10) is already quite small ( 0.1 arcmin2). Pro-
M ≃
using the NIRSPEC near-infrared spectrograph on the vided, as seems reasonable ( 1), that the slope of the
§
Keck II telescope. The survey follows logically from a faint-end slope of the luminosity function is very steep,
very similar, successful one undertaken at z 4 6 us- thenstronglensingsurveysshouldbeaveryeffectiveway
ingtheLRISopticalspectrographontheKeck≃Ite−lescope to characterize the amount of star formation at z >7.
Observersareconsideringseveralwaysto conductsur-
A Keck Survey for Gravitationally-LensedLyman-α Emitters in at 8.5<z<10 3
∼ ∼
veys for z >7 Lyman-α emitters. Before describing the imaged systems (Kneib et al. 1996; Broadhurstet al.
presentsurvey,wethereforeconsideritinstructivetoex- 2005). The most well-understood clusters in our sam-
aminethe benefits anddrawbacksofthe differentstrate- ple contain 10-30 multiply-imaged systems, a signi-
gies. fication fraction of which have spectroscopic redshift
The primary survey techniques used thus far are (e.g. Abell 1689 has 33 such systems of which 21 are
narrowband imaging (Hu et al. 2002, 2004) and spec- spectroscopically-confirmed,Richardetal,inprep). This
troscopy(Martin et al. 2006). Neither technique has yet dataisnecessarytoastrometricallypinpointthelocation
been routinely used in the near-infrared spectral region of the critical line along which the NIRSPEC slit is ori-
appropriate for z >7 and both methods can, in prin- ented.
ciple, be combined with strong lensing by focusing on Deep HST optical imaging is also essential for verify-
fields containing rich clusters. At optical wavelengths, ing, via the absence of any detectable continuum, that
unlensednarrow-bandimagingtechniquestypicallyreach any lensed emitters are genuinely at high redshift. Due
flux limits of 2 10−17 ergs cm−2 s−1, whereas spec- toneutralhydrogenabsorptionwewouldnotexpectany
troscopic techniq×ues probe to 5 10−18 ergs cm−2 s−1 of our z 10 emitters to be seen at optical wavelengths
× ≃
(Santos et al. 2004). and HST provides the deepest verification.
In the near-IR, a longslit spectrograph such as NIR- A final consideration is the availability of deep opti-
SPECoffers a 8˚A resolutionoveramoderate redshift cal spectroscopy which is helpful in order to check for
∼
interval (∆z=2). Although bright atmospheric emission associated emission lines that might arise if any NIR-
linesrestrictvisibilityfor50%oftheredshiftrangeinthe SPEC line detection arises from a lower redshift source.
J-band, for a resolving power R 1500, the gain in red- Given the comprehensive critical line survey undertaken
≃
shift coverage is still a factor of 10-100 over the narrow by (Santos et al. 2004) using LRIS, where possible we
band technique (∆z=0.01-0.1). Per exposure, there is chose the same clusters and explored the same critical
howeveraverysmallfieldofview( 30arcsec2)c.f. nar- line regions.
rowband imaging ( 10 arcmin2 fo≃r a cluster but much In practice, telescope scheduling and weather losses
≃
larger for panoramic field surveys). compromised a perfect adoption of the latter criteria.
Assuming the characteristics for current instrumenta- However, for each of the 9 clusters, the mass model, the
tion and their detectors, we compare the efficiencies of critical line location and associated magnification prop-
the two methods in the lensing case in Figure 1. Details erties are well understood.
of the computation are given in the Appendix. Both
lensed surveytechniques probe atleast anorder ofmag- 3.2. Observations and Data Reduction
nitude deeper than conventional near-IR Lyman-α nar- We utilized the Near Infrared Spectrometer (NIR-
rowband surveys. Of the two lensing methods, long-slit SPEC, McLean et al. 1998) mounted on the Keck II 10
spectroscopy is better suited for detecting the faintest mtelescopeatMaunaKeatoperformoursurvey. Obser-
Lyman-αemittersfordetectors1024 1024pixelsinsize. vations were performed in the J-band (1.143-1.375 µm)
This can be explained as follows. W×hile a narrowband with a slit 42′′ long and 0′.′76 wide. We used the low-
imager coversa largerarea on the sky, only a very small resolution mode of NIRSPEC with the 75 line mm−1
fractionofthisareaishighlymagnified(e.g. >10 ). The grating which offers a net resolving power of R 1500
small gain in the highly-magnified survey area is×insuffi- and a spectral resolution of 8 ˚A. At the wavele≃ngths
cient to compensate for the significantly smaller redshift sampledwithintheNIRSPEC≃J-bandfilter,anydetected
coverage. Moreover, the 100 ˚A bandwidth typical for Lyman-αemissionwouldcorrespondtosourceswithred-
≃
many narrowband filters results in significantly poorer shifts between z =8.5 and z =10.4.
sensitivitylimits. Takingthe variousfactorsintoconsid- Figure 2 shows how the targeted slit positions match
eration, a long slit spectroscopic survey represents the the predicted location of the z 9 critical lines for each
more efficient technique for detecting sources with star of the 9 clusters in our survey.≃To maximize efficiency,
formation rates below 1042 erg s−1, and hence is the op- where a choice was available we selected regions where
timal strategy for determining whether reionization was the critical line is straight enough to provide high mag-
predominantlycausedbylargenumbersoffeeblesources. nification across the entire slit.
Typically, 2-5 slit positions were observed per cluster.
3. NIRSPECCRITICALLINESURVEY
Foreachslitposition,wetook6-10tenminuteexposures
3.1. Cluster Sample
usinga three pointdither patternin whichthe telescope
Our goal is to constrain the abundance of sources up was offset 3′′ along the slit. Table 2 summarizes the
±
to an order of magnitude below current survey sensitiv- details of our NIRSPEC observations. In four observing
ity limits. Accordingly, following the discussion above, runs, we observed 35 slit positions corresponding to a
we have conducted a spectroscopic survey for Lyman- total sky area of 0.3 arcmin2.
α emission along the critical lines of 9 clusters at z Spectrawereflat-fieldedandsky-subtractedusingIDL
≃
0.2 0.5. We limited our observations to those clusters scripts written by G. Becker (2004, private communica-
−
with mass models that are well-constrained by the com- tion). Following techniques described in Kelson (2003),
binationofHSTimagingandground-basedspectroscopy the two-dimensional spectra were not straightened prior
of multiply imaged systems. The list of survey clusters to sky subtraction, thus ensuring sub-pixel sampling of
andtheassociatedmassmodelsaresummarizedinTable atmospheric emission lines which, in turn, led to a sig-
1. nificantly improved removal of the sky background.
