Limits on the LyC signal from z~3 sources with secure redshift and HST coverage in the E-CDFS field

2.9 MB·

by  L. Guaita

Astronomy&Astrophysicsmanuscriptno.LGuaita˙LyC (cid:13)c ESO2016
January14,2016
Limits on the LyC signal from z ∼ 3 sources with secure redshift
and HST coverage in the E-CDFS field (cid:63)
L.Guaita1,L.Pentericci1,A.Grazian1,E.Vanzella2,M.Nonino3,M.Giavalisco1,5,G.Zamorani2,A.Bongiorno1,
P.Cassata7,M.Castellano1,B.Garilli8,E.Gawiser4,V.LeBrun6,O.LeFe`vre6,B.C.Lemaux6,D.Maccagni8,E.
Merlin1,P.Santini1,L.A.M.Tasca6,R.Thomas6,7,E.Zucca2,S.DeBarros2,N.P.Hathi6,R.Amorin1,S.
Bardelli2,andA.Fontana1
(Affiliationscanbefoundafterthereferences)
Accepteddate:January5th,2016
6
1
ABSTRACT
0
2
Context.DeterminingthestrengthoftheLymancontinuum(LyC)andthefractionofLyCescapehaveimplicationsforthepropertiesofthe

n emittingsourcesatanyredshift,butalsoforthere-ionizationoftheUniverseatz>6.
a Aims.WeaimtomeasuretheLyCsignalfromasampleofsourcesintheChandradeepfieldsouth.Wecollectstar-forminggalaxies(SFGs)and
J activegalacticnuclei(AGN)withaccuratespectroscopicredshifts,forwhichHubbleSpaceTelescope(HST)coverageandmulti-wavelength

2 photometryareavailable.
1 Methods.Weselectedasampleofabout200sourcesatz ∼ 3.TakingadvantageofHSTresolution,weappliedacarefulcleaningprocedure
and rejected sources showing nearby clumps with different colours, which could be lower-z interlopers. Our clean sample consisted of 86

] SFGs(including19narrow-bandselectedLyαemitters)and8AGN(including6detectedinX-rays).WemeasuredtheLyCfluxfromaperture
A photometryinfournarrow-bandfilterscoveringwavelengthsbelowa912Årestframe(3.11 < z < 3.53).Weestimatedtheratiobetween
G ionizing(LyCflux)and1400Ånon-ionizingemissionsforAGNandgalaxies.
Results. Byrunningpopulationsynthesismodels,weassumeanaverageintrinsicL (1400Å)/L (900Å)ratioof5astherepresentativevalue
. ν ν
h foroursample.Withthisvalueandanaveragetreatmentofthelinesofsightoftheinter-galacticmedium,weestimatetheLyCescapefraction
p relativetotheintrinsicvalue(fescrel(LyC)).WedonotdirectlydetectionizingradiationfromanyindividualSFG,butweareabletoseta1(2)σ
- upperlimitoffescrel(LyC)<12(24)%.Thisresultisconsistentwithothernon-detectionspublishedintheliterature.
o Nomeaningfullimitscanbecalculatedforthesub-sampleofLyαemitters.WeobtainonesignificantdirectdetectionforanAGNatz=3.46,
tr withfescrel(LyC)=(72±18)%.
s Conclusions. Ourupperlimitonfescrel(LyC)impliesthattheSFGsstudiedheredonotpresenteitherthephysicalpropertiesorthegeometric
a conditionssuitableforefficientLyC-photonescape.
[

Keywords.Techniques:imaging–Galaxies:Star-FormingGalaxies,LymanAlphaEmitters,ActiveGalacticNuclei
1
v
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5 1. Introduction only theLyC signal coming from2.5 < z < 4 sourcescan be
0 detectedfromtheground.
3 Theradiationatwavelengthsshorterthan912Å(Lymancon-
Theoretically, the production, propagation, and escape of
0 tinuum, LyC) is produced by massive OB-type stars in young
LyCphotonsarerelatedtothephysicalpropertiesofthegalax-
. star clusters (e.g. Avedisova, 1979) and by active galactic nu-
1 ies. Firstly, the production of LyC radiation implies the pres-
clei (AGN). At z > 2.5, it is redshifted into the optical light
0 ence of young, massive stars, and therefore of on-going star
6 spectralregionand,inprinciple,itcouldbedetectedfromthe formation.Becauseofthefastrecombinationtimescaleofthe
1 ground. HI atoms, previous episodes of star formation have no signif-
v: Stellar models (e.g. Bruzual & Charlot, 2003; Leitherer icantimpactontheproductionofionizingphotonseventually
i et al., 1999) predict that the intrinsic ratio between ionizing escapingthegalaxy(e.g.Paardekooperetal.,2015).Secondly,
X and non-ionizing radiation from star-forming galaxies is less the propagation of LyC photons within the ISM is favoured
r than0.2-0.5(dependingonthestellarpopulationage,metallic- by a negligible amount of dust and low column density of
a
ity,AGN-galaxyfraction).Asaresultofitsenergy,theLyCis HI (N(HI) ≤ 1018 cm−2) in a 10-pc scale region around the
verylikelytobeabsorbedbyneutralhydrogen(HI)anddustin emitting star clusters. This could be the case for galaxies em-
theinter-stellarmedium(ISM)orbytheHIintheinter-galactic bedded in dark-matter halos with masses less than 108 M
(cid:12)
medium (IGM). At z >> 4, the IGM is completely opaque to (Yajima et al., 2011; Wise et al., 2014; Paardekooper et al.,
thisradiation,andthechanceofdetectionfromEarthbecomes 2015).However,evengalaxiesresidinginmoremassivehalos
negligible (e.g. Madau, 1995; Inoue et al., 2014). Therefore, can have lines of sight favourable to the propagation and the
escapeofLyCphotons(Gnedinetal.,2008;Royetal.,2015).
Send offprint requests to: Lucia Guaita, e-mail: Supernova explosions could have cleared their ISM, and star-
[email protected] formation episodes could occur in their outskirts. In addition,
(cid:63) BasedondataobtainedwiththeEuropeanSouthernObservatory ’runaway’OBstarsupto1kpcawayfromtheinitial-originre-
Very Large Telescope, Paranal, Chile, under Programs 170.A–0788, gionsareproposedtosignificantlycontributetotheamountof
074.A–0709,171.A–3045,275.A–5060,and185.A–0791. LyCphotonsfinallyemittedintotheIGM(Conroy&Kratter,
1
L.Guaita:LyCsignal
2012). Thirdly, LyC photons emitted into the IGM can affect which can be responsible for a false LyC signal. Siana et al.
the galaxy environment, changing the ratio of neutral vs ion- (2015) reached the same conclusion after following up five
izedgas,eventuallyfuellingtheISM(e.g.Martinetal.,2012, LyC emitter candidates that were previously photometrically
forastudyofionized-metaloutflowsandinflows).Simulations selectedthroughnarrow-bandimaging(seealsoMostardietal.,
at intermediate redshift have shown that the LyC escape frac- 2015).
tion(fesc(LyC))steeplydecreasesasthedark-matterhalomass
ThedirectdetectionofLyCphotonsalsohasimplications
(M )increasesat3 < z < 6(e.g.Yajimaetal.,2011)andthat
thehmedian fesc(LyC) also changes with redshift at z = 4−6 forunderstandingtheoriginofthere-ionizationoftheUniverse
at z > 6 and which objects keep it ionized at z ∼ 6. It is still
(e.g.,Cen&Kimm,2015).Itisworthstressingthatwhilesome
uncertain if galaxies or AGN were the main drivers of the re-
authorsfindthattheLyCescapefractiondecreaseswiththein-
ionization. The uncertainty depends on the fact that we still
creaseinthehalomass(seealsoFerrara&Loeb,2013),other
havenotobservedexactlyhowionizingradiationescapesfrom
worksfindtheoppositetrend:fesc(LyC)isfoundtorangefrom
individualgalaxies.However,theaverageionizationrateofthe
a few percent (e.g. Gnedin et al., 2008) up to 20−30% (e.g.
