Table Of Content

MNRAS000,1–6(2017) Preprint2February2017 CompiledusingMNRASLATEXstylefilev3.0 α Direct evidence for Ly depletion in the protocluster core Rhythm Shimakawa,1,2⋆ Tadayuki Kodama,1,2† Masao Hayashi,2 Ichi Tanaka,3 Yuichi Matsuda,1,2 Nobunari Kashikawa,1,2 Takatoshi Shibuya,4 Ken-ichi Tadaki,5 Yusei Koyama,1,3 Tomoko L. Suzuki1,2 and Moegi Yamamoto1 1Department of Astronomical Science,SOKENDAI, Osawa, Mitaka, Tokyo181-8588, Japan 7 2National Astronomical Observatoryof Japan, Osawa, Mitaka, Tokyo181-8588, Japan 1 3Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720, USA 0 4Institute for Cosmic Ray Research, The Universityof Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan 2 5Max-Planck-Institut fu¨rExtraterrestrische Physik, Giessenbachstrasse, D-85748 Garching Germany b e F Accepted2017January31.Received2017January18;inoriginalform2016December22 1 ABSTRACT ] A We have carried out a panoramic Lyα narrowband imaging with Suprime-Cam on Subaru towards the known protocluster USS1558–003 at z =2.53. Our previous nar- G rowbandimagingatnear-infraredhas identified multiple dense groupsof Hα emitters . h (HAEs) within the protocluster. We have now identified the large-scale structures p across a ∼50 comoving Mpc scale traced by Lyα emitters (LAEs) in which the pro- - tocluster traced by the HAEs is embedded. On a smaller scale, however, there are o remarkably few LAEs in the regions of HAE overdensities. Moreover, the stacking r t analyses of the images show that HAEs in higher-densityregions show systematically s lower escape fractions of Lyα photons than those of HAEs in lower-density regions. a [ These phenomena may be driven by the extra depletion of Lyα emission lines along ourlineofsightbymoreinterveningcoldcircumgalactic/intergalacticmediumand/or 1 dust existing in the dense core. We also caution that all the past high-z protoclus- v ter surveys using LAEs as the tracers would have largely missed galaxies in the very 0 densecoresoftheprotoclusterswherewewouldexpecttoseeanyearlyenvironmental 0 effects. 1 0 Key words: galaxies: formation – galaxies: evolution – galaxies: high-redshift 0 . 2 0 7 1 INTRODUCTION can becapturedbyoptical instruments.Thistechniquehas 1 been widely used to identify galaxies at high redshifts both v: High-zgalaxyprotoclusters(Sunyaev& Zeldovich1972)are in the general fields and in overdense regions such as pro- i idealtestbedswherewecanunderstandhowclustergalaxies X toclusters (Ouchiet al. 2003; Venemanset al. 2007). How- form and grow during the course of cosmic mass-assembly ever, we know that only a small fraction of star-forming r history and the build-up of large-scale structures (LSSs) in a galaxies show detectable Lyα emission lines (Hayeset al. the Universe (White& Frenk 1991; Cole et al. 2000). They 2010; Matthee et al. 2016; Hathiet al. 2016). Furthermore, directlyinform usofwhatisoccurringin theearlyphaseof theenvironmentaldependenceof Lyαemitters (LAEs) has cluster formation and galaxy formation therein,which then largely been unexplored yet. Therefore, understanding the tells us what the physical mechanisms are that lead to the dependence of physical properties and the selection effects galaxy diversity depending on the environment seen in the of LAEs across various environmentsis strongly desired. local Universe(Dressler 1980; Cappellari et al. 2011). The observational limitation due to the Earth’s atmo- In this respect, the dual emitter surveys of HAEs and sphere has created a gulf between high-z galaxy surveys at LAEs for the known protoclusters at z = 2.1–2.6 can play z <2.6 and those at z >2.6. At redshifts greater than 2.6, akeyrolein