Many criteria entered into the final selection of our The camera distortion and spectral curvature were
9 clusters. An essential criteria was the availability of computed by fitting standard star traces along the slit
deep HST imaging for constructing samples of multiply- andatmosphericemissionlinesacrossthedispersionaxis.
4 Stark et al.
Knowledge of the distortion and spectral curvature was lengthandsourceredshift. Convertingthisfluxtoalim-
used to construct arrays that provide proxies to the slit iting source luminosity requires knowledge of the mag-
position and wavelength for each exposed pixel on the nification across the NIRSPEC slit. In practice, we will
NIRSPEC detector. Using this information, sky back- quote our limit in terms of that line flux we can expect
groundmodelingwasperformedusingatwo-dimensional to detect, at 5σ confidence, for an unresolved emitter
b-splinefit,wherealow-orderpolynomialisfittotheslit whoserest-framelinewidthistypicalofafaint,butvery
illuminationandab-splineisfitalongthedispersionaxis. well-studied, z 5.7 system (Ellis et al. 2001).
≃
Thewavelengthcalibrationissubsequentlycomputedus- To accomplish this, we computed the total noise as a
ing atmospheric emission lines. For each exposure, we function of dispersion and slit position using the vari-
alsocompute2D-variancearraysbysummingthecontri- ance arrays obtained in the data reduction ( 3.1). This
§
bution from the dark current, flat-field, read noise, sky comprises the sum of the variance from the sky back-
background, and source counts to the variance. ground, read noise, flat-fielding, dark count as well as
FollowingtechniquesintroducedbySantosetal(2004), the source counts. Flux calibration of the variance ar-
we determined the astrometric position of each spectral rays was performed using observations of spectroscopic
exposure by registering images from the NIRSPEC slit- standard stars. The 5σ limiting Lyman-α line flux was
viewing camera (SCAM) to HST images of the same then computed as a function of slit position and wave-
field. WCS coordinates were computed for each exposed length by calculating the signal that is 5 times the root
pixelonthedetectorusingtheslitpositiongriddescribed ofthevarianceinanaperturewhosespatialdimensionis
above. Since these are galaxycluster fields, many bright twicethemeasuredseeingdisk(typically0′.′7)andwhose
objects appear in the 46 46 arcsec2 field of view, en- spectraldimensionequalstheLyman-αlinewidth, 300
abling accurate registratio×n on to the WCS of the HST km s−1 of the lensed system studied with high prec∼ision
images with a typical rms of 0′.′15 (less than 25% of the by Ellis et al. (2001).
slit width). Offsets between exposuresof a givenslit po- We find the median 5σ limiting line flux acrossa typi-
sition are calculated from the registered SCAM images, calslitpositionwith1.5hoursofintegrationis2 10−17
and the 2D-spectra and their associated 2D-variance ar- erg cm−2 s−1 (see Figure 3); the sensitivity va×ries be-
rays are subsequently shifted and combined. To remove tween 0.9-3 10−17 erg cm−2 s−1 depending on the
×
cosmicraysandbadpixels,wemediancombinethe data proximity to atmospheric OH lines. This flux limit is
rejecting the brightest and faintest frame at each pixel. similartothatreachedinthez =8.8narrowbandsurvey
Our ability to detect faint emission lines is strongly ofGOODS-SdiscussedinCuby et al.(2006)(1.3 10−17
dependent upon the accuracy with which the offsets be- ergcm−2 s−1)andslightlylesssensitivethanthe×z =8.8
tweenthedifferentexposuresofaslitpositionareknown. narrowbandsurveypresentedinWillis & Courbin(2005)
If a bright emission line from a foreground galaxy lies (3.3 10−18 erg cm−2 s−1); however the lensing magni-
×
serendipitouslyontheslit,theaccuracyoftheoffsetscan fication enables the detection of sources that are signifi-
be determined by comparing the shifts derived from the cantly less luminous than those detectable in either con-
SCAM images to the offsets between the centroid of the ventional narrowbandsurvey.
emission line. Suitable emission lines are present in sev-
eralofourslitpositionsandapplyingtheaforementioned 3.4. Cluster Magnification
test, we find the offsets determined from the SCAM im-
As discussed in our precursor optical paper
ages are typically good to 0′.′1. At less than 15% of the
(Santos et al. 2004), the cluster mass models listed
slitwidth,thisuncertaintydoesnotcompromisetheS/N
in Table 1 provide a 2-D map of magnifications for
of faint emission features.
any given source redshift. The details by which these
One additional concern in dealing with faint emission
magnifications are realized follow techniques described
line objects is the possible transverse drift of the object
in detail by (Kneib et al. 1996). Further discussion of
across the slit from frame to frame when the seeing is
the code, LENSTOOL, used to create the mass models
lessthanaslitwidth(Pell´o et al.2004;Weatherley et al.
for systems for which rich detail is available Jullo et
2004). Thiswouldcausethecentroidoftheemissionline
al, (in prep and Smith et al. (2005). From each mass
to drift in the dispersion direction of the spectrun. Un-
model, the magnificationcan be computed as a function
corrected, this would increase the area over which the
of source redshift and position.
emission line flux is spread, thereby reducing the ob-
Inpractice,foreachpositionandwavelength,themass
servedS/Nofthe emissionline in the finalstackedspec-
models produce a matrix of the convergence and shear.
trum. We quantify the magnitude ofthe transversedrift
The magnification is then determined from the conver-
bymeasuringtheframe-to-framecentroidofaverybright
gence κ and shear γ using the following equation:
[OIII]emissionlineinNIRSPECobservationstakenwith
exceptionalseeing(0′.′45). We findthatthe standardde- 1
viationinthecentroidpositionover9framesis0.8pixels, M(Ω,z)= [1 κ(Ω,z)]2 γ(Ω,z)2 (1)
or 0′.′2. Adding this measured tranverse drift in quadra- − −
ture to the seeing FWHM only increases the FWHM of where Ω is the position on the sky and z is the source
the line by 5%. Given that the seeing was never better redshift.
thanthevaluequotedabove,itisclearthatthedrifthas A key question that often arises in the consideration
a negligible effect on the detectability of emission lines. of critical line surveys is the accuracy with which the
location of the line can be pinpointed and also that of
3.3. Survey Sensitivity
the associated magnification map. To address this ques-
We nowdetermineoursurveysensitivityfunction. We tion,wehaveusedaMarkovchainMonteCarlosampling
first discuss the limiting line flux as a function of wave- algorithm within LENSTOOL (Jullo et al., in prep) to
A Keck Survey for Gravitationally-LensedLyman-α Emitters in at 8.5<z<10 5
∼ ∼
maptheprobabilitydensityinthemassmodelparameter we are now in a position to calculate the limiting source
space for each cluster. We then generated a magnifica- Lyman-α luminosity, L (Ω,z). This we define as the
lim
tionmapforeachmodelandinturncomputedthemode least luminous source detectable (with 5σ confidence) in
of the distribution at each pixel as well as the range of a given volume element, allowing for the cluster magni-
magnification factors which bracket the mode by 34%. fication .