UniversehasbeenestimatedbystudyingtheLyαforestonthe
Mitraetal.,2013)orevenhigher(e.g.Wise&Cen,2009).
line of sight of bright sources and the physical properties of
Searches for LyC emission from galaxies have been
thehigh-redshiftIGM(e.g.Haardt&Madau,2012;Becker&
very difficult so far. In the local Universe, two galaxies
Bolton, 2013). Depending on the redshift, either galaxies or
have been observed to be ionizing-radiation emitters. One is
AGNhavebeenconsideredthemainsourcesoftheionization.
Haro11, a blue compact and metal poor (oxygen abundance
12+log(O/H)=7.9)galaxyatz=0.02,characterizedbyM(HI) A Lyman continuum from an AGN is observed in many
<108 M (Bergvalletal.,2006).Itiscomposedofthreemain cases (e.g. Cowie et al., 2009; Stevans et al., 2014; Worseck
(cid:12)
young (< 40 Myr old) star-formation clumps, probably expe- etal.,2014;Lussoetal.,2015;Tiltonetal.,2015),butthenum-
riencing a merging event. Leitet et al. (2011) estimated that ber of bright AGN significantly decreases at z > 3 (e.g., Fan
∼ 17% of the intrinsic LyC flux escapes Haro11 (see also et al., 2006). However, Giallongo et al. (2015) evaluated the
Bergvalletal.,2013,forthemethod).Acomparablevaluewas luminosityfunctionoffaintAGNatz=4−6.5.Theydemon-
estimatedbyLeitetetal.(2013)forasecondbluegalaxy,Tol stratedthattheAGNpopulationintheirsamplecouldproduce
1247-232, which is also characterized by low metallicity and ionization rates comparable with those required to keep the
lowdustcontent. IGM as ionized as the observed Lyα forest of similar-redshift
Atz ∼ 0.24,Borthakuretal.(2014)discoveredasimilarly quasar spectra. Despite this, the majority of the studies still
blue galaxy, J0921+4509, that could have a LyC escape frac- supporttheideathatlow-massstar-forminggalaxies(belowthe
tionof(21±5)%.J0921+4509ischaracterizedbyamoderately currentsurveydetectionlimits)arethemostprobablesources
highstellarmass(1010.8M ),metallicity(12+log(O/H)=8.67), of the re-ionization (Duncan & Conselice, 2015; Dayal et al.,
(cid:12)
and a moderate star-formation rate, SFR = 55 M yr−1. 2015; Robertson et al., 2015). A few rare cases of more mas-
(cid:12)
However,theanalysesofhardX-rayandradioemissionsofthis sive galaxies with active phenomena of feedback could also
galaxy did not exclude the possible contribution by an AGN contribute, and we may have the chance to find galaxies like
(Jiaetal.,2011;Alexandroffetal.,2012). them by exploring large samples at z < 4 (see also Chardin
etal.,2015).
At0.3≤z≤3,notoneundisputeddetectionofLyCfroma
purestar-forminggalaxyexists.Bridgeetal.(2010)foundone Finally,itisimportanttoconsiderthefactthatthephysical
LyCleakinggalaxywithanactivegalacticnucleusat z ∼ 0.7 properties of galaxies/AGN and their LyC escape fraction at
(∼15%oftheintrinsicLyC),butnoLyCsignalfrompurestar- theepochofre-ionizationinturncouldbedifferentfromthose
forminggalaxies.Sianaetal.(2010)searchedforLyCemission at2<z<4.Duncan&Conselice(2015),forinstance,showed
instarburstgalaxiesatz∼1.3,butfoundnoemissionfromany thatstar-forminggalaxiesatz>6couldbebluerandtherefore
ofthem.Iwataetal.(2009),Nestoretal.(2013),andMostardi able to produce a larger number of ionizing photons than at
etal.(2013)compiledsamplesofafewLyCemittercandidates intermediateredshift.
among z ∼ 3 UV-continuum-selected star-forming galaxies
In this work, we start from sources with secure redshift,
andnarrow-band-selectedLymanalphaemitters.ALyCsignal
since photometric-redshift samples always contain some in-
from individual sources was estimated by measuring the flux
terlopers. This can be achieved by starting with large spec-
innarrowbands,coveringthe∼900Årestframe.However,the
troscopic samples, such as VUDS (the VIMOS Ultra Deep
contamination from nearby lower-z neighbours was not con-
Survey, Le Fe`vre et al., 2015) and the ESO-GOODS master
sideredontheobject-by-objectbasisinthesesamples,andthis
catalogue; then we analyse images at the resolution of HST
couldhaveledtofalsedetections(Sianaetal.,2015;Mostardi
to eliminate close-by possible lower-z contaminants, and take
etal.,2015).Vanzellaetal.(2010a),Boutsiaetal.(2011),and
advantageofmulti-wavelengthphotometryandHSTimagesto
Grazian et al. (2016) analysed large samples of z ∼ 3 star-
correlatetheLyCsignalwithphysicalandmorphologicalprop-
forming galaxies with secure redshift, without providing any
erties(seealsoBogosavljevic´,2010).Thescopeistomeasure
significant detection. A promising LyC detection comes from
eitheradirectLyCsignalorareliableLyCupperlimitforour
Ion2, a galaxy at z ∼ 3.218, studied in Vanzella et al. (2015);
sample of star-forming galaxies, free of contamination from
deBarrosetal.(2016),Vanzellaetal.inprep.
low-zinterlopers.
The total number of sources investigated in these high-
redshift studies is quite high, but not all of the analysed star- Thepaperisorganizedasfollows.InSect.2wepresentthe
forminggalaxieshavehomogeneousarchivaldata.Inparticu- data and the selection of our sample of star-forming galaxies
lar, only some of them have Hubble Space Telescope (HST) andAGN,inSect.3wedescribethemethodusedtoestimate
coverage and multi-wavelength photometry, which is essen- theLyCsignal,inSect.4wecollectthearchivalpropertiesof
tialtoidentifynearbylow-redshiftcontaminants.Asshownby oursample,inSects.5and6weshowourresultsanddiscuss
Vanzellaetal.(2012a),oneofthemainlimitationsofLyCstud- them,andwesummarizeourworkinSect.7.Throughoutthe
ies is the difficulty of identifying low-redshift contaminants, paperweuseABmagnitudes.
2
filter(FWHM), exp7me, PSF, 3σ,det,limit, op7mal,
instrument/telescope, (PSF/2,radius), aperture,
NB3727(50,Å), 35.8,h, 1.4”, 26.7, 1.4”,
MOSAICII@CTIO4m,
NB387(110,Å), 8.5h, 0.7”, 28.8, 0.8”,
SuprimeCam@SUBARU,
NB388,(37,Å), 16.7h, 0.8”, 29.8, 0.8”,
FORS@VLT,
NB396,(129,Å), 13h, 0.9”, 27.4, 1.0”,
[email protected],
L.Guaita:LyCs3ign0al’x30’%ECDFS% ground+based%
WFI_U50, MUSYC U
0.5 VVIIMMOOSS__UB
NB3727, MOSAICII@CTIO
0.4 NB387, SupCam@SUBARU
T()λ0.3 NNBB339868 WFOFRI@[email protected]
70,spectroscopically,confirmed,
0.2
SFGs,at,3.11<z<3.53,,,0,.1(+,17,LAEs,at,z(cid:1)3.1),
3500 3600 3700 3800 3900 4000 4100 4200
λ [Å]
in,clean,regions,
NB3727,,,,,NB387
0.5 ,
(NB,PSF!), 0.4 ,
0.3 NB388,,,,,,,NB396,,
+  about%50%%of%the%sources%are%lost%for%“cleaning”%
0.2
NB3727, MOSAICII@CTIO
0.1 NspBe3c9t6ru WmF aI@t zL=aS3i.l5la32.2m
spectrum at z=3.30
3500 4000 4500 5000 5500 6000
λ [Å]
+  V~25,%mNB~27%(fesc(LyC)=30%,% %%
Inoue2014)
Fig.1: Upperpanel: Narrow-band filter transmission curves, con-
%%%%%%V~27,%mNB~29%%
volved with the instrument characteristics. They are not affected by
theredleakintheopticalwavelengthcoverage.Lowerpanel:NB3727
andNB396togetherwithtwosyntheticspectraofstar-forminggalax-
ies, obtained by running BC03 models at sub-solar metallicity and
extractingastellarpopulationof10Myr.Thesolid(dashed)redline
correspondstoaspectrumredshiftedtoz=3.30(3.53).