testingthe Lyαselection effect forhigh-z pro- bright Hαλ6565 emission line is no longer observable from toclustersearch.Thisletterstudies Lyαemissivitiesof Hα- ground-basedtelescopes,andthe Lyαλ1216lineisthemost emittinggalaxiesinaknowndenseprotocluster,USS1558– commonlyusedspectralfeatureofstar-forminggalaxiesthat 003 at z =2.53 (αJ2000 = 16h01m17s δJ2000 = −00d28m47s, hereafter USS 1558) discovered by Kajisawa et al. (2006); Kodama et al. (2007); Hayashiet al. (2012) with MOIRCS ⋆ [email protected] on the Subaru telescope. Our recent deep Hα narrowband † [email protected] surveyofthisregionsucceededindetectingmoreprotoclus- (cid:13)c 2017TheAuthors L2 R. Shimakawa et al. The observation was executed on June 10 in 2015 un- der a photometric condition but with a relatively bad see- 2 ing (FWHM=0.8–1.4 arcsec). The science frames with see- 8 2 ing sizes worse than 1.3 arcsec were trashed and we used 4 B 1 19 frames of 700 sec exposures each, amounting to 3.7 hrs N − of net integration time in total. The data were reduced in B exactly the same way as in Shimakawa et al. (2016) based 0 on a data reduction package for the Suprime-Cam, sdfred (ver.2;Yagi et al. 2002; Ouchiet al. 2004).Thepipeline in- 20 21 22 23 24 25 cludes the standard procedures. The sky subtraction was NB428 conducted with the mesh size of 13 arcsec. We additionally implemented a cosmic ray reduction using the algorithm, Figure 1.Thecolour–magnitude diagram.Theblackshowsthe L.A.Cosmic (van Dokkumet al. 2011). The final combined NB-detected sources. The purple circles indicate narrow-band image has a seeing size of FWHM = 1.24 arcsec, and the emittersthatmeetourcriteria,namely(1)thelineEWisgreater limiting magnitudeof 25.13 mag at 5sigma with 2.5 arcsec than >15 A˚inthe rest-frame(the horizontal line), (2) lineflux aperturediameter reckoningwith thegalactic extinction. excess is larger than 3σ (the black curve), and the detection at Combining the reduced NB428 data with the exist- NB428ismorethan5σ levels(theverticalline). ing counterpart B-band image (B5σ = 25.72 mag with 2.5 arcsec aperture diameter) provided by Hayashiet al. (2016),we select LAE candidatesin thesame manneras in termembersamountingto100HAEsintotal(Hayashiet al. Shimakawaet al.(2016).Here,theseeingFWHMofB-band 2016), allowing us to characterise sub-structures such as istunedtothatoftheNB428image.Theobjectphotometry clumpsassociatedtoUSS1558.Givensuchauniquelabora- is performed by SExtractor (ver.2.19.5; Bertin & Arnouts toryofoverdenseenvironmentatz=2.5,wehavemanufac- 1996).Photometricmeasurementsaredoneinthedoubleim- tured a dedicated narrowband filter for this specific target agemodeusingthenarrowbandimageforsourcedetections, andinstalleditontheSuprime-Camsothatwecanalsotar- and we employed aperture photometries with 2.5 arcsec di- get LAEs associated with this region. Also, the wider field ameter. This work imposes the selection criteria as follows; of view of Suprime-Cam (32’×27’) compared to MOIRCS (1) narrowband flux excess is greater than 3σ with respect (7’×4’) enables usto map LSSsin and around USS1558. tothephotometricerror,(2) Lyαequivalentwidthishigher ΩΛ =W0e.7asasunmdeht=he0c.7osamnodloegmicpallopyaaraCmheatebrrsieorf(Ω20M03=) s0te.3l-, thanEWLyα=15˚Aintherestframe,and(3)NB5σ <25.13 mag.Thecorrectionfactorofthecolourterm(i.e.zeropoint lar initial mass function, and the AB magnitude system (Oke& Gunn1983) are used throughouttheLetter. Galac- ofB−NB)isassumedtobe−0.1,whichistailoredtothatin our past study of another field using the same NB428 filter ticextinctionsatNB428andB-bandareassumedtobe0.55 (Shimakawaet al.2016).2σlimitingmagnitudeareassumed