± M
Thiswetakeastheuncertaintyinthemagnification. The Allowing each NIRSPEC pixel to correspond to a vol-
errors derived for each cluster are applied in the deter- ume element, we can calculate the limiting luminosity
minationofboththecandidateluminosities( 5)andthe across each slit in the survey according to the following:
§
survey volume ( 6.1).
Taking each e§xposed pixel on the NIRSPEC detector 4π(1+z)2D2(z) f (z)
L (Ω,z)= c lim . (2)
asanelementofsolidangle,theconvergenceandshearis lim T(Ω) (Ω,z)
computed as a function of position via interpolation be- M
tweentheNIRSPECcoordinategridandthatofthecon- where D (z) is the comoving distance of the volume ele-
c
vergence and shear maps. For each NIRSPEC pixel, the ment and T(Ω) is the slit transmission.
associated redshift at which a Lyman-α emitter would Intheabsenceofthelensingmagnification,thetypical
be located is calculated using the wavelength solution limiting Lyman-α luminosity is 2 1043erg s−1 in be-
×
described above. The convergence, shear and Lyman-α tween atmospheric OH lines. However, more than 50%
redshift is then inserted into equation 1 and the mag- of the survey area is magnified by > 30, enabling the
nification at each pixel determined. Likewise, the detectability of sources as faint as 7 ×1041erg s−1 over
magnificatMion uncertainty map is interpolated onto the 0.15arcmin2 intheimageplane. Co×nventionalnarrow-
≃
NIRSPEC coordinate grid enabling the computation of band Lyman-α surveys at z >6 typically reach limiting
the error as a function of slit position. Both the magni- luminosities between 1042 and 1043 erg s−1. Our NIR-
fication and associated error maps were smoothed using SPEC survey probes over an order of magnitude fainter
a 1′.′0 median box filter to ensure that our results are and thus can clearly provide constraints on star form-
not corrupted by sharp high peaks in the magnification ing sourcesthat are otherwise out of reachof traditional
distribution. The changein the magnificationerrorwith methods, at least with current facilities.
redshift is negligible; hence we adopt the uncertainity
at z = 9 to be the magnification error for all Lyman-α 4. IDENTIFICATION ANDREALITYOFTHECANDIDATE
redshifts probed by our survey. LYMAN-αEMITTERS
As an example of the magnifications sampled, the Atthefaintlimitsnowbeingprobed,wehavefoundthe
wavelength-independent magnification along one of our reliable identification and verification of distant Lyman-
surveyslitpositionsisdisplayedinFigure4forthreedif- α emitters to be a very challenging endeavor, even with
ferent redshifts. The magnification is > 20 over nearly the most powerful facilities available to us. The bur-
×
the entire slit and fairly constant with redshift. We also den of proof that a detected line is (a) real and (b)
plot the associated 1σ uncertainty as a function of posi- truly arises from a highly-redshifted Lyman-α emission
tion along the slit at one of the redshifts. Even allowing line is very great given earlier controversies (Pell´o et al.
for the uncertainty in the lensing model, the magnifi- 2004; Bremer et al. 2004; Smith et al. 2006). So far as
cation is still expected to be uniformly high across the the identification process is concerned, the key issue is
entireslit. Thisislargelyaresultofthefactthatweonly to ensure candidate emission features are not spurious
select clusters with well-defined mass models. Further- anddonotarisefromdetectorartefactsWeatherley et al.
more, in regions where the location of the critical line is (2004). Concerningtheredshiftverification,anewsetof
known less precisely, there is generally a region 2′.′ in challenges emerge at z 10 since additional (confirma-
width around the critical line where the magni≃fication tory) emission lines are n≃ot available from ground-based
factor is greater than fifteen, so the positioning of the facilities (see 5).
NIRSPEC slit is always in a region of high magnifica- Beginning w§ith the identification process, each of the
tion. sky-subtracted2-Dspectrawereindependentlyinspected
Figure 5 presents a histogram of the magnification in for Lyman-α candidates by three of the authors (D.P.S,
our survey. Throughout the entire survey area, the me- R.S.E,and J.R.). We demanded that any candidate fea-
dian magnification provided to background sources is ture be both extended over multiple pixels (in order to
20, a factor of two larger than in the optical sur- differentiate a signal from bad pixels and other detec-
≃ ×
vey conducted by Santos et al. (2004). The increased tor artifacts) and located away from contaminating OH
magnification is due to several factors. First, ultra-high airglowlines. Thethreelistsoflinecandidateseachcon-
magnification regions ( 15-30 throughout the slit-area) tained 25 possible features with many being in com-
werepreferablyselected×toallowverylow-luminositysys- monbe≃tweentwoofthe threelists. Afinalcatalogofsix
tems (< 1042ergs−1) to be probed. Second, since the promising candidates was constructedby selecting those
NIRSP∼EC longslit covers a factor of three smaller an- featureswiththehighestsignificancecommontoallthree
gular extent on the sky than the LRIS longslit used in observers. These are displayed in Figure 6. The S/N ra-
Santos et al.(2004),wewereabletoavoidregionswhere tios of these candidate emission lines, defined as in 3.2,
§
thecriticallinecurvedofftheslit,resultinginhighmag- varies between 5 and 8 (see Table 3).
nification uniformly across the slit. A spurious feature could arise if the reduction process
does not adequately remove a cosmic ray or bad pixel.
3.5. Limiting Lyman-α Luminosity
Obviouslyditheringthetelescopesothecandidatesmove
Given our knowledge of the limiting Lyman-α flux upanddownthesliteliminatesmostdetectoranomalies.
