arXiv:1001.0005v1 [astro-ph.CO] 30 Dec 2009Astronomy& Astrophysics manuscriptno.akari˙RXJ1716˙v5 c∝circlecopyrtESO 2018 October30,2018 Environmentaldependenceof 8 µmluminosityfunctionsof galaxiesatz ∼0.8 Comparison between RXJ1716.4 +6708 andthe AKARI NEP deep field.⋆,⋆⋆ Tomotsugu Goto1,2,⋆⋆⋆, Yusei Koyama3,T.Wada4,C.Pearson5,6,7,H.Matsuhara4,T.Takagi4, H.Shim8, M.Im8, M.G.Lee8, H.Inami4,9,10,M.Malkan11, S.Okamura3,T.T.Takeuchi12, S.Serjeant7, T.Kodama2, T.Nakagawa4, S.Oyabu4,Y.Ohyama13, H.M.Lee8, N.Hwang2, H.Hanami14, K.Imai15,and T.Ishigaki16 1Institute for Astronomy, University of Hawaii,2680 Woodla wnDrive, Honolulu, HI,96822, USA e-mail:tomo@ifa.hawaii.edu 2National Astronomical Observatory, 2-21-1 Osawa,Mitaka, Tokyo, 181-8588,Japan 3Department of Astronomy, School of Science,The University of Tokyo, Tokyo113-0033, Japan 4Institute of Space and Astronautical Science, JapanAerosp ace Exploration Agency, Sagamihara,Kanagawa 229-8510 5Rutherford Appleton Laboratory, Chilton, Didcot,Oxfords hire OX110QX, UK 6Department of Physics,Universityof Lethbridge, 4401 Univ ersity Drive,Lethbridge, AlbertaT1J 1B1, Canada 7Astrophysics Group, Department of Physics, The OpenUniver sity, MiltonKeynes, MK76AA, UK 8Department of Physics& Astronomy, FPRD,Seoul National Uni versity, Shillim-Dong,Kwanak-Gu, Seoul 151-742, Korea 9Spitzer Science Center,California Institute ofTechnolog y, Pasadena, CA91125 10Department of Astronomical Science,The Graduate Universi tyfor Advanced Studies 11Department of Physicsand Astronomy, UCLA,Los Angeles, CA, 90095-1547 USA 12Institute for Advanced Research, Nagoya University, Furo- cho, Chikusa-ku, Nagoya 464-8601 13Academia Sinica,Institute of Astronomyand Astrophysics, Taiwan 14Physics Section,Facultyof Humanities and SocialSciences , Iwate University, Morioka, 020-8550 15TOMER&D Inc. Kawasaki, Kanagawa 2130012, Japan 16Asahikawa National College of Technology, 2-1-6 2-joShunk ohdai, Asahikawa-shi, Hokkaido 071-8142 Received September 15, 2009; accepted December 16, 2009 ABSTRACT Aims.Weaim to reveal environmental dependence of infraredlumin osity functions (IR LFs)of galaxies at z ∼0.8 using the AKARI satellite. AKARI’s wide field of view and unique mid-IR filter s help us to construct restframe 8 µm LFs directly without relying on SEDmodels. Methods. We construct restframe 8 µm IR LFs in the cluster region RXJ1716.4 +6708 at z=0.81, and compare them with a blank field using the AKARI North Ecliptic Pole deep field data at the same redshift. AKARI’s wide field of view (10’ ×10’) is suitable to investigate wide range of galaxy environments. AKARI’s 15 µm filter is advantageous here since it directly probes restfr ame 8µm at z∼0.8, without relyingona large extrapolation based ona SEDfi t,which was the largestuncertainty inprevious work. Results. We have found that cluster IR LFsat restframe 8 µm have a factor of 2.4smaller L∗and a steeper faint-end slope than that of the field. Confirming this trend, we also found that faint-e nd slopes of the cluster LFs becomes flatter and flatter with de creasing local galaxy density. These changes in LFs cannot be explain ed by a simple infall of field galaxy population into a cluster . Physics that canpreferentiallysuppress IR luminous galaxies inhi gh density regions is requiredtoexplain the observed resul ts. Keywords. galaxies: evolution, galaxies:interactions, galaxies:s tarburst, galaxies:peculiar, galaxies:formation 1. Introduction It hasbeenobservedthat galaxypropertieschangeas a funct ion of galaxyenvironment;the morphology-densityrelation re ports that fractionof elliptical galaxiesis largerat highergal axyden- sity(Gotoetal.,2003);thestarformationrate(SFR)ishig herin lower galaxy density (G´ omezet al., 2003; Tanakaet al., 200 4) . However, despite accumulating observational evidence, w e ⋆This research is based on the observations with AKARI, a JAXA project withthe participationof ESA. ⋆⋆Based on data collected at Subaru Telescope, which is operat ed by the National Astronomical