Trident: A three-pronged galaxy survey. I. Lyman alpha emitting galaxies at z~2 in GOODS North

2.8 MB·English

by  A. Sandberg

Astronomy&Astrophysicsmanuscriptno.aadem (cid:13)cESO2015
May21,2015
Trident: A three-pronged galaxy survey
∼ ⋆
I. Lyman alpha emitting galaxies at z 2 in GOODS North
A.Sandberg1,L.Guaita1,G.Östlin1,M.Hayes1,andF.Kiaeerad2,3
1 TheOskarKleinCentre,DepartmentofAstronomy,StockholmUniversity,AlbaNova,10691Stockholm,Sweden.
e-mail:[email protected]
5 2 AarhusUniversitySchoolofEngineering,Finlandsgade22,8200Aarhus
1 3 NordicOpticalTelescope,RamblaJoséAnaFernándezPérez7,38711,BreñaBaja,Spain
0
2

y ABSTRACT
a
M Context.Lymanalpha(Lyα)emittinggalaxies(LAEs)areusedtoprobethedistantuniverseandarethereforeimportantforgalaxy
evolutionstudiesandforprovidingcluestothenatureoftheepochofreionization.However,theexactcircumstancesunderwhich
0 Lyαescapesagalaxyarestillnotfullyunderstood.
2 Aims.TheTridentprojectisdesignedtosimultaneouslyexamineLyα,Hα,andLymanContinuumemissionfromgalaxiesatredshift
z∼2,thuslinkingtogetherthesethreeaspectsofionisingradiationingalaxies.Inthispaper,weoutlinethestrategyofthisprojectand
] examinethepropertiesofLAEsintheGOODSNorthfield.
A Methods.We performed a narrowband LAE survey in GOODS North using existing and two custom made filters at the Nordic
OpticalTelescopewithMOSCA.Weusecomplementarybroadbandarchivaldatainthefieldtomakeacarefulcandidateselection
G
andperformopticaltonear-IRSEDfitting.Wealsoestimatefar-infraredluminositiesbymatchingour candidatestodetectionsin
. Spitzer/MIPS24µmandHerschel/PACScatalogs.
h
p Results.Wefindatotalof25LAEcandidates,probingmainlythebrightendoftheLAEluminosityfunctionwithLLyα∼1−15×1042
ergs−1.Theydisplayarangeofmassesof∼0.5−50×109M ,andaverageagesfromafewtensofMyrto1Gyrwhenassuminga
- ⊙
o constantstarformationhistory.Themajorityofourcandidatesalsoshowsignsofrecentelevatedstarformation.Threecandidateshave
r counterpartsintheGOODS-Herschelfar-IRcatalogue,withluminositiesconsistentwithultra-luminousinfraredgalaxies(ULIRGs).
t
s Conclusions.ThewiderangeofparametersderivedfromourSEDfitting,aswellaspartofoursamplebeingdetectedasULIRGs,
a seemstoindicatethatattheseLyαluminosities,LAEsdonotnecessarilyhavetobeyoung dwarfs,andthatalackofdust isnot
[ requiredforLyαtoescape.

2 Keywords. Galaxies:photometry,Galaxies:structure
v
7
1
1. Introduction It quickly became apparentin the earliest studies that Ly α
0
isnowherenearasbrightandubiquitousinhighredshiftgalax-
6
0Few emission lines have been studied as extensively in iesaswasfirstimagined(see forexamplePritchet(1994) fora
.the distant universe as 1216 Å (Ly α). First suggested by review of non-successfulLy α surveys). Recombinationtheory
1
Partridge&Peebles (1967) as a means of finding high red- tellsusthatroughlyonethirdofalloftheionizingfluxfromhot
0
shift galaxies, it became a signature to look for in galaxy stars should be reprocessed into the Ly α emission line as the
5
surveys for decades. It was however not until deep ded- atomichydrogengasrecombines(Osterbrock1962),butsome-
1
:icated surveys were performed that any significant quan- thingiskeepingLyαfromescapingmostgalaxies.
vtities of these Ly α emitting (LAE) galaxies were found It was natural to assume that interstellar dust would play
Xi(Cowie&Hu 1998). Ever since, Ly α has been success- an important role in absorbing Ly α photons, as dust absorbs
fully used to find galaxies across redshifts z ∼ 2 − 7 (e.g. rest-frame ultraviolet light very effectively. Indeed, early stud-
r
aRhoadsetal. 2000; Kudritzkietal. 2000; Malhotra&Rhoads iesshowedatentativeanti-correlationbetweendustcontentand
2002; Ouchietal. 2003, 2005; Gawiseretal. 2006; Ajikietal. Lyαemission(Meier&Terlevich1981).However,itwassoon
2006;Gronwalletal.2007;Pirzkaletal.2007;Finkelsteinetal. shown that this could not be the only factor governing Ly α
2008; Nilssonetal. 2009; Hayesetal. 2010a; Yumaetal. escape, as there are examples of strongly emitting Ly α galax-
2010; Ouchietal. 2010; Guaitaetal. 2011; Adamsetal. 2011; ies rich in dust, and nearly dust-free galaxies where Ly α is
Blancetal. 2011; Shibuyaetal. 2012; Vargasetal. 2014), completelyabsorbed(Kunthetal.1994;Giavaliscoetal.1996).
even probing the end of the epoch of reionization (e.g. Kunthetal. (1998) showed the importance of the interstellar
Malhotra&Rhoads2004;Dijkstraetal. 2007;Onoetal. 2010; medium(ISM)kinematics,astheLyαphotonsresonantlyscat-
Jensenetal.2013;Mattheeetal.2014). ter on neutral Hi gas. Ly α radiation at the rest wavelength is
opticallythickalreadyatcolumndensitiesNHi ∼ 1014 cm−2,so
⋆ Based on observations made with the Nordic Optical Telescope, evenatinyamountofneutralhydrogenwillaffecttheemergent
operatedbytheNordicOpticalTelescopeScientificAssociationatthe line significantly. With multiple scatterings, the path length is
ObservatoriodelRoquedelosMuchachos,LaPalma,Spain,oftheIn- greatlyincreased,andevena smallamountofdustcaninprin-
stitutodeAstrofisicadeCanarias. cipleabsorbalargeamountofphotons.However,iftheneutral
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HigasisinrapidmotioncomparedtotheLyαsources,forex- (Hayesetal. 2010a,b), in which these emission lineswere suc-
ample due to supernovafeedbackor strong stellar winds, Ly α cessfully studied in galaxies in the GOODS-South field, us-
can beshifted outof resonanceandescape moreeasily.With a ing the FORS1 and HAWK-I instruments at ESO VLT. Being
large range of observational studies, combined with advanced a relatively deep study (reaching line fluxes at 5σ of 6.8 and
simulations of Ly α radiative transfer through galaxies (e.g. 7.8×10−18ergs−1cm−2 inHαandLyα,respectively),itprobed
Verhammeetal. 2006, 2012; Laursenetal. 2013; Duvaletal. the faint end of the Ly α luminosity function (LF) at z ∼ 2
2014), we are only now starting to build a coherent picture of to an unprecedentedaccuracy.A similar approachwas used by
howLyαisproducedandescapesfromgalaxies. Nakajimaetal. (2012) who studied Ly α, [Oii] λ3727and Hα
There are still, however, many questions that remain unan- galaxiesatz∼2.2intheSXDSfield.