Fig.2: Area in E-CDFS covered by the four narrow-band images.
TheimageobtainedwithNB3727coverstheentireE-CDFSareaof
30’x30’,theotherimagesapartofit.Theblueandgreensquaresin-
2. Dataandsampleselection dicatetheareaofE-CDFSandarecolouredasthetransmissioncurves
inFig.1.
WemeasuredtheLyCfluxinnarrow-bandfiltersthatcoverthe
rest-framewavelengthrangeof860 < λ < 912Å.Westarted
fromasampleofphotometricallyselectedstar-forminggalax- Lyαemissionlinesatz ∼ 2.2.Wefocusedonthecentralarea
ies that are spectroscopically confirmed. This ensures that we of NB387 image, where astrometry is of better quality (see
avoid the contamination that is often present when only pho- Sec. 2.1.1). This narrow band samples the LyC for sources at
tometricredshiftsand/ornarrow-bandselectionsareavailable. 3.33<z<3.43.
Withthis,webuiltasampleofstar-forminggalaxiesandAGN The NB388 image was obtained with the FORS1 instru-
with accurate redshift, and we required the availability of at ment at the VLT for a total exposure time of 17 hours; the
least two HST broad bands to be able to identify low-z inter- NB388 filter was also used by Hayes et al. (2010) to probe
lopersbyeye. Lyαemissionlinesatz ∼ 2.2.ItsamplestheLyCforsources
at3.28<z<3.49.Theimageareaisabout12arcmin2.
The NB396 image was obtained with the WFI instrument
2.1. Narrow-bandfilters
attheESO2.2metertelescopeatLaSillaforatotalexposure
Thenarrow-bandfiltersweconsideredformeasuringtheLyC time of 13 hours; this filter covers the Lyα emission line at
fluxareNB3727,NB387,NB388,andNB396.Theycoverthe z∼2.25(likeinNilssonetal.,2009).Wefocusedonthecentral
wavelengthrangebetweentheUandBbands(Table1andFig. area of the NB396 image (see Sect. 2.1.1). This narrow band
1).Fortheimagesobtainedinthesefilters,wemeasuredasee- samplestheLyCforsourcesat3.41<z<3.53.
ingPSFbetween0.7”and1.4”,and3σdetectionlimitbetween Theareacoveredbythefournarrow-bandimagesisshown
26.7and29.1magnitudes.ThePSFanddetectionlimitswere inFig.2.
obtained by following the prescription described in Gawiser
etal.(2006)andGuaitaetal.(2010).
2.1.1. Reductionofarchivalnarrow-bandimages
The four narrow-band observations were originally de-
signedfordifferentsurveys,asexplainedbelow TherawimagesintheNB387,NB388,andNB396bandswere
The NB3727 image was obtained with the MOSAICII processedasdescribed byNoninoet al.(2009).Theonly dif-
instrument at the Cerro Tololo Interamerican Observatory ference is the final stacking of the individual frames because
(CTIO) for a total exposure time of 36 hours (36 exposures we here adopted the SWARP software (Bertin et al., 2002).
of 1 hour each); it was reduced and analysed by Guaita et al. Astrometry calibration was performed by considering a cata-
(2010). It covers a total area of 31.5’ x 31.5’. The E-CDFS logueofsourcesdetectedintheVIMOSB-bandimage,which
was imaged with NB3727 to detect Lyman alpha (Lyα) emit- isavailableaspartofCANDELS.
tersatz (cid:39) 2.1.ThisnarrowbandsamplestheLyCforsources Fluxcalibrationwasperformedbyusingasetofstarsfrom
at3.11<z<3.30. the CANDELS catalogue (Guo et al., 2013), together with a
Fortheotherthreenarrowbands,rawimageswererecov- set of star templates (Pickles, 1998; Gunn & Stryker, 1983).
eredfromtelescopearchivesandreducedasexplainedinSect. Thestartemplateswereconvolvedwiththenarrow-bandfilter
2.1.1. transmission curves to obtain their narrow-band magnitudes.
The NB387 image was obtained with SuprimeCam at ThespectralenergydistributionsofCANDELSstarswerefit-
Subarutelescopeforatotalexposuretimeof8.5hours;NB387 ted with the templates. From the best fit, we inferred the true
isthenarrow-bandfilterusedinNakajimaetal.(2012)toprobe narrow-band magnitudes of the stars. We measured observed
3
L.Guaita:LyCsignal
Table1:PropertiesofthenarrowbandscoveringLymancontinuumsignalat860<λ<912Å
NBfilter INSTR/TEL 3σdetlim 3σdetlim Aperdiam(”) fractionalsignalofPS PSF
(FWHM)
(1”radius) (PSF/2radius) highestS/NofPS withinAperdiam
NB3727 MOSAICII/CTIO4m 26.19 26.71 1.4” 0.390 1.4”
(50Å)
NB-L-387 SupCam/Subaru 26.90 28.20 0.8” 0.374 0.74”
(110Å)
NB388 FORS1/VLT 27.64 29.08 0.8” 0.338 0.80”
(37Å)
NB396 WFI/LaSilla2.2m 26.53 27.43 1.0” 0.398 0.94”
(129Å)
narrow-band magnitudes of CANDELS stars by running the Table2:Numberofstar-forminggalaxieswithrobustredshift
IRAFtaskPHOT(apertureradiusequalto5”)onournarrow-
bandimages.Thenarrow-bandzeropointswasthencalculated NBfilter zrange N.galwithin N.galwithin N.galin
CDF-S zrangea NBarea cleanregions
as the difference between the true and the observed narrow- NB3727b 3.11<z<3.30 88 88 45
bandmagnitudes.WeappliedaGalacticcorrectionoftheorder NB387 3.33<z<3.43 29 21 7
of0.02-0.03magnitudes(Schlafly&Finkbeiner,2011)forthe NB388 3.28<z<3.49 68 22 9
NB396 3.41<z<3.53 59 47 25
reddest and bluest filter, respectively, at E-CDFS coordinates.
Thezero-pointerrorisabout10%. Notes. a Some of the sources corresponding to different NB ranges
overlap.
bThenumberincludescontinuum-selectedstar-forminggalaxiesand
2.2. SpectroscopicsampleandHSTcoverage narrow-bandselectedLAEs(19incleanregions).
A large number of measured redshifts is available in the ex-
tended Chandra Deep Field South (E-CDFS). We considered
previouslystudiedbyVanzellaetal.(2010a,2012b,c),includ-
redshiftsofcontinuum-selectedstar-forminggalaxiesfromthe
ing the LyC candidate, Ion2 (Vanzella et al., 2015), which is
ESOGOODS/E-CDFSmastercatalogue1,includingthecom-
at the redshift covered by our shallowest narrow-band image,
pilations of ESO-GOODS/FORS2 (Vanzella et al., 2008, and
however.
referencestherein)andESO-GOODS/VIMOS(Popessoetal.,
GOODS-SispartofCANDELS3andthuseverysourcehas
2009; Balestra et al., 2010, and references therein). We in-
multi-wavelengthphotometry,fromVIMOSU andB,through
cludedredshiftsfromtheGOODS-MUSIC(Multi-wavelength
HST F435W to F160W, through HUGS (Hawk-I UDS and
southern infrared catalog, Grazian et al., 2006; Santini et al.,
GOODSsurvey)KstoSpitzerIRAC8.0µmbands.