and0.57mag,respectively(Schlegelet al.1998;Fitzpatrick 1999; Schlafly & Finkbeiner2011)1. for thesources with B-band detection levels lower than 2σ. As a result, in total 162 objects satisfied our selection criteria (Fig. 1). This Letter employs these samples as LAE candidates at z = 2.5. However, it is noted that a 2 OBSERVATION AND DATA ANALYSES considerable number of foreground contaminations such as We performed Lyα line imaging of USS 1558 at z = [Oii]λλ3727,3730,Civλλ1548,1551wouldbecontainedeven 2.53 with the Subaru Prime Focus Camera (Suprime-Cam; though we are targeting the region that hosts the known Miyazaki et al.2002) ontheSubarutelescope. Weused the protocluster (c.f. ∼ 60 % in the random field according custom-madenarrowband filter NB428 that has thecentral to Sobral et al. 2016). The currently available photometric wavelengthof4297˚AandFWHMof84˚A.Thisisdesigned data that cover the entire field are only B, r, and z-band so that the filter FWHM neatly captures the Lyα lines at photometries with Suprime-Cam, which are not sufficient z = 2.53 ± 0.03 (emission or absorption) from HAEs at to cleanly decontaminate our LAE samples. This caution z = 2.52±0.02 associated with the USS 1558 protoclus- shouldbekeptinmindwhenourLAEsamplesarediscussed. ter selected by the narrowband filter, NB2315, installed on MOIRCS/Subaru (see Shimakawa et al. 2016)2. The com- binedanalysis ofa resonant LyαlinebyNB428 andanon- 3 RESULTS resonant HαlinebyNB2315enablesustomakethefirstsys- Figure 2a shows the spatial distribution of the LAE candi- tematic comparison of spatial distributions between LAEs dates over the entire field of Suprime-Cam. The protoclus- andHAEs,andtoinvestigatetheenvironmentaldependence ter cores traced by the HAEs with MOIRCS in our previ- ofthe Lyαphotonescapefractionswithintheprotocluster. ousstudies(Hayashiet al.2012,2016)areembeddedinthe muchlarger-scale structurestracedbyLAEs.Hayashiet al. 1 http://irsa.ipac.caltech.edu/applications/DUST/ (2016) have identified 100 HAEs based on the combined 2 The bandpass of NB428 for Lyα does not perfectly match to thatofNB2315for Hα.Thisleadsto10% Lyαfluxlossonav- erageforHAEswhichisconsideredinourstackinganalyses(§4). (Shimakawaetal.2014).Furthermore,NB428coversuptohigher Also, the NB2315 bandpass shifts blueward towards the edge of redshifts for Lyα by & 1500 km s−1 with respect to that of MORICS field of view, however, this effect can be negligible for NB2315 for Hα, and thus a possible redward velocity offset of our HAE samples according to the past follow-up spectroscopy Lyαrelativeto Hα(Shapleyetal.2003)wouldbenegligible. MNRAS000,1–6(2017) Lyα depletion in the protocluster core L3 −0.2 1.0 0 1 2 3 4[σ] 100 150 200 !a"5th[ph-kpc] 0.5 −0.3 0.0 c] p g] M [de−0.4 ph-−0.5 ec. c.[ D e D ∆ −1.0 −0.5 −1.5 −0.6 10co-Mpc HAEs −2.0 HAEs LAEcandidates HAEs+LAEs NB428only 240.6 240.5 240.4 240.3 240.2 240.1 0.5 0.0 −0.5 −1.0 R.A.[deg] ∆R.A.[ph-Mpc] Figure 2. The 2-D maps of USS 1558 protocluster with Suprime-Cam (a: left) and with MOIRCS (b: right). (a) The black crosses represent the LAE candidates, and the blue circles indicate the HAEs identified by Hayashietal. (2016). The filled contours indicate thesignificanceofLAEoverdensities(0–σ, σ–2σ, 2σ–3σ, and3σ–4σ), whicharesmoothedbytheGaussiankernelofσ=1degree. The regionenclosedbytheblacklinescorrespondstothesurveyfieldofMOIRCSforHAEs.