swered. Two galaxies with essentially the same properties in DrawingonourexperiencegainedfromDouble-BlindDeep,
termsofmass,starformationrate,age,gas-massratio,dustcon- westartedourcurrentproject.OneofthedrawbacksofDouble-
tentetc.,canshowcompletelydifferentbehaviorwhenitcomes BlindDeepwasthatitwasdesignedtofindpredominantlyfaint
toLyα(e.g.Hayesetal.2014).Wearestillnotcertainofwhich emitters. However, constraining the bright end of the Ly α LF
parametersaredominantforgoverningLyαescape.Asthenum- requiresalargersurveyvolume,asbrightemittersarerare,and
ber of LAE surveysis rapidlyincreasing, and the volumesand this is one of the goals of the currentproject.In this paper,we
numbersprobedcan nowbemeasuredin wholedegreeson the presentourfirstresultsfroma shallowsurveyovera largevol-
sky and amount to thousands of galaxies, it is more important ume, specifically targeting the brightest Ly α emitters. We are
than ever that we truly understand the mechanisms governing thusnotexpectingtofindlargenumbersofcandidates,butrather
thesegalaxies. weareinterestedintheirHαtoLyαpropertiesanduseancillary
Another outstanding question in the study of high redshift informationin well studied fields to connect these LAE candi-
galaxies is the manner of escape of photons with an energy datestoothergalaxypopulations.
above13.6eV(<912Å),capableofionizingneutralhydrogen. We target well studied extragalactic fields to maximize the
We refer to this as Lyman continuum (Ly C) radiation in this availablemulti-wavelengthdata,includingobservationswiththe
paper. As the universe was gradually reionized somewhere at Hubble Space Telescope (HST) in the ultraviolet, in order to
z & 6 (Fanetal. 2006), the natural ingredient that is assumed add the third prong to our trident. In addition to the GOODS-
responsible are the first stars and galaxies. However, in order North observations presented here, we are currently observing
toreionizeallofspace,theamountofionizingradiationescap- with NOT/MOSCA two Frontier Fields and Abell1689– lens-
ing outof these galaxiesmust havebeen very large.Yet, when ing clusters with existing deep HST UV observations – using
we try to measure this quantity in strongly star-forming galax- severalnewopticalfilterswithdeeperexposuresthanthosepre-
ies("starbursts"),weseerelativelymodestescapefractions(see sentedhere(∼10hourspernarrowbandpointing).Thetotalvol-
e.g. Vanzellaetal. 2012, and referencestherein). The Ly C es- umes probed per cluster will be similar to the central pointing
capefractionremainsdifficulttomeasuredirectly,asthesignal ofGOODS-Northpresentedhere,butreachingfainteremitters,
fromnearbysourcesisabsorbedbyGalacticHigasandrequires also possibly magnified by the foreground clusters. We have
space-basedultravioletobservatorieswith high sensitivity even started gathering Hα observations of Abell1689 in the near-
atredshiftsuptoz.3. infrared, and we plan to observe our two Frontier Fields with
With these unresolved questions in mind, we started the CFHT/WIRCaminthenearfuture.Oncetheseobservationsare
project that we call Trident. In this first paper, we present the completed,theywillbethesubjectoffutureTridentprojectpa-
general outline of the project and our first results, focusing on pers.
Ly α emitting galaxiesin GOODS-N. The Hα to Ly α proper- ObservingfieldswithHSTUVcoverageallowsustoinves-
ties and the Ly C escape will be the subject of future papers. tigate the leakage of Ly C photons from the galaxies we de-
In this paper, we focuson the propertiesof the Ly α emission, tectin thesurvey.Theseionizingphotonsare intimatelylinked
estimating stellar properties from spectral energy districution to the Ly α emission, as they are required for Ly α to form.
(SED) analysis. We also identify three individualcandidatesin Thehighabsorptioncross-sectioninHiforbothtypesofemis-
far-infraredsurveydataanddiscusstheimplicationsontheirdust sion maycause themto escape together,probablythroughion-
content. ized cones or channels with low column densities and/or cov-
Insection2weoutlinethegeneralideaofTridentandhowit ering fractions, as models of porous multi-phase media sug-
isstructured.Insection3wedescribethedatawehavecollected gest (e.g. Clarke&Oey 2002; Neufeld 1991; Laursenetal.
andhowitwasreduced.Section4describeshowweselectedour 2013; Verhammeetal. 2014). Observations and models of in-
candidates from the data. In section 5 we measure far infrared termediate redshift galaxies both support this picture, as LAE
propertiesofourcandidatesfoundintheGOODS-Herschelcat- galaxies appear to emit more Ly C photons than what Lyman
alog. In section 6 we explain how we combined our data with breakgalaxies(LBGs)do(Iwataetal.2009;Nestoretal.2013;
archivalsurveydataofGOODS-N,andhowthiswasthenused Mostardietal.2013).Modelsalsosuggestthatescapefractions
fortheSEDfittingroutine.Insection7wesummarizeanddis- of Ly C are higher in lower-mass haloes (Yajimaetal. 2011;
cusstheimplicationsoftheseresults. Mitraetal.2013).NotethatallLAEsmustshowaLymanbreak,
Throughout the paper, we assume H = 70 kms−1Mpc−1, andthatselectioneffectswillmakeanLAEsurveyfindrelatively
0
Ω =0.3,Ω =0.7 morelow-massgalaxiesonaverageforagivenexposuretime.
M Λ
Adding to this the observed steep UV LFs of faint galax-
iesat z & 2 (Alavietal. 2014), itthereforeseemspossible that
low-mass Ly α emitting galaxies did have large enough Ly C
2. TheTridentsurvey
escape fractions to ionize the universe (see e.g. Bouwensetal.
We chose the name Tridentbecause we are tryingto find three 2009, 2012; Robertsonetal. 2013), and recent observations of
types of emission related to atomic hydrogen from the same thefaintendoftheLyαandUVLFsatz∼6−8arewellinline
galaxiesinonecombinedsurvey.TheideaforacombinedLyα with this picture (Dressleretal. 2014; Ateketal. 2015). How-
andHαstudybuildslargelyonthe Double-BlindDeep project ever, due to the rapidly increasing opacity of the IGM beyond
Articlenumber,page2of24
A.Sandberg etal.:TridentI:LAEsinGOODSNorth
Table1.Narrowbandfilterproperties
62◦25′
Filter ID λ (Å) λ (Å) zrange ZP
c w
6 4 NB392_6 #134 3917 63 2.20−2.25 22.70
NB401_6 #133 4013 64 2.27−2.33 22.72
62◦20′
NB406_6 #121 4058 58 2.31−2.36 22.52
NB410_6 #131 4102 60 2.35−2.40 22.37
NB412_18 #115 4120 180 2.32−2.46 23.66
δ200062◦15′ 3 1 2
Notes.NordicOpticalTelescopeIDnumber,centralwavelength,width
andcorrespondingapproximateredshiftrangeofLyαforthenarrow-
band filters used in this study. The final column lists the average ze-
62◦10′ ropoint for each filter,in AB magnitudes, acquired with a∼ 6 arcsec
5 7
aperture.Theuncertaintyinallzeropointsisroughly0.05mag.
62◦05′ 1.0
Subaru B
12h38m 12h37m 12h36m KPNO
α2000 ction0.8 392_6 401_6 Mayall U NSDOSTS g'
a 406_6
FNiugm. b1e.rFsicdourcrieaslpomnadptoofpopionitnitnignsgsasfodresMcrOibSedCAinTwaibthlefi2l.teDraNshBe4d0l1in_e6s. n fr0.6 412_18
outline the GOODS ACS (red) and the CANDELS-Deep(blue) foot- sio 410_6 HST B
prints. mis0.4 (F435W)
s
n
ra0.2
T
z & 4(Inoue&Iwata2008;Inoueetal.2014),thisLyCescape
canneverbedirectlyobserved.LinkingtheHα,LyαandLyC 0.0
3900 4000 4100 4200 35004000 4500 5000 5500
emissiontogetheratz ∼ 2isthereforeoneofveryfewwaysof λ(A◦)
quantitativelystudyingLyCescapeingalaxies.