2009),CANDELS(cosmicassemblynear-infrareddeepextra-
The HST data in E-CDFS were obtained within GEMS
galactic legacy survey, Grazian et al., 2015) surveys, and the
(GalaxyEvolutionfromMorphologyandSpectralenergydis-
most recent compilation used in this paper. We also incorpo-
tributions,Rixetal.,2004)inHST/ACSF606WandF850LP.
rateddatafromVUDS(VIMOSUltra-DeepSurvey,LeFe`vre
Multi-wavelengthphotometryisalsoavailablefortheextended
et al., 2015) using the VUDS Data Release 1 (Tasca et al., in
area,anditincludesHSTandground-basedfluxes(Cardamone
preparation), which encompasses about one third of the final
etal.,2010;Hsuetal.,2014).
sample, and a few redshifts from the test run of VANDELS2
InTable2welistthenumberofgalaxiesinitiallyselected
(a deep VIMOS survey of the CANDELS UDS and CDFS
in the corresponding redshift bin for each narrow-band filter,
fields),arecentlyapprovedpublicsurvey(PIL.Pentericciand
chosenaccordingtothenarrow-bandfiltertransmissioncurves;
R. McLure). In all cases we only considered the most secure
thenumberofgalaxieswithintheareacoveredbyeachnarrow
redshifts.Forexample,forVUDSweusedthereliabilityflags
band; and the number of galaxies in clean regions (see Sect.
3 and 4, corresponding to a probability greater than 95% for
3.1).
theredshifttobecorrect,supplementedbyflags2and9,cor-
responding to a probability of about 80% for the redshift to
becorrect,onlywhenthephotometricredshiftagreeswiththe
3. MeasuringtheLyCsignal:method
measured spectroscopic redshift (Le Fe`vre et al., 2015). We
finallyaddednarrow-bandselectedLyαemitters,spectroscop- Inthissection,wepresentthemethodusedtoestimatetheLyC
icallyconfirmedduringMUSYC(Multi-wavelengthsurveyby escapefraction.
Yale-Chile, Gawiser et al., 2006) follow up. Throughout the Vanzellaetal.(2010b)discussedindetailtheroleoffore-
paper,weusetheacronymSFGsforcontinuum-selectedstar- ground contamination in estimating the LyC radiation from
forminggalaxiesandLAEsforLyαemitters. galaxies at z ≥ 3. To reduce the contamination from nearby
ThecataloguesbyXueetal.(2011)andFioreetal.(2012) sources, we performed a cleaning procedure (Sect. 3.1). We
wereusedtoidentifyX-raydetectionsandtoisolateAGN. measured LyC fluxes only for sources that passed the three
We compiled an initial sample of about 200 sources at stepsoftheprocedure,andweadoptedasnon-ionizingfluxthe
3.11 < z < 3.53inE-CDFS.Afew(∼ 15)ofthesourceswere HST/ACS F606W band flux from the CANDELS and GEMS
catalogues. This band covers the UV rest-frame wavelength
1 http://www.eso.org/sci/activities/garching/projects/goods/ of 1400 Å at the redshift considered here. The LyC flux was
MasterSpectroscopy.html
2 http://vandels.inaf.it 3 http://candels.ucolick.org/
4
L.Guaita:LyCsignal
measured by running Source Extractor (SExtractor, Bertin & within the size of narrow-band PSF full width at half maxi-
Arnouts, 1996) on the narrow bands at pre-defined positions. mum around the centroid of the candidates. In total, this first
SExtractor configuration parameters for background, source stepofthecleaningremoved70ofthe178inoursample.An
extraction, and optimal photometry were obtained following exampleofasourceremovedduringthisstepisshowninFig
Guaitaetal.(2010).Optimalaperturediametersweredefined 3.
asthosethatallowedthehighestsignal-to-noiseratioforpoint Becausethesuccessorfailureofthefirststepforourpur-
sources, and the corresponding fractional-to-total signal was posesdependssensitivelyonthedeblendingthatissettogen-
calculatedfromthecurveofgrowthofbrightstars. eratethevariouscatalogues,thesecondstepofourprocess is
Narrow-bandphotometrycanbeperformedwithtwometh- the direct inspection of the HST images. For galaxies that al-
ods. In the first method, we can optimize SExtractor detec- low it, we inspect cut-outs in all of the available CANDELS
tion and build narrow-band detection catalogues by adopting HSTbands4Fortheremainderofoursample,weinspectedthe
optimized configuration parameters. We can then search for HST/ACSV606andHST/ACSF850LPfromGEMS.Thepur-
matchesbetweenourtargetsandallthesourcesdetectedinnar- poseofthisinspectionistoseparateisolated,single-component
row band. This method was used in the literature to take into sourcesfromthosewithmultiplecomponents.Theformerare
accountapossibleoffset(observedupto10kpc)betweenion- retained in our sample without further scrutiny, the latter are
izing and non-ionizing emissions (e.g. Mostardi et al., 2013). flaggedandareanalysedinthethirdstageofourcleaningpro-
Theseoffsetscouldbeobservableifspecialchannelsforthees- cess.
capeofLyCphotonsareopenedinanoff-centrestar-formation Thefinalstageofourcleaningprocedurebeginswithgen-
clump. But the method is sensitive to the contamination by eratingcolourimagesofeachmulti-componentsourceselected
nearbysources,whichcouldbelocatedexactlyintheposition in the previous step and examining each sub-component. We
of a matched object. By applying it before cleaning the sam- use the IRAF task IMSTACK to generate colour images in
ple,low-zsourceswithintheground-basedPSFareaareindeed BVi, BVz, BIz, VIz, and V-z for the galaxies that were im-
identifiedasmatches. agedaspartofCANDELS.ForsourcesoutsidetheCANDELS
In the second method, which we adopted here, we mea- area,onlyV-zimagesweregenerated.Anindividualknotwith
surenarrow-bandfluxatthepositionofthesourcewithsecure appreciably different colours with respect to the main galaxy
redshift (e.g. Vanzella et al., 2010a; Boutsia et al., 2011), by componentislikelytobeaninterloper,whichmeansthatgen-
running SExtractor in double mode. As a detection image we erating these images allows us to reject these contaminated
create a narrow-band image by injecting artificial bright stars sources from the sample (Vanzella et al., 2012a; Siana et al.,
(IRAF artdata.mkobject) at the position of the sources. The 2015).However,thesituationisslightlymorecomplexassub-
original narrow-band images are then used for aperture pho- components (hereafter clumps) belonging to the same galaxy
tometry.Wechosetheoptimizedapertures(Table1)thathave canberedderorbluerthanthemaincomponentofthegalaxy,
the advantage to be small enough to additionally reduce the depending on the properties of their stellar populations, dust,
contaminationfromnearbysources. and the presence and strength of emission lines falling within
The optimized-aperture flux was translated into the total the broad-band filters. For this reason, we performed an SED
flux by dividing it by the fractional signal (Table 1). This to- fittingofindividualclumpsforeightstar-forminggalaxiesfor
tal flux is the LyC flux we refer to throughout the paper. The which the analysis was possible (i.e. within the CANDELS
observedfluxratioforeverysourcewasthencalculatedasthe area,seeAppendixAfordetails).Foreachclumpweobtained
ratiobetweenLyCandHST/ACSF606Wbandfluxdensities, thebest-fitphotometricredshiftandcheckedwhethertheSED
fν(900). was consistent with the spectroscopic redshift coming from
fν(14T00h)e ratio was corrected for the effect of the IGM by di- the integrated spectrum of the dominant source. We find that
vidingitbytheIGMtransmissivity,exp(-τ ).Fromtheob- clumpswithcoloursverysimilar(∆(B-V)<0.1,∆(V-z)<0.1)
IGM,z
servedfluxratio,wecanestimatethefesc(LyC)relativetothe to that of the main component are always consistent with be-
intrinsicvalue, ing at the spectroscopic redshift. However, we also find three
cases where, despite relatively large colour offsets (∆(B-V) ∼
fescrel(LyC)= L (1400)/L (900)fν(900)/fν(1400), (1) 0.4 and ∆(V-z) ∼ 0.2), the SEDs of the clumps are consistent
ν ν exp(−τ ) withtheclumpsbeingatthesameredshiftastheirmaincom-
IGM,z
ponent. Three of the eight multi-component objects were re-
wheretheunknownsaretheintrinsicLν(1400)/Lν(900)and movedfromoursamplebecausetheyaresurroundedbydiffer-
exp(-τ )(Sect.3.2).
