arXiv:0811.4158v1 [astro-ph]

27 Pages·2009·3.58 MB·English

Mon.Not.R.Astron.Soc.000,1–??(2008) Printed4October2009 (MNLATEXstylefilev2.2)
Photometric redshift accuracy in AKARI Deep Surveys
M. Negrello1⋆, S. Serjeant1, C. Pearson2,3†, T. Takagi4, A. Efstathiou5, T. Goto4,
D. Burgarella6, W.-S. Jeong4,7, M. Im8,9, H. M. Lee8, H. Matsuhara4, S. Oyabu4,
T. Wada4, G. White1
1Department of Physics and Astronomy, Open University,Walton Hall, MiltonKeynesMK7 6AA, United Kingdom
2Space Science and Technology Department, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UnitedKingdom
8 3Department of Physics, University of Lethbridge, 4401 UniversityDrive, Lethbridge, Alberta T1J 1B1, Canada
0 4Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 229-8510, Japan
0 5Department of Computer Science and Engineering, Cyprus College, 6 DiogenesStr, 1516 Nicosia, Cyprus
2 6Observatoire Astronomique Marseille Provence, Laboratoire dAstrophysique de Marseille, 13376 Marseille Cedex 12, France
v 7Space Science Division, Korea Astronomy & Space Science Institute (KASI), 61-1, Whaam-dong, Yuseong-gu, Deajeon, 305-348, South Korea
o 8Department of Physics and Astronomy, FPRD, Seoul National University,Seoul 151-742, Korea
N 9Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA91125, USA

5
....
2

]
h ABSTRACT
p WeinvestigatethephotometricredshiftaccuracyachievablewiththeAKARIinfrared
- data in deep multi-band surveys,such as in the North Ecliptic Pole field. We demon-
o
strate that the passage of redshifted policyclic aromatic hydrocarbons and silicate
r
t features into the mid-infrared wavelength window covered by AKARI is a valuable
s
a means to recover the redshifts of starburst galaxies. To this end we have collected a
[ sample of ∼60 galaxies drawn from the GOODS-North Field with spectroscopic red-
1 shift 0.5∼< zspec ∼< 1.5 and photometry from 3.6 to 24 µm, provided by the Spitzer,
v ISO and AKARI satellites. The infrared spectra are fitted using synthetic galaxy
8 SpectralEnergyDistributions which accountfor starburstandactive nuclei emission.
5 For ∼ 90% of the sources in our sample the redshift is recovered with an accuracy
1 |zphot−zspec|/(1+zspec)∼< 10%.Asimilaranalysisperformedondifferentsetsofsim-
4
ulated spectra shows that the AKARI infrared data alone can provide photometric
.
1 redshiftsaccurateto|zphot−zspec|/(1+zspec)∼10%(1σ)atz ∼< 2.Athigherredshifts
1
thePAHfeaturesareshiftedoutsidethewavelengthrangecoveredbyAKARIandthe
8
photo-zestimatesrelyonthelessprominent1.6µmstellarbump;theaccuracyachiev-
0
: able in this case on(1+z)is ∼10−15%,providedthat the AGN contributionto the
v infraredemissionis subdominant. Our technique is no more prone to redshift aliasing
i
X than optical-uv photo-z, and it may be possible to reduce this aliasing further with
the addition of submillimetre and/or radio data.
r
a
Key words: galaxies: starburst – galaxies: active – infrared: galaxies.