(b)Thesymbolsarethesameasshowninthe leftpanel,butthepurplecrossesshowthedual Hαand Lyαemitters.ThestarsymbolindicatestheRG.Thefilledcontoursshowsthe meandistanceof,200–150, 150–100, and<100ph-kpc(physicalkpc),smoothedbytheGaussiankernelofσ=0.5arcmin. techniqueofthenarrowbandselectionandtwocolour–colour remarkable that there is no LAE except for the RG in any diagrams (r’JKs and r’HF160WKs). These samples are lim- of the notable dense group cores of HAEs in spite of the itedtothestar-formationrates(SFRs)of>2.2M⊙/yrwith- 4σ overdensity in LAEs in this protocluster as a whole on out dust correction. 41 among those HAEshave been spec- a ∼10 co-Mpc scale. To evaluate the deficiency of LAEs in troscopically confirmed by Shimakawaet al. (2014, 2015). the local overdensities, this work defines a density parame- Also from those spectroscopic analyses, we reckon that the ter, the mean projected distance hai = 2×(πΣ )−0.5 Nth Nth contamination in the rest of our unconfirmed HAEs is less where Σ (= N/(πr2 )) is the number density of HAEs Nth Nth than 10 %. This protocluster core contains four very dense withintheradiusr whichisthedistancetothe(N−1)th Nth groups of HAEs; one in the immediate vicinity of the radio neighboursfromeachHAE.WeuseN=5.Westressthatthis galaxy (RG), two toward the southwest (the further one is densityparametermaintainsarelativeconsistencyevenifwe the densest), and one to the north of RG (Fig. 2b). Within choose different N values. theMOIRCSsurveyfield,weidentifysignificant Lyαemis- Themedianvalueandthescatterofthemeanprojected sion lines for nine HAEs including the RG, which meet our distance (hai ) is 214+140 ph-kpc. We investigate the sig- LAEcriteria.Wealsofindfourmoreobjectswhichhaveboth 5th −88 nificance of the LAE deficiency in the dense groups by di- Lyαand Hαdetectionsinthetwonarrowbands,whichwere viding the HAE samples into high density and low density deselected from our original HAEsamples in Hayashiet al. sub-samplesseparatedatthismedianvalue.Thefractionsof (2016) by their colour–colour criteria. Including those, 104 LAEsamongHAEsare21±11%inthelowerdensitiesand HAEs are now identified in total as protocluster members, only2±4%excludingtheRG(or4±5% withtheRG)in and13outofthosearealsoclassifiedasLAEs.Ontheother the high-density regions, respectively. Fig. 3 represents the hand, three LAE candidates show no Hα emission line in cumulative distributions of hai (upper panel) and stel- ourpreviousnarrowbandimagingatNIR.Someoftheseare 5th lar mass (lower panel) for the HAEs and the HAEs with likelytobeforeground contaminations(otheremittersthan Lyαemissiondetections.StellarmassesoftheHAEsamples Lyα) and therest would betoo faint HAEs. are provided by Hayashiet al. (2016). In the upper panel, LAE density map shows a 4σ excess peak just around the possibility that“HAEs”and“HAEs+LAEs”are drawn themainbodyoftheprotoclusterUSS1558tracedbyHAEs. from the same distribution is only 2 % according to the The notable LSS or a gigantic filament extends toward Kolmogorov-Smirnovtest,suggestingthat Lyαphotonsare northwest. A follow-up spectroscopy is needed to confirm moredepletedinhigh-densityregions.Thistrendisstillsta- the structures since our LAE samples would contain fore- tisticallysignificant(p=0.04)evenifweusethemass-control groundotherlinecontaminations.Surprisingly,however,on samples where we limit the galaxies only with the stellar a much smaller scale (∼300 physical kpc (ph-kpc)), HAEs masses lower than 1010 M⊙. Even if we go further down with Lyα line detections are distributed as if they are try- instellarmasses,wheresuchastatisticaltestwouldbecome ing to avoid the overdense groups of HAEs (Fig. 2b). It is nolongersignificant,theLAEdeficiencyathai <300ph- 5th MNRAS000,1–6(2017) L4 R. Shimakawa et al. r be 1.0 HAEs available, the James Webb Space Telescope (Gardner et al. m HAEs+LAEs 2006) will be very powerful as it probes the rest-frame op- u n tical