IGM,z entredshiftneighbours.Thisprovestheimportanceofhaving
Inthefollowingsubsections,wedescribethecleaningpro-
completeHSTphotometrytostudyLyCcandidates.
cedureindetailandderivetheunknownsofEq.1.
For sources that are only covered by GEMS data, we
were unable to perform the complete SED fitting analysis.
3.1. Cleaningprocedure Therefore,wereliedoncolourcriteriatoestimatethecontami-
nationforthesesources(∆(V-z)<0.2).Theanalysispresented
We applied a procedure to identify the final sample of galax-
inAppendixAsuggeststhatrelyingoncoloursalonemaybea
iesforwhichweperformedmeasurementsofLyCescapefrac-
tooconservativeapproachbecauseittendstooverestimatethe
tionbyremovingthegalaxieswithpossibleprojectedcontam-
number of contaminated sources. Despite this, we decided to
inationonanobject-by-objectbasis.Westartedthisprocedure
adopt it because the purity of our sample is our primary con-
withthetotalsampleof178galaxiesselectedinSect.2.2.Of
cern.Thepresenceofcontaminantsisthemostworryingaspect
these, 35% of the SFGs are only covered by GEMS imaging
inthesearchforLyCemitters(Vanzellaetal.,2012a;Mostardi
and 65% fall within the area covered by CANDELS; 70% of
etal.,2015).
theLAEsfalloutsidetheCANDELSarea.Thecleaningproce-
durewasperformedinthefollowingsteps. 4 Throughout the paper, we abbreviate the HST ACS/F606W and
We first checked that no other source in the F850LPbandsasV606andz850,respectively,andtheACS/F435W
CANDELS/GEMS photometric catalogue is centroided andF814WasB435andI814,respectively,forsimplicity.
5
L.Guaita:LyCsignal
Fig.3:Exampleofasourcerejectedinthefirststepofourcleaningprocedure,objectID CANDELS=5164(z = 3.403).Everystampcovers
6”x6”.Fromthetopleftweshowthefollowingfiltercut-outs:B435,V606,I775,z850,J125,H160,CANDELSdetectionsegmentationmap,
andNB387.ThegreencirclehasaradiusequaltothePSFinNB387.ObjectID CANDELS=5197(ontheleft,zphot CANDELS=2.00±0.05)
wouldbeblendedwithID CANDELS=5164andID CANDELS=5166(objectatthebottomright,zphot CANDELS=0.47±0.13)inaground-
basedimagewithPSFaslargeasthatofNB387.IntheNB387stampweseeemissioncomingmainlyfromthelower-zsourceontheleft-hand
sideofourtarget.
Fig.4: Examples of sources rejected in the third step of our cleaning procedure. Left: BIz colour cut-outs of a narrow-band selected LAE
(ID CANDELS=23527,z = 3.116).ThegreencirclehasaradiusequaltotheNB3727PSFfullwidthathalfmaximum.Thestampcovers
about4”x4.5”.Seetheappendixforthedetailsoneachclump.Right:ObjectID ECDFS=42881(z=3.16).ItisfoundintheGEMScatalogue.
FromtheleftV,z,andV-z,stampstakenfromGEMSimages.Thestampscoveranareaofabout2”x1.5”.Theredcontoursrepresentlevels
equalto4,7,and10timesthevalueofbackground.ThisAGNisexcludedfromoursamplebecauseasourceliesontopofit.
After imposing the colour cuts discussed above, we ex-
cludedanadditional22sources.Figure4showsanexampleof
a narrow-band-selected LAE rejected at this step. The V- and
z-band images and a V-z colour image obtained from GEMS
imagingforanAGNexcludedduringthissteparealsoshown
inFig.4.Itisclearfromtheinspectionoftheimagesthatthere
isanothercomponentinadditiontothemainsource.Thissec-
ond source contaminates the narrow-band (PSF full width at
half maximum equal to 1.4(cid:48)(cid:48)) flux of the main source, which
wasclassifiedasAGNaccordingtothecriteriareportedinHsu
etal.(2014).Figure5showsanexampleofamulti-component
sourcekeptinthesamplebecauseitiscomposedoftwoclumps
withsimilarcolours(seealsoAppendixA).
The number of objects left in our sample from the Fig.5:Exampleofasourcekeptinthethirdstepofourcleaningpro-
cleaning procedure is 86 galaxies (their distribution in cedure,objectID CANDELS=6839(z = 3.2078).Fromthetopleft
CANDELS/GEMSareasislistedinTables4and5).Fromhere BVi, BVz, BIz, VIz stamps taken from CANDELS mosaic images.
Thestampscoveranareaofabout2”x1.5”.Seetheappendixforthe
on,werefertothissampleasthecleansample.Forsourcesat
detailsoneachclump.
a redshift sampled by more than one narrow band, we adopt
the deepest narrow-band image to attempt to measure possi-
bleLyCsignal.WealsostudyeightAGNincleanregions(see
Sect.4.2).
6
L.Guaita:LyCsignal
3.2. IntrinsicL (1400)/L (900)ratioand Table3:Intrinsicnon-ioniziangversusionizingradiationratio
ν ν
intergalactic-mediumtransmissivity
IMF SFH Z age Lν(1400)/Lν(900) EvolTrack,StelAtm
We now proceed to estimate the intrinsic L (1400)/L (900) Z(cid:12) Myr
ν ν BC03 Padova94,KBFA
value and the intergalactic-medium transmissivity, exp(- Chab cSFR 0.02 1 1.76
Chab cSFR 0.02 10 3.41
τIGM,z),at3.11<z<3.53. Chab cSFR 0.02 100 5.97
The effective wavelength of the V606 filter corresponds CChhaabb τ=cS0F.1RGyr 00..0022 1100000 77..0160
to λ = 1400 Å for the redshift covered on average by our Chab τ=−0.1Gyr 0.02 100 5.27
Chab cSFR 1 1 1.94
narrow-bandfilters.TheintrinsicL (1400)/L (900)ofgalaxies Chab cSFR 1 10 4.28
ν ν Chab cSFR 1 100 6.43
dependsoninitialmassfunction(IMF),star-formationhistory Chab cSFR 1 1000 6.86
(SFH),metallicity(Z),stellar-populationage,andstarproper- Salp cSFR 1 100 6.71
BPASSv2 single+binary,BaSeLPoWR
ties.ForAGN,itdependsonthehardnessandtheobscuration Salp cSFR 1 100 4.62
oftheirspectrum.FormixedSFG/AGNpopulations,itdepends Starburst99
Salp cSFR 1 100 5.24 Padova94,PH
on the relative contribution of the two components and on all Salp cSFR ∼1 100 4.62 GenevaV00,PH
theothereffectlistedabove. SSaallpp ccSSFFRR ∼∼11 110000 33..2683 70%RGOeTn+ev3a0V%4n0o,PRHOT,PH
Table 3 summarizes the intrinsic ratios between non- Stel+AGN
2.76 AGN-dominated
ionizing and ionizing flux densities for a variety of galaxy 7.04 obscuredAGN-dominated
physical properties and stellar population models. Since the 2.66 AGN-dominated
2.35 theoreticalAGN(1)
emissionintherest-frameUVcanstronglydependonthestel-
lar evolutionary tracks and on the stellar atmosphere assump- Notes.IntrinsicLν(1400)/Lν(900)ratiosobtainedfromdifferentstel-
larpopulationmodels:theBC03,assumingdifferentinitialmassfunc-
tions,weexploretheintrinsicratioadoptingtheBC03(Bruzual
tions (Salpeter, Salp, and Chabrier, Chab), star-formation histories
&Charlot,2003),theStarburst995(Leithereretal.,1999),and
(constant,cSFR,exponentiallydecliningwithτ = 0.1Gyr,exponen-
theBPASS6(binarypopulationandspectralsynthesis,Stanway tially rising with τ = −0.1Gyr), metallicties (Z and 0.02×Z ), and
(cid:12) (cid:12)
etal.,2016)codes.