1 INTRODUCTION fications (Chapman et al. 2004, 2005). Attention therefore
havebeenfocusinginthelastyearsoninfraredphotometric
The discovery that infrared luminous starbursting galaxies
redshift estimators. Sawicki (2002) showed that the 1.6µm
are significant and possibly dominant contributors to the
spectralfeaturearisingfromthephotosphericemissionfrom
cosmic starformation history of the Universe has had an
evolved stars could be used to obtain crude photometric
enormous impact on the understanding of galaxy evolution
redshift constraints with the 3.6-8.0µm photometry from
(Hughesetal.1998;Lagache,Puget&Dole2005).However,
theIRACinstrumentonSpitzer.Unfortunately,the8-24µm
thesegalaxies areoften toofaint in theopticalforlarge op-
wavelengthgapinSpitzer’swide-fieldsurveycapabilitypre-
tical redshift compaigns, or have ambiguous optical identi-
vented the use of longer wavelength rest-frame features as
redshift indicators, such as the much more prominent Poli-
cyclic Aromatic Hydrocarbons (PAH) and 10µm silicate
∼
⋆ E-mail:[email protected] absorption features.
† visitingResearchFellowattheOpenUniversity
(cid:13)c 2008RAS
2 M. Negrello et al.
AKARI is a Japanese-led infrared space telescope, assumption that the amount of gas in the outflow from the
which was launched successfully in Feb 2006. It has un- starburstregionhasanegligibleimpactonthechemicalevo-
dertaken deep surveys near the Ecliptic Poles, far deeper lution as awhole, thestarburst is characterized bytherate
thanitsall-skysurvey.TheNorthEclipticPole(NEP)field of gas infall and that of the star-formation. However, for
is the largest deep-field legacy survey of AKARI, covering simplicity, THA03 set the infall time-scale equal to star-
0.38deg2, and is its major deep-field legacy (Wada et al. formation time-scale, t . The equations describing the time
0
2008). A wider and shallower survey at the NEP has also variation of gas mass, total stellar mass and gas metallicity
been performed with AKARI, over a 5.8deg2 area (Lee et are solved numerically using in input the unobscured stel-
al. 2008), and further deep photometric surveys have been larSEDsderivedfromtheevolutionarypopulationsynthesis
made of well-studied Spitzer fields (Pearson et al. 2008). codeofKodama&Arimoto(1997).Atop-heavyIMFisas-
AKARI observed in 9 bands from 2 up to 24µm. sumed with a slope x=1.10, flatter than theSalpeter IMF
∼ ∼
Themid-infraredimaging spansthe8 24µm gapbetween (x = 1.35). The initial metallicity of the gas cloud, Z , is
i
−
Spitzer’sIRACandMIPSinstruments.Theuniquediagnos- set to zero. According toTAH03, thechemical evolution as
tic power of AKARI’s Spectral Energy Distribution (SED) a function of t/t as well as the properties of the SED are
0
analysis has been confirmed by Takagi et al. (2007), who almost independentofthepractical choiceoft ,specifically
0
verysuccessfully traced several starburst PAH features and when t > 50 Myr. Therefore, all the reference SED tem-
0
identifiedtheAGNdusttorusexcessoverthestarburstemis- plates us∼ed here for the starburst are calculated adopting
sion, obtainingat thesame timeagood matchbetween the t =100 Myr.
0
photometricredshiftderivedfromHyperZ(Bolzonella etal. THA03 assumes a starburst region in which stars and
2000)andtheoneobtainedbyfittingtheinfrareddatawith dust are distributed within a radius R , and introduce the
t
reliable starburst models. following mass-radius relation
Inthepresentworkweinvestigatethephotometricred-
shift accuracy achievable with AKARI in deep multi-band R M γ
t =Θ ⋆ , (1)
surveys from infrared data alone. Infrared spectra are fit- 1kpc (cid:18)109M⊙(cid:19)
ted using reference SED templates derived from the star-
burst model of Takagi, Arimoto & Hanami (2003, hereafter where Θ is a compactness factor that expresss the mat-
TAH03;seealsoTakagi,Hanami&Arimoto2004, hereafter ter concentration, with the mean density being higher for
THA04),andalreadyexploitedinTakagietal.(2007).How- smaller values of Θ, while M is the (time-dependent) to-
⋆
ever in order to deal with a possible excess over starburst tal stellar mass in the starburst region. The exponent γ is
emission due to an AGN activity we have included a set set to 1 which results in a constant surface brightness of
2
of reference AGN spectra derived from the model of Ef- the straburst galaxy for constant Θ. Dust is assumed to be
stathiou & Rowan-Robinson (1995, hereafter ER95). Our homogeneously distributed within R while a King profile
t
photometricredshift codecombinesthestarburstandAGN is adopted for the stellar density distribution. The optical
componentstoprovidethebestfittoobservedorsimulated depthof thestarburst regions is a function of thestarburst
spectra. We fit the infrared SEDs of a sample of galaxies ageandofthecompactnessfactor,andincreasesfordecreas-
with spectroscopic redshift drawn from the GOODS-North ing valuesof Θ.