Observatory ofJapan. ⋆⋆⋆JSPSSPDfellowstill do not fully understand the underlying physics govern ing environmental-dependentevolutionofgalaxies. Infrared (IR) emission of galaxies is an important probe of galaxy activity since at higher redshift, a sig- nificant fraction of star formation is obscured by dust (Takeuchi,Buat,&Burgarella, 2005; Gotoetal., 2010). Although there exist low-z cluster studies (Baiet al., 2006 ; Shimet al., 2010; Tranetal., 2010), not much attention has been paid to the infrared properties of high redshift cluste r galaxies, mainly due to the lack of sensitivity in previous I R satellites such as ISO and IRAS. Superb sensitivity of recen tly launched Spitzer and AKARI satellites can revolutionize th e infraredviewofenvironmentaldependenceofgalaxyevolut ion.2 Gotoet al.:Environemental dependence of 8 µm luminosity functions ofgalaxies atz ∼0.8 Fig.1.Restframe 8 µm LFs of cluster RXJ1716.4 +6708 at z=0.81 in the squares, and those of the AKARI NEP deep field in the triangles. For RXJ1716.4 +6708, only photometric and spectroscopic cluster member galaxies are used. For the NEP deep field, galaxies with photo-z/specz in the range of 0.65< z <0.9are used. The dot-dashed lines are 8 µm LFs of RXJ1716.4 +6708, but scaled down for easier comparison. Thethindottedlinesarethebest-fitdoublepowerlaws.Vert ical arrows show the 5 σflux limits of deep/shallow regions of the cluster (red) and the NEP deep field (blue) in terms of L8µmat z=0.81. In this work, we comparerestframe8 µm LFs between clus- ter and field regions at z=0.8 using data from the AKARI. Monochromaticrestframe 8 µm luminosity ( L8µm) is important since it is known to correlate well with the total IR luminosi ty (Babbedgeet al., 2006; Huanget al., 2007), andhence,with t he SFR of galaxies (Kennicutt, 1998). This is especially true f or star-forminggalaxiesbecausethe rest-frame8 µm fluxare dom- inatedbyprominentPAHfeaturessuchasat6.2,7.7and8.6 µm (Desert,Boulanger,&Puget, 1990). ImportantadvantagesbroughtbytheAKARIareasfollows: (i) At z=0.8, AKARI’s 15 µm filter (L15) covers the redshifted restframe 8 µm, thus we can estimate 8 µm LFs without using a large extrapolation based on SED models, which were the largest uncertainty in previous work. (ii) Large field of vie w of the AKARI’smid-IRcamera(IRC, 10’ ×10’)allowsustostudy wider area including cluster outskirts, where important ev olu- tionary mechanisms are suggested to be at work (Gotoet al., 2004; Kodamaet al., 2004). For example, passive spiral gala x- ies have been observed in such an environment (Gotoet al., 2003). Unless otherwise stated, we adopt a cosmology with (h,Ωm,ΩΛ) = (0.7,0.3,0.7)(Komatsuet al., 2009). 2. Data & Analysis 2.1. LFs ofclusterRXJ1716.4 +6708 The AKARI is a Japanese infrared satellite (Murakamiet al., 2007), which has continuous filter coverage in the mid IR wavelengths ( N2,N3,N4,S7,S9W,S11,L15,L18Wand L24). The AKARI has observed a massive galaxy cluster,Fig.2.Restframe 8 µm LFs of cluster RXJ1716.4 +6708 at z=0.81, divided according to the local galaxy density ( Σ5th). Thestars,circlesandsquaresareforgalaxieswith logΣ5th≥2, 1.6≤logΣ5th<2,andlogΣ5th<1.6,respectively. RXJ1716.4 +6708, in N3,S7andL15(Koyamaetal., 2008). RXJ1716.4 +6708 is at z=0.81 and has σ= 1522+215 −150km s−1, LXbol= 13.86±1.04×1044ergs−1,kT= 6.8+1.0 −0.6keV.Mass estimate from weak lensing and X-ray are 3.7 ±1.3×1014M⊙ and 4.35 ±0.83×1014M⊙, respectively (see Koyamaet al., 2007, forreferences). An important advantage of the AKARI observation is L15 filter, which corresponds to the restframe 8 µm at z=0.81. With 15 (3) pointings, L15reaches 66.5 (96.5) µJy in deep (shal- low) regions at 5 σ. Here flux is measured in 11” aperture, and coverted to total flux using AKARI’s IRC correction table (2009.5.1)1.ClusterstudieswiththeSpitzerareoftenperformed in 24µm and thus needed a large extrapolation to estimate ei- therL8µmor total infrared luminosity ( LTIR,8−1000µm). Note that we do not claim the L8µmis a better indicator of