stellar-population ages; the BPASS (Binary Population and Spectral
Stars with rotations that are located in binary systems Synthesis code Stanway et al., 2016); the Starburst99 models. We
can produce more ionizing photons than is expected for non- considerarangeofwavelengthsaround900Åand1400Å,compa-
rotational, isolated stars (e.g. Levesque et al., 2012; Eldridge, rabletothatcoveredbythenarrow-bandandtheHST/ACSF606W
2012). Therefore, they can in principle reduce the intrinsic filters in the rest frame, to estimate the average L . As described in
ν
L (1400)/L (900)value. Bruzual & Charlot (2003), BC03 provides the spectral energy dis-
ν ν
Evolutionarytracksofstarswithrotationareimplemented tributionofstarsobtainedfromacomprehensivelibraryoftheoreti-
cal model atmospheres (KBFA in the table). The library consists of
inStarburst99.However,asexplainedinLeithereretal.(2014),
Kurucz(1996)spectraforO-Kstars,Besselletal.(1991)andFluks
those models consider rotational velocities that might be too
et al. (1994) spectra for M giants, and Allard & Hauschildt (1995)
extreme. A more realistic experiment might be obtained by
spectraforMdwarfs.InBPASSthestellarevolutionarytrackscontain
considering 70% of evolutionary tracks with high rotational
acontributionfromisolatedstarsandbinarysystems.Also,thestellar
velocity and 30% of evolutionary tracks without rotation atmospheremodelsareselectedfromtheBaSeLv3.1library(Westera
(70%ROT+30%noROT in the table, Levesque et al., 2012). et al., 2002), supplemented by Wolf-Rayet stellar atmosphere mod-
Weestimatedtheintrinsicratiosbyvaryingevolutionarytracks els from the Potsdam PoWR group (Hamann & Gra¨fener, 2003).
(from the Padova1994 tracks to the Geneva tracks with the With Starburst99, it is possible to generate SEDs assuming a bunch
GenevaV40 and without the GenevaV00 implementation of of stellar evolutionary tracks, including stars with and without rota-
stellarrotations,Leithereretal.,2014),andwealsoperformed tion(GenevaV40andGenevaV00respectively,Leithereretal.,2014),
andstellaratmospheres(thecombinationofmodelatmospherefrom
sometestsbyvaryingmetallicity,IMF,andstellaratmospheres.
Pauldrach et al., 1998; Hillier & Miller, 1998, is the recommended
AspointedoutinLeithereretal.(2014),50%ofthemas-
option).The70%ROT+30%noROTentryindicatesamodelthatisa
sive stars are in binary systems and experience evolutionary
combinationofGenevaV40forthe70%andGenevaV00forthe30%
processes that might alter the expected number of ionizing
(Levesqueetal.,2012).TheGenevaV40andGenevaV00trackswere
photons.Binaryevolutionalsoextendstheperiodofionizing- releasedformetallicityequaltoZ=0.014(∼Z ,Eldridge,2012).The
(cid:12)
photon production, mainly at low metallicities, and so it is changeintheintrinsicratioduetothedifferentevolutionarytracksand
worth considering them in the studies of the re-ionization of stellaratmospheresisshownforaSalpeterIMF,Z ,cSFR,and100
(cid:12)
theUniverse.AsdescribedinStanwayetal.(2016,andrefer- Myroldstellarpopulation.Inthebottompartofthetable,wereport
encestherein),theincreaseintheproductionofionizingpho- theratioscalculatedfromthebestfittemplatesofAGN(Bongiorno
tons that is due to binary systems is significantly lower than etal.,2012),inwhichtheSEDsaredominatedbytheradiationcom-
ing from an (un-)obscured AGN. (1) The intrinsic L (1400)/L (900)
thatproducedbyrotationmodels,suchastheGenevaV40.We ν ν
ratioforanunobscuredTypeIIAGNcanbeaslowas2.35(Richards
followed Nestor et al. (2013) and compared the previous in-
etal.,2006).Intheestimationoftheratioswetakeintoaccountthe
trinsic ratio estimations with those from BPASS version v2.0
HIabsorptionoccurredinstaratmospheres,butweneglectthatwithin
(SalpeterIMF,constantSFR,Z ,100Myrage).
(cid:12) theinter-stellarandcircum-galacticmedium.
Our tests show that the strongest change in the intrinsic
L (1400)/L (900)ratioisduetotheageparameter.Variations
ν ν
with metallicity are only lower than 10% in BC03. Different
star-formation histories create variations of the order of 20- (Padova94 vs GenevaV00) produces variations of 20% at the
30%. sameage,whileincludingrotationmodelsreducestheintrinsic
Including binary stars in BPASS produces changes in the ratioof30(70%ROT+30%noROT)to40(GenevaV40)percent.
intrinsic ratio of up to 60% with respect to BC03 for ages of ThevariationcausedbytheIMFisnegligiblewithinBC03and
100Myr.WithinStarburst99,changingtheevolutionarytracks is up to 20% in general. Finally, the change of stellar atmo-
spheremodelscausesL (1400)/L (900)toincreaseordecrease
ν ν
5 http://www.stsci.edu/science/starburst99/docs/parameters.html by less than 20% within Starburst99 and between BC03 and
6 http://www.bpass.org.uk/ Starburst99.
7
L.Guaita:LyCsignal
In conclusion, as a result of different evolutionary tracks
and stellar-population models, the intrinsic ratio can present 1.0
a 60% uncertainty. In particular, for a Salpeter IMF, constant
SFR,solarmetallicity,and100Myrage,itcanvaryfrom3to
0.8
6,whileforagesofafewhundredsofMyr,itcanincreaseupto
8.Wewouldliketostressthattheintrinsicratioisamultiplica-
tiveconstantintheexpressionofthefescrel(LyC).Bychanging 0.6
itsvalue,itispossibletosimplyre-scalethefescrel(LyC),de- τe−
pendingontheassumptions,tocomparewithotherworks.For 0.4
thisreason,weassumedanintermediatevalueof5asrepresen- z=3.20, <exp(-τ)>=0.41
tativeforourentiresampleofstar-forminggalaxies.Thevalue z=3.38, <exp(-τ)>=0.34
0.2
of5isstrictlyrelatedtotheassumptionsofaSalpeterIMFand z=3.47, <exp(-τ)>=0.35
constant SFR, which is most frequently used in the literature z=3.39, <exp(-τ)>=0.35
0.0
andistheeasiesttobecomparedwith. 3000 3500 4000 4500 5000 5500 6000 6500 7000
observed-frame λ[Å]
Wealsocalculatedtheintrinsicratioscorrespondingtothe
physical parameters listed in Tables 4 and 5. To estimate the Fig.6: Mean IGM transmissivity as a function of wavelength at the
galaxy physical parameters in the tables, we considered the redshiftcorrespondingtothecentreofournarrow-bands(Inoueetal.,
BC03 models (Chabrier IMF) because they are among those 2014). Solid, dotted-dashed, dashed blue, and green curves indi-
thatbetterreproducethemajorityofthegalaxypropertiesboth catetheaverageredshiftssampledbyNB3727,NB387,NB396,and
in the UV and in the NIR (Zibetti et al., 2013, and references NB388,respectively.