Field where deep AKARIobservation have been performed The amount of dust is given by the chemical evolution
at 11 and 18µm, and of a set of simulated spectra in the assuming a constant dust-to-metalratio, δ , with three dif-
0
NEP Deep Field with AKARI. We aim to demonstrate in ferent dust models, i.e. the model for dust in the Milky
thiswaythepowerofthePAHandsilicateabsorptionfeau- Way (MW), the Large Magellanic Cloud (LMC) and the
res to obtain reliable redshift estimates based on infrared Small Magellanic Cloud (SMC). The values of δ derived
0
photometry alone. fromtheextinctioncurveandthespectraofcirrusemission
WestartinSection2describingthereferenceSEDtem- are0.40,0.55and0.75fortheMW,LMCandSMC,respec-
plates. Section 3 presents the sample of infrared sources tively. The difference among the three extinction curves is
drawn from theGOODS-Nfield andSection 4 providesour attributedtothevariationoftheratioofcarbonaceousdust
resultsonthecomparison betweenthephotometricandthe (graphite and PAHs) to silicate grain with the fraction of
spectroscopic redshifts. Section 5 describes the simulations silicate dustgrains increasingfrom MW toSMCmodel,i.e.
andtheresultsonthephotometricredshiftaccuracyforthe asafunctionofmetallicity;thereforethesilicateabsorption
AKARINEPDeepfield.ConclusionsaresummarizedinSec- features are most prominent in the SEDs described by the
tion 6. SMCtypedustmodel.TheSEDfromultraviolettosubmil-
limetrewavelengthsofstarburstregioniscalculatedforeach
starburst age, compactness parameter and extinction curve
usingaradiativetransfercodethatassumesisotropicmulti-
2 INFRARED SED TEMPLATES plescatteringandaccountsforself-absorption ofre-emitted
energy from dust.
2.1 Starburst component
The absorption/emission properties of the dust are re-
Weadopt theStarBUrst with RadiativeTransfer (SBURT) sponsiblefortheinfraredSEDfeaturesofstarforminggalax-
modelofTHA03andTAH04todealwiththeinfraredemis- ies: the PAHs emissions at 3.3, 6.2, 7.7, 8.6 and 11.3µm
sion dueto starburst activity. and thesilicate resonances at 9.7 and 18µm. An example
The model deals with the star formation and chemi- of starburst SED template drawn from the Takagi
cal evolution in thestarburst regions using theinfall model et al. model, showing the main infrared features, is
of chemical evolution of Arimoto et al. (1992). Under the presented in Fig. 1. The efficiency of these features as
(cid:13)c 2008RAS,MNRAS000,1–??
Photo-z accuracy in AKARI Deep Surveys 3
onviewsofthetorus(θ =0◦),butitisratherfeatureless
view
when the torusis seen face-on (θ =90◦).
view
2.3 Reference SED templates and fitting
procedure
Using the models described above we have created a set
of reference SED templates meant for fitting any set of in-
frared photometric data (both real and simulated). These
templates havebeen derived as follows (see TAH04).