thetotalIRluminositythanotherindicators(Brandlet al. ,2006; Calzetti et al., 2007; Riekeet al., 2009), but it is importan t that theAKARIcanmeausureredshifted 8µmfluxdirectlyinoneof thefilters. Thanks to the AKARI’s wide field of view (10’ ×10’), the total area coverage around the cluster is 200 arcmin2, which cover larger area than previous cluster studies with the Spi tzer, allowingustostudyIRsourcesintheoutskirts,whereimpor tant galaxyevolutiontakesplace(e.g.,Gotoet al.,2003).Prev iously, Koyamaet al. (2008) reporteda high fractionof L15sourcesin the intermediatedensity regionin the cluster,suggesting a pres- enceofenvironmentaleffectintheintermediatedensityen viron- ment. Thissameregionwasimagedwith Suprime-Camin VRi′z′ and has a good photometric redshift estimate (Koyamaet al., 2007).Usedinthisworkare54 L15-detectedgalaxieswhichare well identifiedwithopticalsourceswith 0.76≤zphoto≤0.83. With the L15filter covering the restframe 8 µm, we simply convert the observed flux to 8 µm monochromatic luminosity 1http://www.ir.isas.jaxa.jp/ASTRO- F/Observation/DataReduction/IRC/ApertureCorrection 090501.htmlGotoet al.:Environemental dependence of 8 µm luminosity functions ofgalaxies atz ∼0.8 3 Table 1.Best doublepower-lawfit parametersforLFs Sample L∗ 8µm(L⊙)φ∗(Mpc−3dex−1)α β NEPDeepfield 6.1 ±0.5×10100.0010±0.0003 1.1 ±0.3 5.7 ±1.2 RXJ1716.4 +67082.5±0.1×10100.74±0.04 2.6 ±0.1 5.5 ±0.4 (L8µm) using a standard cosmology. Completeness was mea- sured by distributing artificial point sources with varying flux withinthe field andby examiningwhat fractionofthem wasre- coveredasafunctionofinputflux.Sincewehavedeepercover - age at the center of the cluster, the completeness was measur ed separately in the central deep region and the outer regions o f the field. More detail of the method is described in Wada et al. (2008). Oncethefluxisconvertedtoluminosityandcompletenessis takenintoaccount,it is straightforwardto construct L8µmLFs, which we show in the squares in Fig.1. Errors of the LFs are assumedtofollowPoissondistribution.Here,wetakeanang ular distance of the most distant source from the cluster center a s a cluster radius ( Rmax= 6.2Mpc). We assumed4 3πR3 maxas the volume of the cluster to obtain galaxy density ( φ). This is only one of many ways to define a cluster volume, and thus, a cautionmustbetakentocompare absolute valuesofourLFsto other work such as Shimet al. (2010). This cluster is elongat ed inangulardirection(Koyamaet al.,2007),andthus,thevol ume mightnotbespherical.Yet,comparisonofthe shapeoftheLFs isvalid. 2.2. LFs inthe AKARI NEP Deepfield Our field LFs are based on the AKARI NEP Deep field data. The AKARI performed deep imaging in the North Ecliptic Pole Region (NEP) from 2-24 µm, with 4 pointings in each field over 0.4 deg2(Matsuharaet al., 2006, 2007; Wada etal., 2008). The 5 σsensitivity in the AKARI IR filters (N2,N3,N4,S7,S9W,S11,L15,L18WandL24) are 14.2, 11.0, 8.0, 48, 58, 71, 117, 121 and 275 µJy (Wada etal., 2008). Flux is measured in 3 pix radius aperture (=7”), then correct ed tototal flux. AsubregionoftheNEP-Deepfield(0.25deg2)hasancillary datafromSubaru BVRi′z′(Imaiet al.,2007;Wada etal.,2008), CFHTu′(Serjeant et al. in prep.), KPNO2m/FLAMINGOs J andKs(Imaietal., 2007), GALEX FUVandNUV(Malkan et al. in prep.). For the optical identification of MIR source s, we adopt the likelihood ratio method (Sutherland&Saunders , 1992).Usingthesedata,weestimatephotometricredshifto fL15 detectedsourcesintheregionwiththe LePhare (Ilbertet al., 2006; Arnoutset al., 2007).Themeasurederrorsonthephoto -z against 293 spec-z galaxies from Keck/DEIMOS (Takagi et al. in prep.) are∆z 1+z=0.036 at z≤0.8. We have excluded those sourcesbetterfit with QSO templatesfromtheLFs. To construct field LFs, we have selected L15sources at 0.65< zphotoz<0.9. There remained 289 IR galaxies with a median redshift of 0.76. L15flux is converted to L8µmus- ing the photometric redshift of each galaxy. LFs are com- puted using the 1/ Vmaxmethod. We used the SED templates (Lagache,Dole,&Puget, 