Fig.2.TransmissioncurvesforthenarrowbandNOTfiltersusedinthis
work(leftpanel)andhowtheyrelatetobroadU,Bandg’filters(right
3. Observationsanddatareduction panel).NotethattheHSTF435Wfilterisscaledtotheentireinstrument
throughput.
3.1.Customfilters
The generalidea of finding a galaxyin a narrowbandemission MOSCA’s CCDs are especially sensitive in the blue (with a
line surveyis to do photometryof a large piece of the sky, us- quantumefficiencynear97%at430nm),andwithfewinternal
ing a narrowband filter where the wavelength is centered on a surfacesitisanidealinstrumentforourpurposes.
redshiftedemission line. When comparingthe brightnessof an We observed GOODS-North, using a central pointing of
objectinthisnarrowbandfiltertoanearbybroadbandfilter,any- 12h36m54s,+62◦14′50′′ (J2000). For NB401_6, we observed a
thing with a strong emission feature in the narrowband filter grid of positions shown in Fig. 1, coveringa surface area of ∼
willshowanarrowbandcolorexcess(seee.g.Guaitaetal.2010; 400squarearcminutes.Table2liststheexposuretimesandsee-
Nakajimaetal.2012;Sobraletal.2013,forrecentdescriptions ing conditions for each pointing. The other narrowband filters
ofthenarrowbandtechniqueandhowitcanbeused).Thisthere- wherecenteredonthesamecoordinatesbutrotated45degreesto
fore gives a signature that can be used to find candidates for bettermatchwiththeHST/ACSdatafootprint.Thecorrespond-
follow-upstudies. ing volume that we cover in the NB401_6 filter is roughly 83
One can of course combine narrowband filters at different 000comovingMpc3,with anadditional∼ 60000Mpc3 in sup-
wavelengthsinordertoseedifferentemissionlines.Inourcase, plementing filters. This is comparable to e.g. the Guaitaetal.
we want to look for galaxies not only in Ly α but also in Hα, (2010) MUSYCstudy,whichcovers∼135000Mpc3 ofLAEs
inawaythattheredshiftrangeisthesameforbothsurveys.We atz∼2.07,butgoesconsiderablydeeper.
thereforeattemptedtomatchnear-IRnarrowbandfilterswithop- The Hα data for GOODS-North cover circa 22 arcmin2
tical filters in terms of the redshiftsof these emission lines. To (roughly the size of the view in Figure 1) in a narrowband fil-
this end, we had two custom filters made to exactly match ex- termatchingtheNB401_6filterinredshiftspace.Thedetailsof
istingnear-IRfilters.Wehavealsousedthreeother,pre-existing theseobservationsaredescribedinaforthcomingpaper.
opticalfilters,whichextendsourLyαredshiftrangeandthusin-
creasesthevolumeprobed.Werefertothenarrowbandfiltersas
e.g.NB392_6,where392and6arethecentralwavelengthand 3.2.Datareduction
width,innanometers.NB392_6andNB401_6weredesignedto
The NOT/MOSCA data discussed in this paper were taken in
match H and Brackett γ near-IR filters. The properties of all
2 MarchandMay2011andFebruaryandMay2012.Table2sum-
the narrowband filters, and how they give an almost complete
marizestheseobservations.
redshiftcoveragebetweenz = 2.2and2.46,aresummarizedin
Many of the basic reduction steps were developed from
Table1. Tapio Pursimo’s notes1 on reducing MOSCA data using the
Our Ly α observations were performed at the 2.56 me- IRAF2MSCREDpackage.
ter Nordic Optical Telescope (NOT) on La Palma, Spain, us-
ing the MOSCA instrument. MOSCA is a mosaic camera that 1 http://www.not.iac.es/instruments/mosaic/reductions/reduction.
is mounted at the Cassegrain focus, and it consists of four 2 IRAFisdistributedbytheNationalOptical AstronomyObservato-
CCDslaidouttoformaroughly7.5arcminwidefieldofview. ries,whichareoperatedbytheAssociationofUniversitiesforResearch
Articlenumber,page3of24
A&Aproofs:manuscriptno.aadem
Table2.Observinglog backgroundis considereda detection.Thiscanbe comparedto
theseeingdiskwhichforafullwidthathalfmaximumof1arc-
Date Filter Pointing Exp.time Seeing secwouldcorrespondtoanareaofabout17pixels.Definingthe
2011-03-01 NB406_6 1* 2.5h 0′.′9−1′.′1 thresholdsthiswayallowsforlowsurfacebrightnessdetections
2011-03-01 NB410_6 1* 1.5h 1′.′1−1′.′2 withunknownmorphologies.
2011-03-02 NB412_18 1* 1.5h 0′.′8−1′.′0 WeestimatethedepthofourNB401_6imagingtoreach24.3
2011-03-02 NB406_6 1* 1.5h 0′.′8−0′.′9 (24.6) AB mag at the 90% (50%) completion level, for the ar-
2011-03-02 NB410_6 1* 1.5h 0′.′7−0′.′8 easwithatotalexposuretimeof∼4.5hours.Thiscorresponds
2011-05-02 NB401_6 1 2.5h 0′.′8−0′.′9 to a limiting Ly α line flux (assuming a 20 Å EW cut in the
2011-05-03 NB392_6 1* 3.5h 0′.′7−0′.′8 rest-frame)of 9.8(7.3)×10−17ergs−1cm−2. In theNE andSW
2011-05-04 NB401_6 1,2,3 2h,2h,0.5h 0′.′8−1′.′0 outskirtsoftheNB401_6image,andin ourotherfilterstheef-
2011-05-05 NB401_6 2,3 0.5h,2h 0′.′8−1′.′1 fectiveexposuretimeis∼2-3hours,andthedepthreachedcon-
2012-01-27 NB401_6 5 3h 0′.′9−1′.′1 sequently0.2-0.4magnitudesshallower.
2012-02-23 NB401_6 4 4h 0′.′9−1′.′0
2012-05-20 NB401_6 2 2h 1′.′3−1′.′5
2012-05-21 NB401_6 3,5 1.5h,1h 0′.′9−1′.′5 3.3.Standardstars
2012-05-22 NB401_6 6,7 2h,2h 0′.′7−0′.′8
Our custom made filters require the use of spectrophotometric
standard stars for the photometric calibration. We used white
Notes. Log of the NOT/MOSCA Observations. The filters listed are
dwarfstarsfromOke(1990),asthesestarsdonotshowastrong
onlythenarrowbandfiltersusedinobservations;complementingbroad
band observations (usually in the SDSS g’ band) where always per- Balmerbreakaround4000Å.WecorrectedtheOke(1990)ab-
formed. The"pointing" columncorresponds tothenumbersinFig.1. solutefluxesaccordingtoColina&Bohlin(1994,usuallyashift
Anasteriskisusedtomarkifthepointingwasobservedwitha45de- on the orderof +0.05AB magnitudes).We thenconvolvedthe
greefieldrotation. standard spectra with our measured narrowband transmission
curves and calculated the AB magnitude of each standard star
ineachofourfilters.
After the basic bias, flat and dark reduction steps in MS- The CFHT/WIRCam/WC8305 narrowband image was re-
CRED, the IRAF FIXPIX task was used to apply a bad pixel ducedusingtheSimple-WircampipelinedevelopedbyWei-Hao
masktothedata.Next,theMSCSETWCStaskfromMSCRED Wang 4. NB401_6 is designed to match this filter, which was
wasusedtocorrectforindividualrotationanddistorsionsinthe usedforthematchingHαsurvey,inredshiftspace.TheHαre-
four CCDs of MOSCA by changingthe WCS projection.This sults will be discussed in a forthcomingpaper, but the postage
stepusesaplatesolutionwhichweconstructedwiththeMSCT- stampsinFig3showacutoutforreferencepurposeshere.