STARBURSTcomponent:foreachtypeofextinction
•
curve, we adopt 10 different starburst ages in the interval
t/t = 0.1 6.0 (or equivalently t = 10 600 Myr, being
0
− −
t =100 Myr), and 17 different compactness factors in the
0
range Θ=0.3 5.
AGN com−ponent:we vary theviewing angle between
0◦•and 90◦, with 11 discrete values.
The resulting SED templates are convolved with the re-
sponse functions of the instruments for different values of
theredshift intherangez =0 7andinstepsof0.02. The
−
fluxes derived for the two SED components (i.e. starburst
Figure 1. Example of a starburst SED template (rest-frame) andAGNtorus)arethenlinearly combinedtogivethepre-
drawn from the Takagi et al. model, showing the main infrared dictedfluxatthepassbandsoftheinstruments.Thebestfit
features exploited as redshift indicators: PAH emissions at 3.3,
parameters of the SED models, including the redshift, are
6.2, and 7.7µm, and silicate absorption at 9.7µm. The 1.6µm then obtained by minimizing the χ2
stellar bump is also visible. In the central and in the bottom
panels are shown the transmission curves of the infrared filters Nfilters F (i) 2 f F (i) 2
exploited in the GOODS-N field and in the AKARI NEP deep χ2 = obs − k=1 k× k , (2)
field,respectively,normalizedtoamaximumvalueofone:Spitzer Xi=1 (cid:20) Pσ(i) (cid:21)
3.6,4.5,5.8,8.0,16and24µmfilters(green),ISO6.5and15µm
filters (blue), AKARI filters (the 11 and 18µm filters in yellow, whereFobs(i)isthemeasuredfluxinthepassbandiandσ(i)
theothers inlightblue). is its corresponding uncertainty. Fk(i) represents the pre-
dicted flux in the passband i contributed by the SED com-
ponent k, with k =1, 2 denoting “Starburst” and “AGN”,
respectively.f isthereforetherelativecontributionofthek
redshiftindicatorsiswhatwearegoingtoputtothetestin k
componenttothetotalbolometricluminosityofthegalaxy.
thepresent work.
Note that for each set of values of the SED model param-
eters, the values of f and f are fixed by the condition of
1 2
minimizationwiththerestrictionthattheymustbeallnon-
2.2 AGN component negative.Whenanegativevalueisobtained foreitherf1 or
f ,thenthatparameterissettozeroandtheprocessofmin-
2
The infrared spectrum of the AGN dusty torus is modelled imization is performed on theother SED component alone.
as in ER95. They combine an accurate solution for the ax- We descard those sets of values of the SED model parame-
ially symmetric radiative transfer problem in dust clouds tersforwhichthebolometricluminosity1 resultingfromthe
with the multigrain dust model of Rowan-Robinson (1992) minimization overtheparameters f and f lies above1014
1 2
and three different geometries for the dusty torus that sur- L⊙ orbelow108 L⊙.Sincewearedealingwith photometric
round the central supermassive black hole. Herewe assume datafrom different telescopes we haveset a minimum error
their thick tapered disc model, which is a disc-like torus of 10% on the measured fluxes before performing the SED
whose height increases with the distance from the central fitting.
source but tapers to a constant height in the outer parts. The goodness of the fit is described by the probability for
This choice has been found to provide the best agreement aχ2-distribution,with thenumberofdegree offreedom set
with the observational constraints on AGN. The tapered by the problem, to produce a value of χ2 higher than the
disc is assumed to have a r−1 density distribution. The ra- one obtained by the best SED-fit. The number of degree
tiobetweentheouterandtheinnerradiusofthetorusisset of freedom is given by ν = n n where n is the
data par data
to20,inthemiddleofthevaluesconsideredbyER95,while number of fitted data (ranging fro−m 8 to 10 depending on
we assume a value of 45◦ for the half-opening angle of the theavailabilityofISOand/orIRSphotometry)whilen is
par
torus(thevalues assumed byER95 for thenucleusof NGC thenumberofparametersofthemodel,i.e.theredshift,the
1068). The equatorial optical depth of the torus is fixed to normalization of the SED components and the SED model
1000.ThereforetheSEDoftheAGNtorusdependsonlyon
theviewing angle of thetorus, θ .
view
With the above choice of parameters the SED shows deep 1 Thebolometricluminosityisobtained byintegratingtheSED
absorptionfeaturesat 10µmduetosilicatedustforedge- intherest-framewavelength interval0.1-1000µm.