2003) for k-corrections to obtain the maximumobservableredshiftfromthefluxlimit.Completene ss of theL15detection is corrected using Pearsonet al. (2009b). Thiscorrectionis25%atmaximum,sincewe onlyusethesam- plewherethecompletenessisgreaterthan80%. The resulting field LFs are shown in the dotted line and tri- angles in Fig.1. Errors of the LFs are computed using a 1000Monte Caro simulation with varying zand flux within their er- rors. These estimated errors are added to the Poisson errors in eachLFbinin quadrature. We performed a detaild comparison of restframe 8 µm LFs to those in the literature in Gotoetal. (2010). Briefly, there is an oder of difference between Caputiet al. (2007) an d Babbedgeetal. (2006), reflecting difficulty in estimating L8µm dominatedbyPAHemissionsusingSpitzer24 µmflux.Ourfield 8µm LF lies between Caputi etal. (2007) and Babbedgeet al. (2006). Compared with these work, we have directly observed restframe 8 µm using the AKARI L15filter, eliminating the un- certaintlyinfluxconversionbasedonSEDmodels.Moredetai ls andevolutionoffieldIRLFsaredescribedinGotoet al.(2010 ). 3. Results& Discussion 3.1. 8µmIRLFs In Fig.1, we show restframe 8 µm LFs of cluster RXJ1716.4 +6708 in the squares, and LFs of the field re- gion in the triangles. First of all, cluster LFs have by a fact or of∼700 higher density than the field LFs, reflecting the fact the galaxy clusters is indeed high density regions in terms o f infraredsources. Next, to compare the shape of the LFs, we normalized the cluster LF to match the field LFs at the faintest end, and show in the dash-dottedline. In contrast to the field LFs, which sh ow flattening of the slope at log L8µm<10.8L⊙, the cluster LF maintainsthesteepslopeintherangeof 10.0L⊙ L∗) (2) Free parameters are: L∗(characteristic luminosity, L⊙),φ∗ (normalization, Mpc−3),αandβ(faint and bright end slopes), respectively.ThebestfitvaluesforfieldandclusterLFsare sum- marisedinTable1andshowninthedottedlinesinFig.1. The bright-end slopes are not very different, but L∗of the cluster LF is smaller than the field by a factor of 2.4, and the faint-endtailofclusterLF issteeperthanthatoffieldLF. To further examine the difference at the faint end of the LFs, we divide the cluster LF using the local galaxy density (Σ5th) measuredbyKoyamaet al. (2008). Thisdensityis based on the distance to the 5th nearest neighbor in the transverse di- rection using all the optical photo-z members, and thus, is a surface galaxy density. We separate LFs using similar crite ria, logΣ5th≥2(dense),1.6≤logΣ5th<2(intermediate), and logΣ5th<1.6(sparse), then plot LFs of each region in the4 Gotoet al.:Environemental dependence of 8 µm luminosity functions ofgalaxies atz ∼0.8 stars, circles, and squares in Fig.2. A fraction of the total vol- umeofthe clusteris assignedto eachdensitygroupin invers ely proportionaltothe sumof Σ3/2 5thofeachgroup. Interestingly, the faint-end slope becomes flatter and flatt er with decreasing local galaxy density. This result is consis tent with our comparison with the field in Fig.1. In fact, the lowes t densityLF(squares)hasaflatfaint-endtailsimilartothat ofthe fieldLF.SincetheseLFsarebasedonthesamedata,changesin the faint-end slope are not likely due to the errors in comple te- ness correction nor calibration problems. The completenes s of the deep and shallow regions of the cluster are measured sep- arately. The changes in the slope is much larger than the maxi - mumcompletenesscorrectionof25%.Wealsocheckedtheclus - ter LFs as a function of cluster centric radius, to find no sign ifi- cantdifference,perhapsduetotheelongatedmorphologyof this cluster. At the same time, assuming the same cluster volume, Fig.2 shows that a possible contamination from the field gala x- ies to cluster LFs is only ∼0.1% in the dense region and ∼1% eveninthe sparseregion. It is interesting that not just the change in the scale of the LFs, but there is a change in the L∗and the faint-end slope ( α) of the LFs, resulting in the deficit in the 10.2L⊙