( 3.2) and the spatially-dependent magnification ( 3.3), Howeverartefactscanremainifthespectraarerebinned
§ §
6 Stark et al.
during the wavelengthcalibration;inthis case,suchbad a rate of 1 M yr−1. For f 1 (e.g. Shapley et al.
⊙ esc
pixelswouldbesmoothedoutandappearsimilartoemis- 2006), this results in a conversi≪on factor of 1 M yr−1
⊙
sion lines (Weatherley et al. 2004). Fortunately, we do = 4.2 1042 erg s−1. With these assumptions, the star
not face this problem in our Keck dataset since our re- forma×tion rates range from 0.038 to 1.2 M yr−1. If a
⊙
duction technique altogether avoids rebinning the spec- ScaloIMFwasadoptedinstead,the starformationrates
tra (see 3). Nevertheless, one may still worry that the would be a factor of three greater (Loeb et al. 2005).
§
measured emission line flux originates from one or two We note that these star formation rates are uncertain
frames with overlapping cosmic rays or bad pixels. We for several reasons. If dust in the galaxy absorbs any
havetested this possibility by median combinationofall Lyman-α photons, then the intrinsic luminosities and
of the exposures of slit positions containing our candi- corresponding star formation rates will be larger. Like-
datesandrejectingthethreehighestpixels. Wealsome- wise,ifonlyafractionoftheLyman-αlineistransmitted
dian combined half of the exposures (chosen randomly) through the IGM due to the presence of neutral hydro-
for each candidate. In both tests, all six candidates are gen, then Lyman-α luminosity and star formation rates
still clearly visible (albeit at lower S/N for the latter will again be underestimated. On the other hand, if the
test), suggesting that our emission features do not arise stellar IMF at these early times is more top-heavy than
from flux in only one or two frames. a Salpeter or Scalo IMF, more ionizing photons are pre-
Finally, it is conceivable that the noise distribution of dictedper unitofstarformation. Similarly,ifthemetal-
the spectraldataissuchthat5σ noisefeaturesaremuch licityofthesegalaxiesislessthanwhatisassumedabove,
more common than would be expected under Gaussian the efficiency of ionizing photon production (and hence
statistics. Such a curious situation might arise if there Lyman-α luminosity) is increased (Tumlinson & Shull
wassomeunforeseenpropertyoftheNIRSPECdetector. 2000; Tumlinson et al. 2001; Schaerer 2003). Hence, in
Inthiscaseourcandidateemissionfeaturesmightsimply these cases, the star formation rates tabulated will be
be peaks in the noise distribution. We investigated this overestimates.
possibilitybycomputingthefluxinaperturescenteredat But how can we be sure, with a single emission line,
random positions in areas of the spectra where the sky- that we have truly detecteda Lyman-α emitter? Nor-
subtraction is clean. The aperture size was matched to mally,foranarrow-bandimagingsurvey(Hu et al.2002,
the expected size of a Lyman-α feature at high redshift: 2004; Malhotra & Rhoads 2004; Shimasaku et al. 2006;
the spatial FWHM was taken to be the size of a typical Kashikawa et al.2006)oranunlensedspectroscopicsur-
seeing disk, 0′.′6 and spectral FWHM corresponding to vey(Martin et al.2006),contaminationfromforeground
typical line width of Lyman-α emitters, 300 km s−1) [O II] and Hα emitters can be significant. However, it
Anormalizedhistogramofthedistributionofsummed is worth pointing out that such foreground contamina-
aperture fluxes is presented in Figure 7. The frequency tionislikelytobemuchreducedforacritical-linesurvey
withwhichafeaturewithS/N=5appearsinourspectra designed to probe faint emitters since, for lensing clus-
is 0.02%, which corresponds to just under a 4σ detec- ters at z 0.2-0.5, all contaminating emitters will also
≃ ≃
tion for a Gaussian distribution. Though the frequency be lensed and thus their respective most-likely location in
offeatureswithfluxescomparabletoourcandidatesmay theimageplanewillbespatially offsetwithrespecttothat
be slightly higher than indicated by Gaussian statistics, for a source at z 2. Thedegreeto whichthisbenefits a
≃
the test clearly illustrates that features of prominence lensedsurveywithrespecttoablankfieldsurveywillde-
similar to those of our candidates are extremely rare in pend on the relative magnifications at each redshift and
clean regions of the spectra. the shape of the faint end of the respective line emitting
luminosity functions.
5. VERIFYINGTHECANDIDATEREDSHIFTS
A redshift that the general community normally ac-
If the emission features discussed above are Lyman-α cepts is usually one in which at least two, preferably
,thenthe sourceredshifts forour6candidatesrangebe- more,featuresarereliablyidentified. Single line identifi-
tween z = 8.7 and z = 10.2 and the observed fluxes cations are quite naturally questioned as unreliable. As
span 2-5 10−17 erg cm−2 s−1. Converting these ob- we probe to z 10 with ground-based telescopes, how-
servedflu×xesintointrinsicluminositiesiscomplicatedby ever, we reach≃the situation where multiple-line identifi-
uncertaintiesinthefractionofemittedLyman-αphotons cations are simply not practical. With Lyman-α in the
that reach the observer. We can compute a lower limit J-bandnoadditionalfeatureisvisibleinthefullpractical
to the unlensed Lyman-α luminosity by converting the spectroscopic range from the UV limit to the beginning
fluxes to luminosities and dividing by the magnification ofthethermally-dominatedbackgroundat2µm. TheIn-
factor. With these assumptions, the derived unlensed frared Spectrograph (IRS) on Spitzer in principle offers
luminosities range from 1.6-50 1041 erg s−1. the possiblity of detecting Hα emission from z 10 ob-
We convert these Lyman-α×luminosities to star for- jects. However, sensitivity calculations (Lacy e≃t al., in
mation rates assuming case B recombination: Lα = prep.) suggest that the Hα/Lyman-α flux ratio would
2hν (1 f )Q(H)M˙ where hν is the energy of a have to be significantly greater than that expected for
3 Lyα − esc ⋆ α
Lyman-α photon, Q(H) is the Hydrogen ionizing pho- caseBrecombination(perhapsduetoLyman-αsuppres-
tons flux, and f is the fraction of ionizing photons sionfromresonantscatteringandabsorption)inorderfor
esc
that escape from the galaxy (and hence are not emitted this to be an effective verification.
in recombination emission lines). Assuming a Salpeter A second-best verification of Lyman-α that might be
initialmassfunction,1/20thsolarmetallicity,andamin- convincing would be the identification of an associated
imumandmaximumstellarmassof1and20M ,respec- Lyman break in broadband imaging of the continuum
⊙
tively, the hydrogen ionizing photon flux is 4 1053 s−1 distribution (Stanway et al. 2004) or the presence of an
(Schaerer 2003) for galaxyforming stars contin×uously at asymmetriclineprofile(Ellis et al.2001;Hu et al.2002).