regime where many nebular emission lines other than e Lyα are located. It should be noted, however, that bright v 0.5 rti Lyαblobs can bealso used as a tracer of thecentral galax- ea mbul ies in massive haloes as suggested by the past studies (e.g. m Steidelet al. 2000; Matsuda et al. 2011). u nCu 0.0 Wemeasuretheescapefractionof Lyαphotonsbycom- e 0 100 200 300 400 v paring the Lyα and Hα fluxes. This can quantify the de- ati !a"5th[ph-kpc] ficiency of LAEs in the protocluster’s dense cores and thus mul 1.0 provide insight into its physical origins. Unfortunately, the HAEs current datasets are not deep enough to estimate the es- u HAEs+LAEs C capefractionof LyαphotonsforindividualHAEs,andthus we conduct the stacking analysis and derive Hα and Lyα 0.5 luminosities with high precision. The entire HAE samples are divided into two sub-samples by their local 2-D den- sities at the median value of the fifth mean distance (214 0.0 ph-kpc). The narrowband images are then combined with 8 9 10 11 the imcombine task by median on iraf2. We derive Hα log(M⋆/M⊙) and Lyα fluxesin the same way we measure the individual HAEsand LAEs in Hayashiet al. (2016); Shimakawa et al. Figure3.Normalisedcumulativedistributionsof(a:upper)the (2016).Photometric errors are estimated with a similar ap- mean distances and (b: lower) stellar mass. The blue and pur- proach taken by Skelton et al. (2014) where the 1-sigma ple solid lines indicate the entire HAE samples and HAEs with Gaussiannoiseinbackgroundcountsismeasuredasafunc- significant Lyαemission,respectively.Thedottedlinesshowthe tion of variable aperture size. Our error measurements are mass-controlsamplewithM⋆<1E1010 M⊙. thus performed independently of the SExtractor photometries since the SExtractor does not consider the pixel-to- pixel correlation and thus underestimates the errors espe- kpc would still remain. In fact, we do not see a significant cially for photometries with large aperture sizes. More de- difference in stellar mass distributions between the HAEs tailsofthestackingmethodandthemeasurementsoffluxes and those with Lyα emission lines; p-value is 0.07 for the and errors will be presented in a forthcoming full paper entire sample and is 0.18 for the mass-control sample, re- (Shimakawaet al.2017).Weassumea10%fluxcontamina- spectively.Suchaninsignificantorsmalldifferenceinstellar tion from [Nii]linetothenarrowbandfluxfor Hα,and0.7 mass distributions between LAEs and non-LAEs is consis- magofdustextinctionin Hαflux.The Lyαescapefraction tentwiththerecentstudies(Hagen et al. 2016;Hathiet al. 2016).Westress that wehereemploy only theLAEswhose is given by feLsycα = (fLyα,obs)/(8.7fHα,int) where 8.7 is the ratio of Lyαto Hαundertheassumption of case Brecom- Hα emission lines are detected (as HAEs) by the indepen- bination(Brocklehurst1971).Thisworkestimates Lyαand dentnarrowbandimagingatNIR.Therefore,thesecompar- Hα fluxes with various aperture radii from 6 to 30 ph-kpc isons are free from the contaminations in our LAE samples taking into account the fact that most of the star-forming which could bea problem only for theLAE-only samples. galaxies show diffuse Lyα components (O¨stlin et al. 2009; Steidelet al. 2011; Hayeset al. 2013). We compare the measured Lyα photon escape frac- 4 DISCUSSION AND SUMMARY tions between the HAEs in high-density regions and those Our dual Lyα and Hα line survey of a protocluster at in lower-density regions. Both composite line images seem z =2.5 presented has provided us with the first critical in- to have diffuse Lyα profiles since the escape fraction in- sightintotheenvironmentaldependenceofthe Lyαstrength creases with aperture