∼
(cid:13)c 2008RAS,MNRAS000,1–??
4 M. Negrello et al.
the6.5µm band. It includes 49 objects, 42 detected only at
Table 1. Some information about the Spitzer and AKARI in-
fraredcataloguesusedinthepresentwork:observingwavelength, 15µm,3atonly6.5µmand4atbothwavelengths.Ausselet
FullWithatHalfMaximum(FWHM)oftheinstrument,number al. also provide an additional, less secure, list of 51 sources
ofsurcesdetectedabove3σ,correspondingminimumsourceflux, ofwhich47aredetectedat15µmonly,4at6.5µmonly,but
andsurveyedarea. noneinbothfilters.Alltogetherthetwocatalogues include
a total of 100 objects.
instrument waveband FWHM ♯sources Smin Area
(µm) (arcsec) (µJy) (arcmin2)
Spitzer/IRAC 3.6 1.6 5792 0.52 230
Spitzer/IRAC 4.5 1.7 5576 0.53 230
3.2 Spitzer
Spitzer/IRAC 5.8 1.9 2328 2.74 226
Spitzer/IRAC 8.0 2.0 2186 1.80 249
AKARI/IRC 11.0 4.8 242 44.04 101 The Spitzer data for the HDF-N are part of the GOODS
AKARI/IRC 18.0 5.7 233 96.29 115 Spitzer Legacy Data Products3 and are in the public do-
Spitzer/MIPS 24.0 6.0 1199 80.00 254 main. The Spitzer data sets include the images of both
GOODSfieldsat3.6,4.5,5.8,8.0µmfromtheInfra-RedAr-
rayCamera(IRAC)andat24µmfromtheMultibandImag-
parameters.Asaresult2 npar =6ifbothf1 andf2 arepos- ingPhotometerforSpitzer(MIPS),plusalistofsourcesfor
itive, otherwise npar = 4 (f2 = 0) or npar = 3 (f1 = 0). A the MIPS 24µm imaging. The MIPS catalogue consists of
99% confidence interval on the estimated photometric red- 1199sourcesdetectedat3σ,withfluxdensitiesgreaterthan
shift is derived by the ∆χ2 method (Avni 1976), being the 80µJy (see Table 1), a limit where the source extraction is
number of “interesting parameters” equal to 1, i.e. zphot. statedtobehighlycompleteandreliable.Sourcelistsforthe
In this case the value of ∆χ2 defining the confidence inter- IRAC imaging have not yet been released by the GOODS
val at 99% is 6.63. A best SED-fit is considered “good” if consortium.ThereforewederivedourownIRACsourcecat-
Pχ2 >1%. alogues in GOODS-N using SExtractor (Bertin & Arnouts
1996) with the default set of values for the configuration
parameters. The sourcefluxesandthecorresponding errors
3 DATA SETS havebeenderivedfromtheautomaticaperturemagnitudes.
Sources were extracted down to a minimum signal-to-noise
In order to show the reliability of the redshift estimates ratio of 3. The resulting catalogues consist of 5792, 5576,
based on the PAH and silicate infrared features, we have 2328, 2186 sources at 3.6, 4.5, 5.8 and 8.0µm, respectively
assembled a sample of galaxies with flux measurements in (see Table 1).