A Keck Survey for Gravitationally-LensedLyman-α Emitters in at 8.5<z<10 7
∼ ∼
The Lyman break is due to intergalactic absorption of by a number of factors. First the resolution of NIR-
flux shortwardof the Lyman-α line and causes all z >7 SPEC is only moderate; R 1500c.f. R 15,000for the
≃ ≃
sources to be undetected at optical wavelengths. How- ESI spectrum obtained at z=5.7 (Ellis et al. 2001). Sec-
ever, the lack of an optical broad-band detection need ond, stacking assumes that all 6 lines are Lyman-α with
not necessarily rule out a lower redshift source, for ex- similar profiles and, most importantly, requires an accu-
ample those with very strong emission line spectra and rateregistrationusingthecentralwavelengthwithineach
weak continuum emission. low signal/noise profile. Finally, as the adjacent OH sky
Asymmetric line profiles are expected because of res- lines are distributed differently for each emitter, wave-
onant scattering by neutral hydrogen within the host length regions that are clean in one candidate become
galaxy and in the IGM. This leads to absorption on the averaged with noisy regions in another so the contrast
blue side of the line as the light redshifts into resonance, of the stacked line does not increase as √N. Even with
leaving a sharp cutoff on the blue side of the line and weighted addition, we did not find the test to be very
an extended red damping wing. Unfortunately, the de- conclusive.
tection of an asymmetric line profile requires very long
integrations and high spectral resolution in order to de- 5.2. Searching for Lensed Pairs
tect the damping wing. In several of the lensed emitters and drop-outs we
A key factor in our consideration of how to verify the havepublishedinthepast(Ellis et al.2001;Kneib et al.
Lyman-α label for our 6 candidates is the importance of 2004), the location of a second or third counter-image
exquisite conditions to see such faint emitters. The re- hasbeenaparticularlyconvincingdemonstrationofboth
quiredseeingandtransparencyoccuronly40-50%ofthe the lensed hypothesis and, via the mass model, the ap-
time evenon an excellent site such as Mauna Kea. Con- proximate redshift. Indeed, the accurate recovery of the
servatively, we estimated that to measure the Lyman-α thirdimageforthez 6.8drop-outsourceinAbell2218
profile of any one of our 6 candidates with adequate sig- (Kneib et al. 2004) w≃as one of the major arguments jus-
nal/noise would take 8-10 hours of integration on Keck tifying its location beyond z 6.
II. Allowing for weather, to follow-up all 6 candidates in Generallyspeaking,imagep≃airsareexpectedinstrong
this fashion would take 24 nights; clearly this is not a lensing configurations depending on the degree of align-
practical proposition! ment between the background source and the cluster
One might conclude we have reached new territory caustics,the physicalsize ofthe sourceand, inthis case,
where we may never quite know with certainty whether the ground-based seeing. The key feature which led to
an object is at z 10. In practice, however, there is a the satisfactory recovery of image pairs in our earlier
≃
wayforwardalthoughitislessdefinitiveandmorestatis- studies was a broad-band detection with HST. As an
tical in nature. The approach we will adopt is to search example, in the case of the z = 5.7 pair discussed by
spectroscopically in various passbands for lines associ- Ellis et al (2001), only a single emitter was recoveredby
atedwithalternative,lowerredshiftexplanationsforthe LRIS.Thelocationofthesecondimagerequiredadetec-
detected J-band emissionline. In ruling out a particular tion by HST. Indeed, pairs were not detected for any of
alternative identification, we must make some assump- the other lensed sources in the subsequent LRIS survey
tions about the likely emission line spectrum of the in- (Santos et al. 2004).
terloper. As such this is then a statistical process since Some true pairs may simply not be resolved in our
we cannot cover all possible emission line ratios. While ground-basedspectraand,quitepossibly,thesecondim-
this method will never confirm the Lyman-α interpreta- ageliesoutsideourNIRSPECslitcoverage. Forthemost
tion with 100% certainty, it is clearly an essential pre- well-constrained mass model, the location of a counter-
requisite to spending 2 nights per candidate measuring image can usually be predicted with an uncertainty of
the line profiles. 0′.′5-1′.′, i.e. to within 1-2 NIRSPEC slit widths. Al-
In the following subsections, we proceed to apply the though it is practical to consider chasing the second im-
tests discussed above to our candidates in order to con- ages, this could still be fairly time-consuming and we
strain their redshifts. In 5.1, we examine the stacked concluded that priority should be given in eliminating
§
line profiles of the six candidates to see if we can collec- low-redshift interlopers from our catalog of candidates.
tively assess the likelihood the bulk represent Lyman-α
emission; in 5.2, we test for the presence of a Lyman 5.3. Broadband Photometry of Candidates
§
break in deep broadbandphotometry; finally in 5.3, we The next test is to determine whether optical contin-
§
discuss our follow-up spectroscopic program to test for uum emission is visible for any of our candidates. Such
low-redshift interlopers. a detection would clearly rule out a high z interpreta-
tionalthoughnon-detectiondoesnotimplytheopposite.
5.1. Stacked Line Profile of Candidates
Deep ACS/WFPC-2 (Kneib et al. 2004; Smith et al.
Asymmetriclineprofilescannotbediscernedinthein- 2002) images are available for all 9 clusters so we have
dividual emission features, because of the low spectral examinedtheseimagesatthepreciselocationsofeachof
signal/noise. As discussed, much longer integrations (8- our 6 emitters.