radius, although photometric errors ascomparedto Hα.Thebroadagreementinthespatialdis- arequitelarge.Inaddition,wesee Lyαabsorptionfeatures tributions between LAEs and HAEs on a large scale (& 10 within the small aperture radii, which are consistent with co-moving Mpc) indicates that Lyα line would be a good theindividualdetectionof LyαabsorptioninmassiveHAEs tracer of LSSs in the high-z Universe. On a smaller scale, in the random field (Shimakawaet al. 2016). Most impor- however, we see that LAEs, except for the RG, completely tantly,wefindsystematicallylower Lyαphotonescapefrac- avoidtheprotocluster’sdensecores,whicharetracedbythe tionsforthedenserregions.Thismeansthatthe Lyαemis- overdensitiesofHAEs.ThismeansthattheLAEsurveysof sionlinesaresystematicallymoredepletedindenserregions protoclusters would inevitably miss the particularly dense than in lower-density regions in the protocluster (Fig. 4). regions of protoclusters which are likely to be the most in- However, we should note that the Lyα photon escape frac- teresting and critical environments where we expect to see tion is considered to depend on various physical proper- anyearlyenvironmentaleffectsastheprogenitorsofpresent- ties such as dust, SFR, and metallicity (Hayeset al. 2010; dayrichclustercores.Inotherwords,wearenotreallyable Mattheeet al.2016)thatcoulddependontheenvironment. tostudyenvironmentaleffectswith theLAEsaloneasthey The discrepancy between the two composite HAEs may can trace only the outskirts of protocluster cores or even larger-scale structures around them. In order to search for truly dense structures at z > 2.6 where Hα is no longer 2 http://iraf.noao.edu MNRAS000,1–6(2017) Lyα depletion in the protocluster core L5 5 ACKNOWLEDGEMENTS higherdensities 4 lowerdensities ThedataarecollectedattheSubaruTelescope,whichisop- eratedbytheNationalAstronomicalObservatoryofJapan. 3 ] ThisworkissubsidizedbyJSPSKAKENHIGrantNumber % 2 15J04923.ThisworkwasalsopartiallysupportedbytheRe- [ αLyfesc 1 streoanrcohmFicuanldScfioerncSet,uSdOenKtsE(N2D01A3I).oWfethtehaDnekptahretmaennotnyomf Aouss- 0 referee for useful comments.R.S.and T.S. acknowledge the −1 Lyαabsorption support from the Japan Society for the Promotion of Sci- ence (JSPS) through JSPS research fellowships for young scientists. T.K. acknowledges KAKENHINo. 21340045. 0 10 20 30 Radius[ph-kpc] REFERENCES Figure4.Basedonthestackinganalysis,the Lyαphotonescape fractions(feLsycα)ofthecompositeHAEsareshownasafunctionof BertinE.,ArnoutsS.,1996, A&AS,117,393 photometricapertureradiuswithinwhichweintegratelinefluxes. BrocklehurstM.,1971,MNRAS,153,471 Here,theHAEsamplesaresplitintotwosub-samplesattheme- CappellariM.,etal.,2011,MNRAS,416,1680 dianvalueofthemeanprojecteddistance(hai5th =214ph-kpc). ChabrierG.,2003,PASP,115,763 ThecompositeHAEsinthehigher/lowerdensityregionsarepre- ColeS.,Lacey C.G.,BaughC.M.,FrenkC.S.,2000, MNRAS, sented by blue/grey zones, respectively. The errorbars represent 319,168 thephotometric errorsin Lyαand Hαlinefluxes. Thenegative DekelA.,etal.,2009a,Nature,457,451 feLsycα valuesmeanthat Lyαisseenasanabsorptionline. DekelA.,SariR.,CeverinoD.,2009b, ApJ,703,785 DresslerA.,1980,ApJ,236,351 FitzpatrickE.L.,1999,PASP,111,63 GardnerJ.P.,etal.,2006, SpaceSci.Rev.,123,485 also contain such secondary factors on top of the environ- HagenA.,etal.,2016, ApJ,817,79 ment. Moreover, the narrowband technique cannot resolve HathiN.P.,etal.,2016,A&A,588,A26 thedetailedspectralfeaturesaroundthe Lyαline.Thusour Hayashi M., Kodama T., Tadaki K.