therein).TheywellreproducetheentireSEDofintermediate-
massgalaxies,likethosestudiedhere(Santinietal.,2015).In
addition to this, with the BC03 code we are able to consider
24 24
cinocnlsutdaentq,uriitseinagw, aidnedvdaercielitnyinogfhsitsatro-rfioersm(athtieoynahlsisotomriiemsi,cwmhoicrhe 2265 NNUvBBi33m77o22s77,,, LLL111444000000///LLL999000000===115 2265 NNBB338877,, LL11440000//LL990000==15
thanUonndeesritnhgeleaspsoupmulpattiioonn)o.faBC03decliningtaumodel,the LyC mag222897 LyC mag222897
rangeofintrinsicLν(1400)/Lν(900)ratiosis4-16forthegalax- 30 ffeesscc((LLyyCC))==15000%% 30
ies(SFGsandLAEs)studiedhere,characterizedbyamedian 31 fesc(LyC)=30% 31
fesc(LyC)=10%
ageof300Myr. 3228 27 26 25 24 23 3228 27 26 25 24 23
V606 V606
We refer to Bongiorno et al. (2012) to infer the expected 24 24
L (1400)/L (900)ratioforAGN.Theypresentedtheobserved 25 NNBB338888,, LL11440000//LL990000==15 25 NNBB339966,, LL11440000//LL990000==15
ν ν 26 26
SttyiEopnDic(satlhoefteiurmnF-piolgabsts.ecs4urvaeandrdya2na)ds.oaTbhrseecsuluarlertgdoeAfstGthpNeoshasnoibdslteaglAsaolGasxNhyocwoobensdtcrhuiboruaw-- LyC mag222897 LyC mag222897
tionintheLyCregimeproducesaL (1400)/L (900)=2.35.A 30 30
ν ν 31 31
typical AGN-dominated source can have a ratio of 2.6. Since 3228 27 26 25 24 23 3228 27 26 25 24 23
theobservedvaluesalreadycontaintheIGMcontribution,we V606 V606
adopt 2.35 throughout the paper. Sources in which the galaxy
emissiondominatesthatfromtheAGNareexpectedtopresent Fig.7: Expected LyC mag as a function of the magnitude in the
L (1400)/L (900) ratios similar to those calculated for galaxy V606 band. For each narrow-band, we consider the redshift corre-
ν ν
sponding to 890 Å at the centre of the filter. Thick (thin) lines cor-
templates.
respondtothecalculationsundertheassumptionoffesc(LyC)=100%
By following the analytical prescription by Inoue et al. (10%),whilethecyan(black)linesindicateL (1400)/L (900)=1(5).
(2014), we determine τ (Fig. 6). In the range of redshift ν ν
IGM,z Horizontaldotted-dashedlinesarelocatedatthe3σdetectionlimitof
studiedhere,exp(-τIGM,z) (cid:39) 0.36.Eventhoughweconsidered eachnarrow-bandfilter.Thereddashedlineindicateswhatisexpected
narrow-band filters, we derived the average transmissivity by iftheLyCmagismeasuredintheVIMOSUbroad-band.
applyingtheequation
(cid:82)
exp(−τ )T(λ)dλ In this exercise, we considered a LyC escape fraction
<exp(−τIGM)>= (cid:82) IGM , (2) of 10-100%, a galaxy template characterized by an intrin-
T(λ)dλ sic L (1400)/L (900) value between 1 and 5, and an average
ν ν
treatment of the IGM lines of sight. We estimated narrow-
where T(λ) represents the narrow-band filter transmission bandmagnitudes(LyCmag)asafunctionofV606,asshown
curve. Measuring LyC for a number of sources allowed us to in Fig. 7. Given the depth of the available data, we expect
use a mean value for <exp(-τIGM) >, such as that provided more significant detections or upper limits from the NB388
by the analytical prescription by Inoue et al. (2014). We refer filter. For a LyC emitter at z = 3.4, characterized by an
tothispaperforunderstandingthelimitationsanduncertainty fesc(LyC)=100% and V606=25 (the average magnitude of
relatedtotheapplicationofthisprescription.SeealsoThomas our sample of continuum-selected SFGs), we expect a LyC
et al. (2014) as an example of a study of IGM variability for (NB388) mag = 25.5-27.5 (depending on the assumptions on
sourcesatthesameredshift. theintrinsicL (1400)/L (900)value).Thisisbrighterthanthe
ν ν
Finally,weperformedacalculationtodemonstratethatwe 3σ detection limit of the narrow-band image and therefore in
couldexpectsomedirectLyCmeasurementfromthesourcesin principle detectable. For fesc(LyC)=30%, we expect a LyC
oursample,giventheirUV-continuummagnitude(V606)and (NB388)mag=26-28,wichismoredifficulttobedetecteddi-
thedepthofournarrow-bandfilters. rectly.WemightalsoexpectaLyC(NB387)mag=25.6-27.6
8
L.Guaita:LyCsignal
(26.3-28.3) for an fesc(LyC)=100% (30%). For the sources
with LyC emission covered by NB396, an fesc(LyC)=100%
and V606=25, we expect a LyC (NB396) mag = 25.7-27.7, 12
barely above the narrow-band detection limit. On the other
hand,foragalaxyatz=3.2weexpectaLyC(NB3727)mag= 11
25.4-27.4,whichisevenbelowtheNB37273σdetectionlimit.
GalaxiesthatarecharacterizedbyanSEDsuchastheoneas-
10
sumed in this exercise would show a LyC signal in NB3727
)
(fesc(LyC) > 30%) if they were brighter than V606=24. The ⊙
M
same calculation performed with the VIMOS U broad band M/ 9
would produce a LyC mag up to 0.6 magnitude fainter than (
g
thevaluecalculatedforNB3727.Thisdemonstratestheadvan- o
l
tage of using narrow instead of broad bands in the LyC mea- 8
NB3727
surements.OfcoursetheU-bandestimationcanchangeifthe
NB387
broadbandissignificantlydeeperthanthenarrowband. 7 NB388
NB396
NB3727_LAE
4. Physicalpropertiesofthecleansample
6
−23 −22 −21 −20 −19 −18
M1400
We cross-correlated our sources in the clean sample with the
E-CDFS multi-wavelength photometric catalogue (Guo et al.,
Fig.8:Stellarmassvsrest-frameUVabsolutemagnitude.Thesym-
2013; Hsu et al., 2014). In this way, we associated a multi-
bolsindicateSFGsintherangeofredshiftcoveredbyNB3727(red
wavelengthSEDfromtheopticalU totheSpitzerbandswith
circles),NB387(redtriangles),NB388(redsquares),NB396(reddi-
everysource,thankstoSEDfitting,physicalparameters,such
amonds),andLAEs(redstars).TheLAEsfoundinGEMScatalogue
asstellarmass(M∗),star-formationrate(SFR),dustreddening
butnotintheE-CDFScataloguefromHsuetal.(2014)arenotshown
(E(B-V),andstellar-populationage. inthisplot.