10hrs)wouldbeneededtocharacterizetheemissionline Nodefinitivebroadbandopticaldetectionisseenatthe
profiles and this seems pointless until lower redshift in- location of any of the candidates to R 27.5 (3σ, Fig-
≃
terlopers have been rigorously tested. ure 8). One of the candidates (Abell 2219 c2) is very
Nonetheless, one can ask whether statistically as a marginally detected in the z′ -band; if this flux is
F850LP
combinedset,thecombinedlineprofileofall6candidates associated with the J-band emission feature, then the
reveals an asymmetric profile suggestive of a Lyman-α high redshift interpretation would seem unlikely. Al-
origin. The practicality of such an exercise is limited though 3 of the candidates (Abell 2219 c1, Abell 1689
8 Stark et al.
c1, c2) are located within 2′.′˜of brighter objects, in all an approach. Predicting the flux of Lyman-α is always
cases, our registration sufficiently accurate to rule out difficult due to the resonant nature of the transition.
the possibility that the J-band emission emanates from While we allow for a factor of three suppression in the
the bright objects. Lyman-α flux, it is possible that a larger fraction of
A detection in deep near-infrared imaging would Lyman-α photons could be absorbed. Furthermore, the
be more interesting. Coupled with the optical non- flux ratios, especially those involving the forbidden oxy-
detections discussed above, this might hint at the pres- genlines,aredependantonthemetallicityandtheeffec-
enceofaLymanbreak,lendingcredencetoaz 10inter- tive temperature of the ionizing stars. However, as we
≃
pretation. In fact, none of the candidates is detected at will show below, our observational flux limits are often
or above the 5σ level ( 27 AB mag) in deep broadband sufficiently tight to rule out low-redshift hypotheses for
≃
HST J , H and Keck J-band imaging. Abell a wide variety of metal abundance and stellar effective
110W 160W
2219 c2 is marginally detected at 2σ in the H - temperature assumptions.
160W
∼
band. Followingtheabovelogic,aftercompletionoftheblind
In fact the lack of definitive near-infrared detections scanning survey, we set out to secure additional optical
for our Lyman- α candidates is not at all surprising and H-band spectroscopy for all 6 candidates to apply
if they are at high-z. If the equivalent width distri- this interloper-rejection method. Sadly, after two sea-
bution of Lyman-α emitters at z 10 is compara- sons of observing, weather and other observing vagaries
≃
ble to that at z 4-6, then we would need to reach meanthatexhaustiveconsiderationofallinterloperpos-
≃
H = 27.7 (1 magnitude below our sensitivity limit) sibilitiesisonlyavailablefor3ofourcandidates. Forthe
160W
to detect our candidates, if they are indeed Lyman- other 3 candidates, only partial coverageis in place.
α emitters (Malhotra & Rhoads 2004; Shimasaku et al. Table 4 summarizes the various hypotheses for the J–
2006). band emission and lists, for each case, which additional
Thus, we can conclude that our deep imaging adds a lines would be expected and at what wavelengths. Op-
furtherimportantcomponentinthe caseforthehypoth- tical spectroscopy is very efficient at constraining the
esis that bulk of our emission line sources lie at z 10. likelihood that our J-band emission arises from either
≃
Hα (z 0.9) or [O III] (z 1.5) since, in these cases,
5.4. Spectroscopically Testing Low Redshift Scenarios the LR≃IS data probes very d≃eep in the associated wave-
Aside from Lyman-α, the most likely alternative iden- length regions where [O II] would be seen (at 7100 ˚A
tifications for the J-band emission features include Hα, and 9300 ˚A respectively). In some cases, the survey J-
[OIII], Hβ, and [OII], in which case the features would band NIRSPEC data can also be used to constrain the
correspondtolensedsourcesatz =0.5 2.5. Sourceslo- hypothesis that the primary detection is Hβ or [O III]
−
catedattheseredshiftswouldalmostcertainlyhaveaddi- wherewewouldexpecttolocateoneorbothofthe[OIII]
tional,associatedemissionlinesbrightenoughtobeseen λ4959,5007pairandHβ. Inpractice,this iscomplicated
in the opticalandnear-infraredspectralregions(see Ta- by the fact that lines can be obscured by the OH night
ble 4). We have constrained each of these lower redshift sky or seen at very low significance (e.g. 2σ). We found
hypotheses by undertaking further spectroscopy,placing thatsuchcasescanbemoreeffectivelydispensedwithus-
stringent limits on the presence of lines. ing H-band spectroscopy to search for Hα which should
This technique has value so long as the expected line be easily observable if it lies in between the sky lines.
spectrumoftheforegroundinterlopersisknown. Inprac-
5.5. Best Candidates: Abell 68 c1 and Abell 2219 c1
tice selective extinction and excitation differences make
therelevantlineratiossomewhatuncertain. Thebestwe The most convincing elimination of foreground inter-
candoistoapproachtheproblemstatistically,assuming loper status has been achieved for the two candidates
a typical HII region spectrum (Figure 9) allowing some Abell 68 c1 and Abell 2219c1. In our subsequentanaly-
leeway in the line ratios. siswewillassumetheseemittersareLyman-αandsowe
We generate the HII region spectrum by computing describe these spectroscopic constraints in some detail.
flux ratios of bright recombination lines (Lyman-α, Hα, Inadditiontothenullbroad-banddetectionsdiscussed
andHβ)assumingCaseBrecombinationandaccounting earlier, deep optical LRIS and H-band NIRSPEC spec-
for the possibility that a signficant fraction of Lyman-α troscopy was secured in good conditions for both candi-
photons are absorbed by dust thereby reducing its flux dates and no definitive features are seen in either case.
by up to a factor of three. With these assumptions, we Wesummarizethepredictedfluxesandlimitingsensitiv-
adopt the following emission line ratios for hydrogen re- ities for each low-redshift interloper case in Table 5.
combination lines: Lyman-α : Hα : Hβ = 7.4-22 : 2.8 In the case of Abell 68, if the J-band line was Hα,
: 1.0. Flux ratios involving the forbidden oxygen lines for our adopted template H II region spectra, [OII] and
([OII]and[OIII])canbedeterminedfromobservationsof Hβ would have been recovered with > 45σ in the LRIS
low- and high-metallicity galaxies (Pilyugin et al. 2000, spectrum.
2001). Hereweadoptaratioof[OIII]λ5007: [OIII]λ4959 If the J-band feature was [OII], then Lyman-α would
: [OII]:Hβ =0.8-7: 0.3-2: 0.8-3: 1. Usingtheseratios have been seen with ease in the blue LRIS spectrum.
andtheconversionbetweenLyman-αluminosityandstar While the resonantnature ofthe transitionmakes it dif-
formation rate ( 5), we generate a template spectrum ficult to predict the Lyman-α flux , if even 0.1% of the
§
(Figure 9); the continuum flux spectrum is determined photons can escape, a 5σ detection would still have
≃
for a given star formation rate via a Bruzual & Charlot resulted. Since Lyman-α emission is often not seen in
(2003) model with identical stellar properties to those distant Lyman break galaxies (Shapley et al. 2003), we
assumed in 5. cansecureadditionalconstraintsfromthe non-detection
§
There are certainly a number of shortcomings in such of Hβ and the two [OIII] lines in the H-band spectrum.