PEAKtask.
The sky level was then subtracted using Source Extractor
(Bertin&Arnouts 1996), and the images were combined to a 3.4.Archivalandcatalogdata
single fits extension using the MSCIMAGE task. Cosmic ray
A large set of multi-wavelengthdata already exists for various
cleaning was then applied using v0.4 of cosmics.py, a python
regionsaroundGOODS-North.Wehavechosentomakeuseof
interface to L.A.Cosmic3. The MSCIMATCH task was used to
ground-basedUBVRIzbroadbandimagingusingtheKPNO4.0
matchtheintensitybetweentheframes,takingintoaccountdif-
metertelescopeandSubaru8.3meter,describedinCapaketal.
ferencesinairmassandseeingconditions,etc.TheWCSofeach
(2004).WeusetheR-banddetectioncatalog,usingautophotom-
framewasmatchedagainsttheUSNO-A2.0starcataloguewith
etry.WechosethisphotometriccatalogovertheHST/ACScata-
theMSCCMATCHtask,whichwassettoshifttheWCStothe
logforseveralreasons.Thecoveredareaontheskyislarger,and
correct position. The science data were then stacked using the
includesthe entireregionthat we examinedwith the NB401_6
MSCSTACK task, which uses the WCS of the frames to align
filter. The resolution is very similar to our data, as they are all
thembeforecombining.
ground-based,whichmakesthephotometryeasiertohandleina
The final astrometric solution was obtained by run- consistentmanner.TheKPNOU-banddataisalsoimportantfor
ning MSCCMATCH with a set of relatively bright HST b- ournarrowbandselectioncriteria.
band sources, in order to match our astrometry to that of For our near-infrared data, we use the deep K and
s
Giavaliscoetal.(2004)whichiscommonlyusedforsourcesin SpitzercatalogfromWangetal.(2010),whocarefullymatched
GOODS-North.WethenusedGREGISTERtoregisterallofour Spitzer/IRAC data to each K source. To fill in the J and H
s
ground-baseddatainallfilterstoacommonsetofphysicalim- bandswhereavailable,wemakeuseoftheMOIRCSDeepSur-
agecoordinates.Thisisacrucialstepfordoingthephotometry veynear-IRcatalog(Kajisawaetal.2010),andcalculateaJ−K
s
with SExtractorindualmode,whereoneimageisusedforde- andH−K color.TheMOIRCSSurveycoversahorizontalstrip
s
tectionandtheotherforphotometry. throughalargeportionofthefield.
For our ground-baseddata, we use aperture corrected pho- We do see a shift between the two K catalogs of about
s
tometry where the correction is calculated from isolated bright 0.2±0.1magnitudes.Weinvestigatedwhetheroneofthetwosur-
sources in the field. We use aperture diameter sizes of 22 and veyswouldgiveabetteroptical-to-infraredcolorbymeasuring
60 pixels, corresponding to radii of 2.3 and 6.3 arcsec. De- theaverageandmediancolorsofallsources,buttheendresultis
tections were made using DETECT_THRESH = 1.2 and DE- somewhereinbetweenthetwowithquitealargeuncertainty.We
TECT_MINAREA = 32 in SExtractor, meaning that anything have therefore chosen to use the average value of the catalogs,
with32contiguouspixelsthatareall1.2timesbrighterthanthe andhaveaddedinquadratureanuncertaintyof0.2magnitudesto
allofournear-infraredandSpitzerdatapoints.Thisreflectsour
inAstronomy,Inc.,undercooperativeagreementwiththeNationalSci- uncertaintyintherelativeopticaltonear-infraredphotometry.
enceFoundation.
3 http://obswww.unige.ch/~tewes/cosmics_dot_py/ 4 http://www.asiaa.sinica.edu.tw/~whwang/idl/SIMPLE/
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A.Sandberg etal.:TridentI:LAEsinGOODSNorth
Filter 392_6, candidate 2
12:36:50.14 +62:14:01.8
U-NB: 2.00 0.60
±
g'-NB: 2.02 0.66
±
B-Z: 0.30 0.21
±
Z-Ks: 1.94 0.27
±
Fig.3. Postage stampsfor anexample candidate galaxy. Thefieldof view is10 ×10 arcsec. ThelargeRGBpicture showsan HSTbvi color
compositemade withthescaleineachcolor channel relatedtothetruetotalintensity, withanarcsinhintensityscalingrelation.Thiscoloring
schemeissimilartothatdescribedinLuptonetal.(2004).Thewhitecirclehasaonearcsecradius-typicallytwicethesizeoftheground-based
seeingdisk.PostagestampsforallcandidatesarecompiledintheAppendix.
3 4
2 3
AB) 1 2
01_6 ( s (AB)
NB4 0 Z-K 1
g'-
0
−1
−1
−2
−1.5 −1.0 −0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4
U-NB401_6 (AB) B-Z (AB)
Fig.4.NarrowbandselectionforourNB401_6filter.Dottedlinesshow Fig.5.BzKselectionforcandidatesintheNB401_6filter.Thedotted
the cut-off. Photometry for all data is indicated by small gray points. lineshowsthecut-off,correspondingtotheDaddietal.(2004)BzKcri-
Largebluepointsindicatecandidatesthatmeetboththeexcessandthe terion.Photometryforalldataisindicatedbysmallgraypoints.Large
BzKcriteria.Thebluedatapointonthetopleftshowsthetypicalsize bluepointsindicatecandidatesthatmeetboththenarrowbandcolorse-
oftheuncertainty. lectionandBzKcriteria.Thebluedatapointonthebottomrightshows
thetypicalsizeoftheuncertainty.
4. Candidateselection f = fEL + fcont = fνEL(λ)λc2TNB(λ)e−τIGM(λ)dλ+ (1)
ν,NB ν,NB ν,NB R cT (λ)dλ
λ2 NB
fνcont(λR)λc2TNB(λ)e−τIGM(λ)dλ
ThereareseveralpossibleapproachestofindingLAEcandidates
R c T (λ)dλ
in a narrowband survey. A common way is to simply measure λ2 NB
a narrowbandto broadband color, where the broadbandcovers R
Where fEL and fcont arethefluxdensitycontributionsfrom
thenearbycontinuum.Ifastrongemissionlinefallswithinthe ν,NB ν,NB
theemissionlineandcontinuum,T isthetransmissioncurve
narrowfilter,thiswillproduceanarrowbandexcessandpossible NB
of the filter, and the IGM transmissivity is described as a sim-
candidates will therefore show a color signature. This method
pleexponentialwithappropriateτ (e.g.Madau1995).Such
is relatively stable if the broadband is centered approximately IGM
IGMabsorptionbluewardsofLyαcanbesignificant,especially
on the narrowband, or if the continuum is expected to be flat
athigherredshifts.Theexpressionforthe broadbandfilter will
throughouttheprobedwavelengthrange.
haveanidenticalappearance.