-i., Koyama Y., Tanaka I., measurements may underestimate the escape fraction due 2012,ApJ,757,15 to absorption by foreground circumgalactic/intergalactic Hayashi M., Kodama T., Tanaka I., Shimakawa R., Koyama medium (CGM/IGM) (Hayes& O¨stlin 2006) and/or blue- Y., Tadaki K.-i., Suzuki T. L., Yamamoto M., 2016, ApJ, shifted denseoutflowing gas (Reddyet al. 2016). 826,L28 Insummary,wefindthatLAEsaremissinginthedense HayesM.,O¨stlinG.,2006,A&A,460,681 HAE cores. We also find that Lyα photon escape fractions HayesM.,etal.,2010,Nature,464,562 HayesM.,etal.,2013,ApJ,765,L27 inthecompositeHAEsinthedenserregionsarelowerthan KajisawaM.,KodamaT.,TanakaI.,YamadaT.,BowerR.,2006, those in lower-density regions. In the general field, it is ex- pectedthatLAEshavelessdustandlower Hicoveringfrac- MNRAS,371,577 KodamaT., Tanaka I., KajisawaM.,KurkJ., Venemans B.,De tions(Shibuyaet al.2014;Reddyet al. 2016)andtherefore BreuckC.,VernetJ.,LidmanC.,2007,MNRAS,377,1717 Lyα photons can relatively easily escape from the galaxies. KoyamaY.,etal.,2013,MNRAS,434,423 However,intheprotoclustercore,wehaveextrasurrounding MatsudaY.,etal.,2011,MNRAS,410,L13 gasanddustcomponentstrappedinthegroup/clusterscale Matthee J., Sobral D., Oteo I., Best P.,Smail I., Röttgering H., haloes, which may prevent the Lyα photons from escaping Paulino-AfonsoA.,2016, MNRAS,458,449 fromthesystems.Suchabundantgroup-scale Higasmaybe MiyazakiS.,etal.,2002,PASJ,54,833 suppliedfrom thesurroundingregions,forexample,bycold OkeJ.B.,GunnJ.E.,1983,ApJ,266,713 streams (Dekelet al. 2009a,b). Moreover, since the mean O¨stlin G., Hayes M., Kunth D., Mas-Hesse J. M., Leitherer C., PetrosianA.,AtekH.,2009,AJ,138,923 projecteddistanceofHAEsismuchsmaller(.200 ph-kpc) OuchiM.,etal.,2003,ApJ,582,60 in higher-density regions than in the lower-density regions, OuchiM.,etal.,2004,ApJ,611,660 Lyα photons emitted from a HAE have a higher chance of ReddyN.A.,SteidelC.C.,PettiniM.,BogosavljevićM.,Shapley penetrating CGM associated to other member galaxy(ies) A.E.,2016, ApJ,828,108 in the foreground located along the line of sight, and thus SchlaflyE.F.,FinkbeinerD.P.,2011, ApJ,737,103 theymaybemoredepleted.Otherwise,mostoftheindivid- SchlegelD.J.,FinkbeinerD.P.,DavisM.,1998,ApJ,500,525 ual galaxies in the dense cores could originally have lower ShapleyA.E.,SteidelC.C.,PettiniM.,AdelbergerK.L.,2003, Lyαphotonescapefractionsdueforexampletohigherdust ApJ,588,65 extinction by a certain environmentaleffect (Koyamaet al. ShibuyaT.,etal.,2014,ApJ,788,74 2013). The current datasets are clearly insufficient to con- ShimakawaR.,KodamaT.,TadakiK.-i.,TanakaI.,HayashiM., KoyamaY.,2014,MNRAS,441,L1 clusivelychoosethemostplausibleexplanation(furtherdis- ShimakawaR.,etal.,2015,MNRAS,451,1284 cussionwillbedescribedinShimakawa et al.2017).Adeep, ShimakawaR.,etal.,2016,preprint, (arXiv:1607.08005) rest-frame FUV spectroscopy of the protocluster galaxies ShimakawaR.,etal.,2017,inpreparation with LRIS and/or KCWI on Keck telescope will be helpful SkeltonR.E.,etal.,2014,ApJS,214,24 for us to constrain the physical origins of the Lyα deple- SobralD.,etal.,2016,preprint, (arXiv:1609.05897) tioneffectandtoresolvetheinterplaybetweenprotocluster SteidelC.C.,AdelbergerK.L.,ShapleyA.E.,PettiniM.,Dick- galaxies and CGM/IGM. insonM.,GiavaliscoM.,2000,ApJ,532,170 MNRAS000,1–6(2017) L6 R. Shimakawa et al. Steidel C. C., Bogosavljević M., Shapley A. E., Kollmeier J. A., ReddyN.A.,ErbD.K.,PettiniM.,2011,ApJ,736,160 SunyaevR.A.,ZeldovichY.B.,1972,A&A,20,189 VenemansB.P.,etal.,2007,A&A,461,823 WhiteS.D.M.,FrenkC.S.,1991,ApJ,379,52 Yagi M., Kashikawa N., Sekiguchi M., Doi M., Yasuda N., Shi- masakuK.,OkamuraS.,2002,AJ,123,66 vanDokkumP.G.,etal.,2011,ApJ,743,L15 Thispaper has been typeset fromaTEX/LATEXfile preparedby theauthor. MNRAS000,1–6(2017)