A Keck Survey for Gravitationally-LensedLyman-α Emitters in at 8.5<z<10 9
∼ ∼
Although some of the relevant regions are affected by here making it a very promising source. However, we
OH emission, [OIII] 5007 ˚A would be strong enough to prefer not to elevate it into the status of the two most
be detected at >5σ under some cases. promising sources because of the potential broad-band
Ifthelinewere[OIII]λ5007˚A,thenHβ,[OIII]λ4959˚A, ACSdetection( 5.2). Inthissenseitrepresentsasource
§
and Hα would all fall in the spectral region covered by of status intermediate between our top two candidates
theH-bandspectroscopy. Hβ andHαwouldeachbeseen andthosediscussedbelowforwhich,due toweatherand
at >5 σ for most of the range of emission line ratios. otherproblems,ourspectroscopyiscurrentlyincomplete.
One seemingly unlikely hypothesis deserves attention. For A1689c1,c2 and c3, we have no optical or H-band
The candidate is quite close (1.8 arcsec) to a z=1.58 spectroscopyandsomajorinterloperhypothesessuchas
galaxywithHβ and[OIII]revealedintheJ-banddiscov- Hα and [O II] cannot yet be excluded.
eryspectrum(Richardetal. 2006,inpreparation). Ifthe In summary, we can conclude that at least two (and
candidate line were Hβ, the rest-frame velocity separa- quite possibly three) of our six candidates cannot be ac-
tion between the two sourceswould only be 400km s−1. counted for as lines from lower redshift emitters. More-
The likelihood of an association thus seems high. How- over, there is no reason to denigrate the status of the
ever, it turns out the combined J+H spectra of the two other 3 candidates either since, so far, every additional
sourcesareverydifferent. Hαisseenfromtheforeground spectrum we have taken has continued to support the
galaxyintheH-bandbutthesamelinecanberejectedfor Lyman-α hypothesis. Our low contamination ratio is
the candidate with 15σ confidence for a Case B Balmer not that surprising when one considers all of the con-
line ratio. Likewise the limiting sensitivity at the loca- sideredinterloperhypotheseswouldstillrepresentlensed
tionof[OIII]emissionline forthe foregroundhypothesis systemsandthus,if highly magnified,notbe seenatthe
is sufficiently deep such that we would have expected to location of the z 10 critical line.
≃
seealinefornearlyallreasonablelineratioassumptions. Onthe other hand, it is clearthat our tests make nec-
In combination, therefore, the non-detection of Hα and essary assumptions about the line ratios expected from
[OIII]λ 5007 ˚A strongly rule out any association of the 0.85< z <2.26 galaxies. It is conceivable that these as-
candidate with the z=1.58 galaxy. sumptions are invalid and that the sources have anoma-
Similarly, in the case of Abell 2219 c1 if the J-band lous line ratios (although it would seem surprising that
line were Hα, [OII], Hβ and [O III] 4959˚A would have thisisthecaseinseveralsystems). Giventhewavelength
been seen in the LRIS spectra at 23-85, 8.5, and 6.4-43 ranges accessible to ground-based spectrographs,we be-
σ respectively. lieve we have undertaken all possible tests.
If the line was Hβ, Hα would have been detected at
22σ in the H-band spectrum.and [OIII] λ4959˚A and 6. IMPLICATIONS
λ5007˚A at 3.5-24 and 21-190 σ respectively in the J- Noting that at least two of our six candidates may in-
band. deedlieatz 10,wenowaddressthe goalofthesurvey:
≃
If the line were [OII], then, as before strong Lyman- what is the volume density of high z emitters? How-
α would be expected in the blue LRIS spectrum, even ever, noting their identification could still be considered
if only a small fraction of photons escaped. And tentative, we will also explore the implications if none
[OIII]λ4959˚A and λ5007˚A would have been seen at up of our candidates is real. After making simple assump-
to 4.5-8.0 and 4.8-11σ respectively in the H-band spec- tions about the effect of dust and resonantscattering on
trum. escapingLyman-αphotons,wewillusethederivedabun-
Finally,ifthelinewere[OIII]λ5007˚A,thenHβ would dance to constrain the star formation rate density. We
be detected at 1.7 15σ,depending on the oxygenabun- then attempt to understand whether the derived abun-
dance. and Hα w−ould been seen in a clean area of the dancerepresentsasignificantcontributiontotheionizing
H-band with 4.8 43σ. photon budget at z 10.
− ≃
All cases described above assume an HII region tem-
6.1. Volume Density of z>8 Lyman-α Emitters
plate spectrum. Assuming the emission lines are in-
steadgeneratedby a harderionizationsourcesuchasan In a blank-field survey, the surface density of sources
AGN, the CIV λλ 1548, 1550 line may be prominent in brighter than a givenlimiting line flux can be computed
the rest-frame UV spectrum. If Lyman-α is suppressed by dividing the number of detected sources by the area
via resonant scattering and absorption by dust, narrow- observed. The calculation is more complex in this case
line AGN can exhibit line ratios of 1< CIV/Lyα < 1.4 because of the varying amplification of the source flux
(Dey et al. 1995; De Breuck et al. 20∼01; Dawson∼et al. overthe surveyvolume andthe distortion,by lensing,of
2003). If these sources are indeed Lyman-α emitters at the survey area.
z 10, then the CIV line may also be observable in The differential comoving volume element at (Ω,z)
the≃H-band. For both Abell 68 c1 and Abell 2219 c1, along one of the NIRSPEC slit positions is
CIV would lie in clean regions (1.60 µm and 1.55 µm,
respectively);however,neitheremissionline isseen,sug- 1 dl (z)
dV (Ω,z)= c dz D2(z)dΩ . (3)
gesting that if the candidates are at z 10, they have c (Ω,z)(cid:20) dz (cid:21)× c
CIV/Lyα< 1. ≃ M (cid:2) (cid:3)
where dl is the comoving length of the volume element
c
alongthe line of sight. We also compute the uncertainty
5.6. Other Candidates
associated with each volume element due to the error in
Comprehensive optical and H-band spectroscopy is the magnification ( 3.4).