Thecontinuumcanbe describedas a powerlawwith slope
Inthissimplifiedcase,thecolorsignatureinthenarrowband α,sothat fcont = Nλα,whereN isanormalizationfactor.Ifwe
ν c
filtercaneasilybeestimated.Thefluxdensityinthenarrowband assumetheLyαlinetobeadeltafunction,andtheIGMeffects
filtercanbedescribedas tobenegligible,wecansimplify(1)to
Articlenumber,page5of24
A&Aproofs:manuscriptno.aadem
forminggalaxiesatz > 1.4.Figures4and5showthesecriteria
appliedtoourNB401_6data.UsingtheBzKcriterionrequiresa
f = FELTNB + Nλα−2TNB(λ)dλ (2) detectioninthesebroadbands,andthiscriterionreducesourto-
ν,NB c T (λ)dλ R c T (λ)dλ talnumberofLAEcandidatesfrom42to30sources.Giventhat
λ2 NB λ2 NB thebroadbanddataisconsiderablydeeperthanournarrowband
R R
data,andthatwearelookingspecificallyforbrightemitters,we
where F is the flux in the emission line and T is the
EL NB
usethisalsoasacriteriontoremovespurioussources.
integralaveragedtransmissivity of the filter. Describing F in
EL
terms of the equivalent width, F = EW× Nλα−2, we arrive We correlate our sample of candidates to the spectroscopic
EL
(with(2))atanexpressionforthefluxinagivenfilterasafunc- catalogues compiled by Bargeretal. (2008) and Adamsetal.
tionofequivalentwidthandthecontinuumparametersN andα. (2011). These catalogsonly contain sourcesfor which the red-
The color for a given narrowband to broadbandis then simply shiftcouldbedeterminedwithseveralspectralfeatures.Wefind
givenbytheusualmagnitudedefinition, five candidates to already have spectra consistent with the ex-
pected narrowband wavelengths (as indicated in Tables 4 - 6),
buttherearealsospectroscopicallydiscoveredLAEswhichare
m −m =−2.5log fν,NB (3) toofainttobedetectedinoursample.Athoroughanalysisofthe
NB BB 10 f ! LyαandHαbrightnessofgalaxieswhicharealreadyspectro-
ν,BB
scopicallyconfirmedwillbeincludedinaforthcomingpaper.
Witha flatspectrumin fν (α = 0)andanobservedEWcut We match our candidatesto the Chandra Deep Field North
of∼65Å(20Åintherest-frameatz∼2.3),(3)andthemethod X-ray survey (Alexanderetal. 2003) and deep VLA data
described above would give us an excess with a color cut g′ − (Morrisonetal. 2010) in orderto removeAGN. Anythingwith
NB≈0.6,orU−NB≈0.7. an X-ray luminosity > 1042 erg/s or with a 20 cm detection
Note thatthe NB412_18filter is three times wider than the > 1024 W/Hz is considered an AGN. We remove 5 candidates
other filters. This means it is probing a three times wider red- based on these criteria, leaving 25 LAE candidates for further
shiftspacebutatthesametimereachesalowersignificancedue analysis.OurAGNfractionof∼17%isbroadlyconsistentwith
todecreasedcontrastbetweencontinuumandlineemission.We similarworks(seeSection5.1).
can only reliably resolve sources in this filter with an equiva-
lentwidthintheobservedframesimilartothewidthofthefilter
(>180Å). 5. Warmdustproperties
For a more realistic calculation, we need to take the com-
plicationsmentionedaboveintoaccount.Ournarrowbandshave We have performed a match against the photometric GOODS-
wavelengthsneartheedgesofourbroadbandfilters, andmore- Herschel catalogs of Elbazetal. (2011) and Magnellietal.
over,thecontinuumoneitherside ofLy αcan notbeexpected (2013). The latter catalog combines the deeper data from
tobeflat.AbsorptionofLyαinHigasintheIGMwilltendto Elbazetal. (2011) with the PEP 100 and 160 µm data of
attenuate the spectrumbluewardsof Ly α. A strong continuum Lutzetal.(2011).19ofourcandidatesareinthefieldofviewof
slopecanthereforecreatefalsecandidates,ormakerealsources thesesurveys.
disappear,ifaclassicnarrowbandexcesstechniqueisused(see We find five candidates with 24 µm Spitzer/MIPS detec-
e.g. Hayes&Östlin 2006). We have therefore chosen to com- tions within 1 arcsecond of the catalog coordinates. These are
binetwobroadbands,theU andg′bands,oneithersideofLyα NB401_6-10,-14,-19 and NB392_6-1,-2. Three of these five
atz∼2. candidates have detections in the Magnellietal. (2013) PEP-
Inordertoestimateourcolorselectioncriteria,wefirstcre- Herschel catalog; NB401_6-19, NB392_6-1 and NB392_6-2
atedmodelspectrausingourSEDfittingcodeGalMC(seeSec- (seeSection6).
tion 6). We convolved these spectra with our measured filter Interestingly, NB401_6-10 has a detection at 160 µm in
transmissioncurvestocalculatetheexpectedbrightnessineach the Elbazetal. (2011) catalog, but with no detection at 100
filter. µm and not being included in the 3σ catalog of the combined
UsinganEW(Lyα) cutof∼20Å (rest-frame),westartby Magnellietal. (2013) data. It seems that the original flux may
acolorcutU−NB>0.7,correspondingtothelimitofnoIGM havebeenoverestimated,andthatthiscandidatenolongermeets
absorption. This color also has a significance criterion, so that thesignificancecutinthe combinedcatalog.Atthe sametime,
the photometric error is not larger than the color itself. To al- NB392_6-1 is not listed in the Elbazetal. (2011) catalog, but
low for reddening up to E(B− V) = 0.5, we set our limits at withthedeeperdataitisdetectedat3σ.
g′ −NB > 0.1 and g′ − NB > −1.2(U − NB)+1.4.Allowing ForthethreematchesinMagnellietal.(2013),wehavefit-
forstrongerreddeningcouldintroduceagroupofinterlopersat ted an M82 template to the far-IR data points, i.e. only to the
slightly different redshift, that may show a strong U − NB ex- datapointsfrom 24 µm or longer wavelengths, to derive an es-
cess duetothe Lymanbreak,buthaveno actualLy αsignalin timate of the total IR luminosity, L , integrated from the rest-
IR
the narrowband.Our limit is therefore chosen to balance these frame 8-1000µm. Figure 6 shows the fit to the data, with esti-
effectsaswellaspossible.Manyofourcandidateshavealarge mates of log(LIR/L⊙) in the bottomright cornerof each graph.
photometricuncertaintyduetothisbeingashallowsurvey,and Also shownare two AGN templates(Pollettaetal. 2007) - one
wethereforealsochosethisdouble-excessmethodtogetamore quasartemplateandaSeyfert2galaxytemplate-aswellasan
reliableselection,althoughwe willalwayshavesomeinterlop- Arp220templateforcomparison.
ersinourcandidatesample.The−1.2slopewascalculatedfrom AssumingtheM82templatefitvalueasourvalueforL ,we
IR
ourGalMCmodels,andissetbytherelativewidthsoftheUand compute the total IR to UV luminosity, IRX = log(L /L ),
IR 1600
g′filters. whereL isthemonochromaticluminosityatrest-frame1600
1600
In order to deal with possible low redshift interlopers with Å, calculatedfromthe photometryin the V band.Meureretal.
[Oii]3727Åemission,wecombinethesecriteriawiththestan- (1999)foundatightcorrelationbetweentherest-frameUVslope
dard BzK ≡ (z− K)−(B−z) > −0.2, designed to select star βandIRX froma localsampleofstarburstgalaxies,indicating
Articlenumber,page6of24
A.Sandberg etal.:TridentI:LAEsinGOODSNorth
104 NB401_6-19 104 NB392_6-1 104 NB392_6-2
103 103 103
y) 102 102 102
µJ
ux ( 101 101 101
Fl
100 100 100
M82: 12.17 0.06 M82: 11.84 0.03 M82: 12.11 0.08
± ± ±
Arp220: 12.27 0.39 Arp220: 11.85 0.49 Arp220: 12.26 0.28
10-1 QSO2: 11.54±0.57 10-1 QSO2: 11.28±0.45 10-1 QSO2: 11.44±0.72
± ± ±
Sey2: 11.83 0.31 Sey2: 11.58 0.26 Sey2: 11.73 0.47
10-2 ± 10-2 ± 10-2 ±
100 101λ(µm) 102 103 100 101λ(µm) 102 103 100 101λ(µm) 102 103
Fig.6.Fitstothefar-infrareddatapointsfromtheElbazetal.(2011)GOODS-Herschelcatalog,usingtwotemplatesofstarforminggalaxies(M
82andArp220insolidredanddashedblue,respectively)aswellastwoAGNtemplates(QSO2andSeyfert2indot-dashedgreenanddotted
cyan).Notethatonlytheinfrared>8µmcatalogdatawereusedinthefit,andtheopticaltonear-infrareddataareonlyshownhereforcomparison.