4.1. Star-forminggalaxies
is obtained from the equations in Sect. 5, assuming an in-
We adopted the physical parameters quoted in Santini et al. trinsic Lν(1400)/Lν(900)AGN = 2.35 (Bongiorno et al., 2012,
(2015) for the galaxies entering the CANDELS area. For and Table 3) and the spectroscopic redshift to account for the
the sources in E-CDFS, we estimated M∗, SFR, E(B-V), and IGM opacity. We also report the hardness ratio for the AGN
stellar-populationagestartingfromtheHsuetal.(2014)pho- in the Xue et al. (2011) catalogue. Some of these AGN may
tometry,thespectroscopicredshift,andwiththesamecodeas be obscured and/or affected by broad absorption region, as
inSantinietal.(2015),adoptedforCANDELS. werethosediscussedbyCivanoetal.(2012)intheCOSMOS
InTables4and5,wereportthephysicalparametersconsid- field. Moreover, the presence of intervening damped Lyman-
eredhere.Forreference,wereporttheIDsfromtheCANDELS alphaorLymanlimitsystemscloseto(orassociatedwith)the
andHsuetal.(2014)cataloguesforeverysource.Itisimpor- AGNcanabsorbtheLyCradiationanddecreasetheirmeasured
tant to note that our sample covers quite a wide range in UV fesc(LyC).Adedicatedworkontheconnectionbetweenobscu-
absolute magnitude (−21.5 (cid:46) M1400 (cid:46) −18.5), stellar mass rationandLyCemissionisongoing.
(108 M (cid:46)M∗ (cid:46)5×1010),star-formationrate(1(cid:46)SFR(cid:46)50
(cid:12)
M yr−1), specific star-formation rate (10−9.5 (cid:46) sSFR (cid:46) 10−7
yr−(cid:12)1), dust reddening (0 (cid:46) E(B-V) (cid:46) 0.2), and stellar popula- 5. Results
tion age (7 (cid:46) log(age/yr) (cid:46) 9.3). Some of the LAEs are too InTables4and5wepresenttheobservedf (900)/f (1400)flux
ν ν
faintinthecontinuumtoentertheCANDELS/E-CDFSphoto- ratiosforSFGsandLAEs,andthe1σphotometricerrorsonthe
metriccatalogue.Therefore,wehaveastellarmassestimation measurements. Figure 9 shows the observed f (900)/f (1400)
ν ν
foronly11ofthe19LAEs. fluxratioasafunctionofV606bandmagnitudeforeachfilter.
Figure8showsstellarmassasafunctionofrest-frameUV In NB3727 we measure LyC signal from continuum-selected
magnitude. The sub-sample of Lyα emitters occupies the re- star-forminggalaxies,narrow-bandselectedLyαemitters,and
gionwiththelowestmassandthefaintestmagnitude.Werefer AGN.InNB387andNB396forSFGsandAGN,andinNB388
to Fig. 7 in Grazian et al. (2015) to compare our sample with onlyforSFGs.ThemeasuredLyCfluxesofallthecandidates
all the sources in GOODS-S. In a similar redshift range, our arewithinthe2σbackgroundlevel,exceptforoneAGN.This
sample occupies the bright-magnitude and high-mass tail of source,identifiedbyID=78,isdetectedinNB396atmorethan
the general distribution. This is expected because the sources 3σ.Inaddition,theAGNidentifiedbyID=9isdetectedat∼2σ
areallspectroscopicallyconfirmed. inNB3727(Fig.10).
Previous works in the literature have shown that LAEs
are characterized by a stronger LyC signal than continuum-
4.2. Activegalacticnuclei
selected star-forming galaxies. For instance, Mostardi et al.
WematchedourinitialsamplewithAGNcatalogues.Wechose (2013) found an observed f (900)/f (1400) of 0.14 for LAEs,
ν ν
theXueetal.(2011)X-raycatalogueandtheHsuetal.(2014) largerthanthevaluetheyobtainedforLBGsof0.01(seealso
AGNphotometriclist.InTable6wepresenttheAGNmatches. the recent work by Micheva et al., 2015). We do not find any
Most them have a counterpart in X-ray (Xue et al., 2011; significant detection for any of the LAEs. One of the reasons
Fiore et al., 2012). In the table we show their absolute mag- might be that the NB3727 image is the shallowest, character-
nitudes, the spectroscopic redshift, the LyC flux and the rel- ized by the largest PSF among the four narrow-band filters.
ative LyC escape fraction. The relative LyC escape fraction Theresultisalsopartiallyduetothesmallnumberofsources
9
L.Guaita:LyCsignal
NB3727(3.11<z<3.30) NB387(3.33<z<3.43)
1.5
0.4
1.0
0.2
0 0.5
0
4
1
f 0.0 0.0
/
0
0
9
f −0.5
−0.2
−1.0
−0.4
−1.5
23 24 25 26 27 28 23 24 25 26 27 28
NB388(3.28<z<3.49) NB396(3.41<z<3.53)
0.2
0.5
0.1
0
0
4
1
f 0.0 0.0
/
0
0
9
f
−0.1
−0.5
−0.2
23 24 25 26 27 28 23 24 25 26 27 28
V606 V606
Fig.9: Observed flux ratio, f (900)/f (1400), as a function of V606 band magnitude. From the top left to bottom right: spectroscopically
ν ν
confirmed sources entering the range of redshift covered by NB3727, NB387, NB388, and NB396. Dashed (dot-dashed)(dot-dot-dashed)
curvesindicatetheratiosbetween1(2)(3)σbackgroundrmsvaluesandV-bandflux.ThesymbolcodingisthesameasinFig.8.Blackdots
representtheeightAGNinoursample.
andtheirfaintaverageV606magnitude(27against25forthe galaxies in each narrow-band redshift range. By assuming
SFGs,seecalculationattheendofSect.3.2). L (1400)/L (900) = 5 and exp(-τ ) (Sec. 3.2), we derive
ν ν IGM,z
We conclude that our whole population of star-forming the following 1σ upper limits on the fescrel(LyC) for each
galaxies does not show any direct LyC signal. Moreover, the narrow-bandsub-sample(Eq.1),
1σ photometric errors on the observed f (900)/f (1400) flux
ν ν
ratios depend on the depth of the narrow bands in which the fescrel(LyCNB3727)<44%
ratios are measured. Since the LyC measurements come from
fourdifferentnarrowbandswithdifferentdetectionlimits,the fescrel(LyCNB387)<25%
contribution of the sample as a whole to the LyC emission
canbeestimatedasaweightedmean(LyC(SFGs)=wmean± fescrel(LyC )<17%
NB388
σwmean).Becauseofthelackofindividualdetections,wmean
is consistent with 0 and σwmean provides an upper limit of fescrel(LyC )<33%.
NB396
theLyCsignalofthewholesample.Bydefinition,σwmean=
1/(cid:113)(cid:80)N(1/w2),whereNisthenumberofsourcesconsideredin These values reflect the different depths of the indi-
i i vidual narrow bands. For the whole sample of 67 SFGs
pthheoctoamlcuetlraitciounnacnerdtathinetiwesei(gwhits=,w1/iσ,a2ir)e.relatedtotheindividual (=<5)z,w>e=ob3t.a3i9n7t,he<foelxlpo(w-τinIGgMt,hz)e1>(2=)σ0.u3p6p,eLrνli(m14it0:0)/Lν(900)
ConsideringthattheV606bandcoverstherest-frameUV
(λ ∼ 1400 Å) forthe entire range of redshift probedhere, we fescrel(LyC)<12(24)%.
also calculated the weighted mean of the V606 fluxes in the
estimation of the observed fν(900)/fν(1400) flux ratio for the As shown in Sect. 3, IGM transmissivity and intrinsic
wholesampleofSFGs. L (1400)/L (900) ratio correspond to multiplicative factors in
ν ν
Following the same approach, we estimate an upper thecalculationof fescrel(LyC).Inparticular,L (1400)/L (900)
ν ν
limit of the observed flux ratio for the sub-samples of the =5isobtainedassumingconstantSFR,SalpeterIMF,andage
10