§
alsoavailableforAbell2219c2andinfacttheconstraints The total survey volume over which sources with
wediscussaboveforAbell2219c1arejustasconvincing Lyman-α line luminosity greater than L is the integral
10 Stark et al.
over all volume elements with limiting Lyman-α lumi- A second question is whether our density is higher
nosity below L, or lower than the Lyman-α luminosity function which
is now fairly well-established at z = 5.7, at least at
the bright end (Santos et al. 2004; Malhotra & Rhoads
Vc(L)= dVc(Ω,z) H[L Llim(Ω,z)], (4) 2004; Shimasaku et al. 2006). It is worth remembering
ZΩZz − thatdespitethefactthatover100Lyman-αemittersare
whereH[L-L (Ω,z)]isastepfunctionequalto1when now catalogued at this redshift, the degree of spectro-
lim
L L (Ω,z) and equal to 0 when L<L (Ω,z). scopicconfirmationisstillquitelimited. Theextantdata
lim lim
≥The uncertainty in the magnification introduces error are well-fit by a Schechter function with φ⋆=1.6 10−4
intothederivedcomovingvolume. Wequantifythiserror Mpc−3 andL⋆=1.6 1043ergs−1 where,forthepu×rposes
×
bycomputingthedistributionofcomovingvolumeforall of discussion, we have adopted the steepest likely faint-
acceptable cluster mass models (see 3.4). The rangeof endslope ofα= 2soastoprovidethe mostoptimisitc
comoving volume that spans 34% o§f the total samples no-evolution scen−ario. Assuming, naively, no evolution
is taken to be the uncertainty±in the volume. between z=5.7 and z=10, it is unlikely that we would
The survey volume and associated errors are plotted find a source brighter than 1040 erg s−1. At face value,
as a function of Lyman-α luminosity in Figure 10. The therefore, our detection of at least two promising cases
comovingvolumedecreasesfrom 30Mpc3atL=3 1042 wouldsuggestanupwardsevolutioninthe abundanceof
erg s−1 to 3 Mpc3 at L=3 10≃41 erg s−1. The×’one- faint emitters with redshift.
sigma’ error≃bars in the volu×me are small enough that The likely evolution of the Lyman-α luminosity func-
theydonotextendoutsidethe datapoints. InFigure11, tion between z=5.7 and z 10 is a complicated func-
≃
we plot the number density of z 9 Lyman-α emitters tion of the star formation efficiency, stellar initial
≃
as a function of Lyman-α luminosity assuming that (i) mass function, relative geometry and kinematics of HII
noneofourcandidatesisatz 8-10(left)and(ii)twoof and HI gas, and dust content of the host galaxy, as
our candidates are Lyman-α e≃mitters (middle) and (iii) well as the ionization state of the IGM. If, for ex-
all six of our candidates are real (right). ample, the dust content of galaxies decreases toward
In the case where two of the candidates are real, the higher redshifts, the percentage of Lyman-α photons
Poisson errors are 100%. This arises because galaxies that escape the host galaxy may increase. On the
need not only be brighter than the surveylimiting lumi- other hand, as the IGM becomes increasingly neu-
nosity but must also lie within the area covered by the tral, observed Lyman-α counts are expected to de-
survey. In lensing experiments, in order for inclusion as- cline (Malhotra & Rhoads 2004; Haiman & Cen 2005),
sociated with some limiting luminosity, the 5σ limiting although the presence of large HII regions around clus-
luminosity at the location of the object must be lower tered star-forming sources may slightly offset the mag-
than the limiting luminosity of the survey. The net re- nitude of this decline (Furlanetto et al. 2004). Clearly,
sult is that objects with fluxes close to the sensitivity it is very difficult to predict exactly how the Lyman-α
limits only are included in “surveys” with a small range luminosity function will evolve over this 500 Myr time
≃
of limiting luminosities. If two candidates are real, as interval.
considered here, the luminosity ranges over which the Bearing these uncertainties in mind, our detection of
objects would be detected do not overlap; hence no bin two promising candidates clearly implies a steep faint-
contains more than one object. end slope and hence a large abundance of intrinisically
To place the inferred abundances in context, we com- faint sources. If more of our candidates are at z 10, it
≃
pare our results with other surveys that have been con- would almost certainly imply that low-luminosity star-
ducted, particularly at z 6 where the luminosity func- forming systems are more abundant at z = 9 than at
tion of Lyman-α emitters≃is more firmly established. z = 5.7. This may be a result of evolution in galaxy
The first observational constraint on the abun- properties(asdiscussed),ortheeffectofapredominantly
dance of Lyman-α emitters at z>8 was presented by neutral IGM which increases star formationefficiency in
(Willis & Courbin 2005). These authors conducted a low-mass halos due to the decrease in the Jeans mass
narrow-band imaging survey in the J-band toward the (Barkana & Loeb2001). Determiningwhichofthesefac-
Hubble Deep Field South and found no z = 8.8 sources tors is primarily responsible for the possible upturn in
withLyman-αluminositygreaterthan1042.5ergs−1over counts is difficult with the available data.
a volume of approximately 990 Mpc−3. More recently, If none of our candidates is real,a number of explana-
Cuby et al. (2006) reported a null detection of z = 8.8 tions are possible. First the faint-end slope may not be
Lyman-α emitters brighter than 1043 erg s−1 over a co- as steep as required for us to detect sources in our ob-
moving volume of 4200Mpc3. Given the large error servedvolume. Thereareanumberoffeedbackprocesses
bars,the upper limi≃ts fromWillis & Courbin(2005)and (galactic winds, supernova explosiona, etc.) that could
Cuby et al. (2006) are consistent with the case in which significantlyreducestarformationefficiency inlow-mass
only two of the candidates are real. If all six of the can- halos, thereby resulting in a relatively shallow faint-end
didates are Lyman-α emitters, it becomes slightly more slope. Alternatively, if the IGM is significantly neutral,
difficult to explain the lack of Lyman-α emitters in the it is possible that faint-end slope is very steep, but the
two narrowbandsurveyswithout resorting to luminosity neutralIGMis blocksour viewofthese systems. We de-
dependent evolution. Hence, in this case, the observa- feradetaileddiscussionoftheimplicationsofoursurvey
tionscouldsuggestthatthedeclineinluminousLyman-α intermsofwhatisnowknownatz 5-6toalaterpaper
≃
emitters relative to fainter sources observedat z 6 7 (Stark, Loeb & Ellis, 2007).
≃ −
(Shimasaku et al. 2006; Kashikawa et al. 2006; Iye et al.
2006) continues to z 10.
≃