Thenumbersinthebottomrightofeachpanelshowtheintegrated8-1000µmluminositiesinsolarunitsforeachtemplate.
3.0 14.0
2.5 2 13.5
)UV 2.0 NB392_6-1 NB392_6- NB401_6-19 13.0
L/LIRF 1.5 /L)⊙12.5
log(= 10 1.0 og(L10bol12.0
X l
IR 0.5 Meurer et al. 1999
Takeuchi et al. 2012 11.5
0.0 Buat et al. 2012
Reddy et al. 2010 (typical) Daddi et al. 2007
11.0
Reddy et al. 2010 (ULIRGs) Reddy et al. 2006
−0.5
This work This work
10.5
−2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
β IRX = log10(LIR/LFUV)
Fig. 7. Left panel: β vs. IRX plot for our three candidates detected in GOODS-Herschel. Note that the Meureretal. (1999) data and relation
havebeencorrectedwiththe1.75factorasquotedbyReddyetal.(2010),toconvertfromL tothetotalL .Alsoshownarethemorerecent
FIR IR
Takeuchietal.(2012) and Buatetal. (2012) β-IRX relations, derived for local galaxies and z ∼ 1−2 sources, respectively. Right panel: IRX
vs.bolometricluminosityforthesamecandidates,comparedtothesampleofBzKselectedgalaxiesfromDaddietal.(2007)andtheempirical
relationforBM/BX-selectedgalaxiesfoundbyReddyetal.(2006).
thattheenergyabsorbedbydustintheUVisbeingproportion- and the relation derived for z ∼ 1−2 sources from Buatetal.
atelyre-emittedatfarinfraredwavelengths.Reddyetal.(2010) (2012).IntherightpanelofFigure7weseehowourcandidates
foundthatthisrelationbreaksforultra-luminousinfraredgalax- compare with the IRX-luminosity ratios of z ∼ 2 − 3 sources
ies(ULIRGs),probablyduetosaturationaspartoftheUV-bright fromDaddietal.(2007)andReddyetal.(2006).Allthreecan-
starformingregionsarebeingcompletelyobscuredbydust. didatesappeartoliewithinthenormalspread,indicatingthatour
FIR-detectedLAEsdonotpossessunusualUVtoIRproperties,
Weestimatedtheβslopesforourthreecandidatesbyfitting
comparedwithothersamplesatsimilarredshift.
a power-law through the observedV, R and I magnitudes, cor-
respondingroughlytothe1600-2600Årangeintherestframe. We estimate the energy budget for our candidate LAEs by
Overplottingour three candidates in the left panel of Figure 7, calculating the amountof flux absorbed by dust in the optical-
they appear to fall into the same locus as the z ∼ 2 ULIRG UV part of the spectrum, and expressit in terms of luminosity
samplefromReddyetal.(2010).Indeed,wefindL & 1012L as L . We create model spectra based on simple SED fit-
IR ⊙ abs,UV
for two of our three candidates, which is the very definition of ting(modellingonlyageandE(B-V)forconstantstarformation
a ULIRG. In this panel we also show the Meureretal. (1999) rate; see model A in Section 6), both with the best fit value of
relation as corrected by Reddyetal. (2010) for comparison. E(B-V) and with a value of 0. We then integrate both spectra
Note that the GALEX IRX ratios used by Meureretal. (1999) foreachcandidate,andestimatetheamountoffluxabsorbedby
are believed to have been overestimated (Overzieretal. 2011; dustasthedifferencebetweenthetwospectra.NotethatL
abs,UV
Takeuchietal.2012).Weshowforcomparisonthenewrelation isnotanobservedquantitybutanestimateoftheamountofdust
derivedforlocalgalaxiesaspresentedbyTakeuchietal.(2012), absorption,expressedin terms of the absorbedluminosity.The
Articlenumber,page7of24
A&Aproofs:manuscriptno.aadem
they estimate AGN to contribute no more than 15% to 35% to
thebolometricluminosity.
12.2
NB401_6-19
Oteoetal. (2012a) perform a match of 56 spectroscopi-
12.1 NB392_6-2 cally detected LAEs at z ∼ 2 − 3.5 in GOODS-South to the
)
⊙ GOODS/Herschel catalogs and find 10 of them to be detected
L
/R byMIPSat24µm,fourofwhichhavedetectionsinthePACS-
LI12.0
log(10 f1o6u0rµcmanbdaidnadtewsisthhoinw2intaergcrsaetceodnldusmoinfotshietieospLtiIcRal>d1e0te12cLti⊙o,np.lAacl-l
11.9
ingthemintheULIRGregime,whiletheothersixgalaxiesde-
NB392_6-1 tectedinMIPSareatleastintheLIRGregime(L > 1011L ).
IR ⊙
11.8 Theirstudy of 23LAEs atz ∼ 0.3with mid-IR/FIRdetections
showedonly onesuch IR-luminousgalaxy,with the rest of the
10.5 11.0 11.5 12.0
log10(Labs,UV/L ) samplehavingLIR < 1011L⊙ (Oteoetal.2011,2012b).Toecho
⊙ theconclusionsofOteoetal.(2012a),thisindicatesthattheLyα
Fig.8.Far-IRluminosityversusdust-absorbedluminosityasestimated selectiontechniquemighttracedifferentgalaxiesatdifferentred-
fromtheE(B-V)valuesderivedfromSEDmodel A.Ourthreefar-IR shifts;asseenforexampleinGALEXLAEswhicharegenerally
detectionsaremarkedwithmagentastars.Bluetrianglesshowestimates
not as luminous as their higher redshift counterparts (see e.g.
atourapproximatedetectionthresholdinthefar-IRofL ∼ 1011.8L .
IR ⊙ Cowieetal.2011).
Blacktrianglesshowupperlimitstothedustabsorbedluminosities,and
havebeenshiftedby-0.02dexalongthey-axisforclarity.Thedotted On the other hand, our results do not appear to agree with
blacklineshowsthe1:1relation.
asimilarstudyrecentlyperformedbyKusakabeetal.(2015)on
LAEsatz=2.18intheGOODS-Southfield.Afterstacking218
LAEsintheHerschelbandstheyreportonlyanupper3σlimit
resultsareshowninFig. 8, plottedagainstthe estimatedfar-IR on L of 1.1 × 1010L . Our study does not include the same
IR ⊙
luminosity.Ourthree(U)LIRGsarefoundamongthecandidates
removalof24µmdetectionsandmaythereforebemorevulner-
that appearto have the highestamountof L , as expected.
abs,UV able to confusionfromnearbysources.However,judgingfrom
All candidates show L < L , which is also expected in
abs,UV IR themulti-wavelengthimagingwehaveavailable,oursourcesare
the scenario where a dust screen model may underestimate the
well-isolated(withthepossibleexceptionofNB401_6-19)inthe
total dust content. Since these estimates of L should cor-
abs,UV Spitzer/IRACimaging,whichwasusedtoidentifydetectionsfor
respond to a lower limit to L , we thus expect at least one of
IR theHerschelsample.AccordingtoKusakabeetal.(2015),none
themtobebrightenoughtobedetectedasaULIRGinthefar-
of the 218 LAEs in the GOODS-South sample were detected
IR (NB401_6-19, which is also our brightest candidate in the
in MIPS 24µm imaging at the 3σ level. As previously stated,
far-IR).Ahandfuloftherestofthecandidateswouldappearto
out of our 19 candidates overlapping with the MIPS footprint,
beatleastintheLIRGregime(11 < log(L ) < 12),andas
UV,abs fivehaveacounterpartinthe24µmbandwithin1arcsec,asre-
discussed, two of these are also detectedas LIRGs or ULIRGs
ported by Magnellietal. (2013). The 218 LAEs discussed by
inthefar-IR.
Kusakabeetal. (2015) are howeverconsiderablyfainter on av-
eragethanthesamplepresentedhere(Table3);theaverageLyα
luminosityis6.6×1041erg/s,andthemaximumisonly6.6×1042
5.1.Comparisontopreviouswork
erg/s(Kusakabe2014,privatecommunication).Theaveragelu-
Ourfindingsarebroadlyconsistentwithseveralpreviousworks. minosityforoursampleis4.2×1042erg/s(seeTable3).
AlthoughtheLAEpopulationisnormallyassociatedwithsmall,
Wardlowetal.(2014)stackedthreesamplesof126,280and
dust-freesystems, evidenceof Lyα emissionfromlarge,dusty
92 LAEs in the Extended Chandra Deep Field-South (ECDF-
galaxiesdoesexistintheliterature,buttheAGNcontributionto
S) at z∼2.8, 3.1 and 4.5 respectively in Herschel/SPIRE 250,
theLyαemissivityisnotalwayswellknown.
350and500µmimagesaswellas870µmdatafromLABOCA
Chapmanetal. (2005) present a spectroscopic follow-up (Weißetal.2009).Similarly,theyput1σupperlimitsonL of
IR
studyof98sub-millimetergalaxiesdetectedat850µmanddeep 5−8×1010L ,correspondingtoroughly2−3×1011L at3σ
⊙ ⊙
VLA1.4GHzdata,andobtainedreliableredshiftestimatesfor forallthreeredshiftranges.Onlyoneofourcandidatesisbright
73ofthese.25ofthosesourcesshowa"strong"Lyαline.The enough to be detected at these longer wavelengths; NB401_6-
AGN fractionis notcertain, as they cannotrule outfaint AGN 19 (at 250 µm), which is also the brightestin the far-IRof our
features,buttheynotethattheLyαto[Civ]λ1549lineratios candidates.
are generallyconsistentwith whatyouwould expectfromnor-
mal starbursts. They conclude that a patchy or inhomogenous Inlightofthesesomewhatconflictingresults,itisclearthat
dustdistributionmightallowLyαtoescapefromthesesystems. large samples of bright LAEs, in particular at the strong star-
formingepocharoundz∼2,withmatchingfar-IRsurveyscould
Caseyetal. (2012) performeda spectroscopic study of 767
testifthisisacosmicvarianceissue.
galaxies individually detected in Herschel/SPIRE data. 36 of
thosegalaxiesarefoundtobeatz>2,withLIR ∼2−6×1012L⊙, As a final note, a preliminary analysis of our
the majority of which show Ly α in emission. 17 of the Ly α CFHT/WIRCam data of Hα candidates matching the red-
emittinggalaxiesdonotshowany[Civ]emission,andtheyes- shift of our NB401_6 Ly α data finds both NB401_6-19 and
timateanAGNfractionofroughly∼25%. NB401_6-10also show significant Hα emission, which would
HST/COS spectroscopy in the ultraviolet of 11 local beexpectedforbrightinfraredgalaxieswithhighstarformation
ULIRGs is presented in recent work by Martinetal. (2015). 8 rates, if the star forming regions are not too obscured by dust.
oftheULIRGsshowLyαinemission,threeofwhichappearto ThedetailsofthefullHαtoLyαmatchwillbediscussedina
be AGN dominated galaxies. For the five remanining ULIRGs forthcomingpaper.
Articlenumber,page8of24
A.Sandberg etal.:TridentI:LAEsinGOODSNorth
6. SEDfitting – ModelAcorrespondstothecommonapproachoffittingfor
theage,reddeningandmassofthegalaxy,usinga constant
We combinecatalogdata fromCapaketal. (2004), Wangetal. starformationrate.Forthismodel,theagevaluesrepresenta
(2010) and Kajisawaetal. (2010) as described in Section 3.4. formofluminosity-weightedage,basedontheentireoptical
In order to perform SED fitting on a relatively homogeneous tonear-IRSED.Thisageisinotherwordsdifficulttointer-
and self-consistent data set, we chose to use archival Sub- pretinaphysicallymeaningfulway.Ahigh(low)ageisusu-
aru+KPNOUBVRIzdataratherthanHST/ACSdata.Acollec- ally associatedwith ared(blue)spectrum,butthisquantity
tionof "postagestamps" fromourimagingdatais presentedas isofcoursestronglydegeneratewiththedustreddening.In
anexampleinFigure3.ThefullsetisincludedintheAppendix. ordertoattempttobreaksomeofthesedegeneracies,andex-
To perform the SED fitting, we use version 2.0 of the ploretheparameterspaceinmoredetail,wealsoattempted
GalMCcode(Acquavivaetal. 2011),with GALAXEVsynthe- twomoreunconventialmodels.
sis models of S. Charlot & G. Bruzual (2007, updated version – ModelBassumesanageofthegalaxyequalto2.2Gyr,cor-
ofBruzual&Charlot2003))andaSalpeterinitialmassfunction responding roughly to the age of the first stars in the uni-
(IMF). We also ran some tests, comparing the original (2003) verseatz∼2.Thisisnotatypicalvaluefoundforthestellar
and the more recent (2007) Charlot-Bruzual model, as well as model age of LAEs, but the idea of this model is to see if
ChabrierandKroupaIMFs,butfoundnostrongdifferencesfor wecanconstrainageneraltrendinthestarformationhistory
thegalaxiesdominatedbyyoungstellarpopulationsthatweare throughoutthe formation of the galaxy. The star formation
mainlyinterestedin.WeuseaCalzettietal.(2000)dustabsorp- rateφisthenallowedtochangeaccordingtoasimpleexpo-
tion model. For an overview on these different models and the nentiallaw;φ=1/|τ|exp(−t/τ).Byallowingtheτparameter
general effects they have on SED fitting, see the discussion in totakeanyvalue|τ| > 1Myr,thestarformationmayeither
Acquavivaetal.(2011).GalMCusestheinformationonthere- decreaseorincreasewithtime,correspondingtopositiveor
processingofionizingphotonsinordertoaddnebularemission negativeτ values, respectively.The exponentialmodelsare
lines, similar to the approach of Schaerer&deBarros (2009). restrictedsothat|age/τ|<13.78.
TheMonteCarloMarkovchainswererunusing20000samples. – ModelC assumes two separate populations,each with con-
TheoutputchainswerethenanalyzedusingtheApril2013ver- stant star formation histories, where one is fixed to an age
sion of the GetDist program from CosmoMC (Lewis&Bridle of2.2Gyr andtheotheris restrictedto beyoungerthan 10
2002). Myr.Thisagecorrespondsroughlytothemaximumagefor
ForallofourSEDfittingwehavechosentoremovetheU- whichasimplestellarpopulationcanbeexpectedtoproduce
band,correspondingtothespectrumbluewardofLyα,fromthe ionizingcontinuumphotonsgivingrise to nebularlines,in-
fit. The code does include a correction for IGM absorption ac- cludingLyα.Thismodelisthussimilartotheideainmodel
cordingtoMadau(1995),butthisisonlyanaveragecorrection. B,wherewewanttoseehowmuchmorerecentstarforma-
Sincethisparameterisnotknownfortheindividualgalaxy,in- tionisneededinordertoreproducetheobservedspectrum.
cludingtheU-bandcouldskewtheresults.Wehavealsochosen We can then estimate the relative contributions of the two
tosubtractthecontributionoftheLyαfluxtotheB-band.This populationsto the total mass as a measurementof whether
isdonesimplybyassumingthecontributionfromthecontinuum theoldoryoungstarsdominatetheSED.
tobethesameinBasinthenarrowband.Theuncertaintyinthe
B-bandfluxwasthenincreasedbyaddinginquadraturethepho- Theideaofusingthesethreemodelsisnottotrytofindthe
tometricuncertaintyofthenarrowbandfluxcontribution. onemodelthatbestrepresentsthespectralshapeofourdata.In
fact, all three models can usually find reasonable fits to all of
The metallicity is a notoriously difficult parameter to con-
the observedphotometry.None of the modelsrepresenta truly
strain in SED fitting, as it has a relatively modest effect on the
realisticviewofthestarformationhistoryandsoarenotmeant
overallSEDshapeandisalsostronglydegeneratewiththedust
to be compared in that sense. Instead, our idea is to try to see
reddening.We thereforechose to use a fixed metallicity of 0.2
whatSEDmodellingcantellusaboutthestarformationratesin
Z andonlyallowedthereddeningtovary.Weheldtheredshift
⊙
general,undercertainassumptions.
fixedattheexpectedvalue,correspondingtoLyαbeingcentered
inthenarrowbandfilter.
6.2.SEDfittingresults
6.1.SelectionofSEDfittingparameters TheresultsofourfitsforallourLAEcandidatesarepresentedin
Tables4,5and6formodelsA,BandCrespectively.Thevalues
Galaxiesthatweobserveatz∼2havealreadylivedthroughover givenasbestfitparametersarethosecorrespondingtothelow-
2 Gyr of cosmic time. In that time, these galaxies have likely estχ2 found.However,theprobabilitydensityspacemayshow
gonethroughseveralmergingeventswithepochsofheavystar a complicated structure, and the following "low" and "high"
formationaswellastimesofmorequiescentevolution.Whatwe columns listed after each parameter represent the 68% confi-
observeatthismomentrepresentsasnapshotintimesomewhere dence intervals marginalized over the entire parameter space.
alongthatevolutionarypath.Toaccuratelyresolvetheentiryhis- Notethatthe least-squaressolutionmayevenin some casesbe
tory ofsuch a complexsystem basedsolely on the spectralen- located outside this region. This represents a case where most
ergy distribution is a challengingtask, which comes down to a reasonablygoodmodelsfavourcertainvaluesforthatparameter
matterofstatisticsandaverages. (e.g.,alowage),whilethesinglemodelwiththelowestχ2hap-
Basedoncertainassumptions,wecanseewhichparameters penstocorrespondtoadifferentvalueforthatparameterinthat
wouldfittheobservedSEDthebest.Ratherthanonlyassuming particularcase (e.g.a highage).Themarginalizedlimitsthere-
the standard SED fitting parameters and simply compiling our fore give a slightly broader picture of which parameter values
results,wewantedtoseeifwecouldsayanythingmoredefini- canstillyieldacceptablefits.
tiveaboutthestarformationhistoriesofourLAEcandidates.We TheR-1valueshowstheGelman&Rubin(1992)Rstatistic,
thereforedecidedtorunSEDfittingusingthreedifferentsetups: which is a measurementof how similar the chains are towards
Articlenumber,page9of24
A&Aproofs:manuscriptno.aadem
500
43.0 400
s)
s/
g
r
e
y ( 300
minosit42.5 A◦W ()Lyα
αy lu E 200
L
g 10
lo42.0
100
41.5 0
8.0 8.5 9.0 9.5 10.0 10.5 8.0 8.5 9.0 9.5 10.0 10.5
log10 Stellar Mass (M ) log10 Stellar Mass (M )
⊙ ⊙
Fig.9.EW andL vs.Stellarmass,derivedfromSEDfittinginmodelA.Downwardpointingtrianglesdenoteupper1σlimits.
Lyα Lyα
9.5 14 2.5
A) B) C)
g Age (yr)109887....0055 bate parameter 116802 bhrate parameter 2011....0505
lo67..50 Birthr 4 og Birt10 0.0
2 l−0.5
6.0
0 −1.0
8.0 8.5 9.0 9.5 10.0 10.5 8.5 9.0 9.5 10.0 10.5 11.0 7.0 7.5 8.0 8.5 9.0 9.5 10.010.511.0
log10 Mass (M ) log10 Mass (M ) log10 Mass (M )
⊙ ⊙ ⊙
Fig.10.Left:Agevs.MassforSEDmodelA,usingconstantstarformationrate.NotethatNB401_6-7isnotincludedinthisgraphastheage
could not be constrained for this candidate. The mass in all three panels is the total mass that has gone into stars, i.e. the integral of the star
formationrateovertheageofthegalaxy.Middle:Birthparametervs.MassforSEDmodelB,usinganexponentialstarformationrate(Eq.4).
Lowerlimitsinthebirthparameter correspond tolowerlimitsin1/τandaredenotedbyupwardpointing triangles.Right:Birthparametervs.
MassforSEDmodelC.Notethelogarithmicscaleonthey-axis.Upperlimits(1σ)aredenotedwithadownwardpointingtriangle.Fourofthe25
candidatesarenotshowninthispanel,forwhichthemassoftheoldpopulationcouldnotberecovered(seetext).Themassisthetotalmassof
thetwopopulationscombined.
thefinalstagesofthefitting.Ideally,allchainswillendupinthe againstthemassforourthreemodels.Thebparameterisdefined
sameregionofparameterspace,indicatingagoodconvergence. as the currentstar formationrate (SFR) dividedby the average
Incasesofstrongdegeneracieswithmultiplelocalminima,this pastSFR. FormodelC, therearefourcandidatesforwhichthe
numbermaybelarger,indicatingthatthedifferentchainsfavour bestfitmassintheoldpopulationisnegligible,correspondingto
differentsolutions.R-1 < 0.1 is typicallyconsideredasaccept- asystemwherea∼10Myrpopulationcompletelydominatesthe
ableconvergence.Anyvalueabove0.1istypesetinboldinTa- spectrum.Thesefoursystemsarealsoclearlyamongthelowest
bles4,5and6forclarity. masssystemsinmodelC(see Table 6). Sincewe fixedtheage
Thegeneralpicturethatemergesfromthedifferentmodelsis of the old populationto 2.2 Gyr,the resultingburstparameters
notunexpected;forcinganoldagegenerallyshiftstheextinction areoverlyhigh(notethelogscaleintherightpanelofFig.10),
valuesdown,makingthespectrumlessreddenedduetodust.For and would for those four cases imply an essentially infinite b
theexponentialstarformationinmodelB,anegativeτvalueis parameter.Thesefourcandidatesarethereforenotshowninthe
oftenfavored,correspondingtoexponentiallyrisingstarforma- figure.NotethatthebparametersderivedforbothmodelBand
tion, again filling in the blue spectrum by adding more newly Carenottobeinterpretedliterally,duetotheheavilyrestricted
formedstars. modeofstarformationinthetwomodels.Rather,weshowthe
InmodelC,wewantedtotesthowimportantrecentstarfor- b parametershere as a way to understandthe ratio of youngto
mation truly is for reproducing the spectrum. Our results indi- oldstarsinourmodels,andseetheresultsonlyasanindication
cate thatmanyoftheseLAEsseem heavilydominatedbya re- that the majority of our LAE candidates show signs of recent
cent burst of star formation, while some are consistent with a elevatedstarformation.
slow,steadyrate.Fig.10showstheageortheburstparameterb
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