the wavelength range 3.6-24 µm and with measured spec- Imaging at 16µm of selected areas within the GOODS-N
troscopicredshifts.Thesamplehasbeenselectedwithinthe field have been obtained as a result of a pilot study with
northernfieldoftheGreatObservatoriesOriginsDeepSur- the Spitzer IRS (Teplitz et al. 2005). The majority of the
vey(GOODS,Dickinson et al. 2001) because ofits richness area (30 of 35 arcmin2) reaches a 3σ sensitivity of 75µJy.
inmultiwavelengthphotometricdataandspectroscopicred- Teplitz et al. (2005) detected 149 objects at 16µ∼m, with
shifts. The GOODS observations covered two fields on the fluxrangingfrom 21µJyto1.24 mJy,andwith photometry
sky: a northern target area (GOODS-N) coincident with in good agreement with the 15µm ISO survey of the same
(butsignificantlylargerthan)theHubble-DeepFieldNorth area.
(HDF-N,Williams et al.1996), andasimilarly sized south-
ern field (GOODS-S) coincident with the Chandra Deep
Field South (Giacconi et al. 2000). The GOODS-N field
has been imaged at infrared wavelengths by different in- 3.3 AKARI
struments: at 6.75µm and 15µm by ISOCAM on board of
The GOODS-N region has recently been imaged at 11µm
the Infrared Space Observatory (ISO, Kessler et al. 1996),
and18µmwiththeAKARIInfra-RedCamera(IRC)within
at 3.5, 4.5, 5.8, 8µ and 24 µm by Spitzer, and, more re-
the FUHYU program. FUYHU is an AKARI mission pro-
cently, at 11 and 18µm by AKARI (Pearson et al. 2008).
gram to follow-up well-studied Spitzer fields (Pearson et
Some subregions of the GOODS-N field (including part of
al. 2008) including the GOODS-N field. This survey has
theHDF-N)werealsoimagedat16µmbytheSpitzerInfra-
mappedtheLockman-HoleandELAISN1fields,otherwell-
Red Spectrograph (IRS) blue peak-up filter (Teplitz et al.
studiedfieldswithsufficientlyhigh β eclipticlatitudesand
2005).Itisthesesetsofinfrareddatawehaveexploitedhere | |
consequently high AKARI visibility. The source extraction
and we provide brief descriptions below.
and flux calibration are described in Pearson et al. (2008),
and are based partly on thesource extraction methodology
3.1 ISO developed for sub-millimeter surveys (Serjeant et al. 2003).
The samples comprises 242 detections at 11µm and 233 at
WeusethecatalogueofISOsourcesintheHDF-Nproduced
18µm respectively, with a signal-to-noise ratio higher than
byAusseletal.(1999).Thesourcecatalogueisclaimedtobe
3 (see Table 1).
95%completeat200µJyinthe15µmbandandat65µJyin
2 Thetypeoftheextinctioncurve(i.e.MW,LMC,SMC)isnot
considered as a SED parameter and therefore it has not been 3 seehttp://data.spitzer.caltech.edu/popular/goods/Documents/
includedinthecalculationofnpar. goodsdataproducts.html
(cid:13)c 2008RAS,MNRAS000,1–??
Photo-z accuracy in AKARI Deep Surveys 5
3.4 Spectroscopic redshifts bands over the whole field and in HK′ over a smaller re-
gion covering the Chandra Deep Field South, down to 5σ
The region around the HDF-N had been the subject of in-
(AB magnitudes) limits of 27.1, 26.9, 26.8, 26.6, 25.6, 25.4
tensive spectroscopic campaigns in the 90s’ by a variety
and 22.1, respectively. The images and the corresponding
of groups using the Low Resolution Imaging Spectrograph
catalogues are available on theWorld WideWeb6.
(LRIS;Okeet al.1995) on theKeck10metertelescopes. A
Here,opticaldataareusedjustforcomparisonwiththe
compilationoftheLRIS-spectrafor671sourcesispresented
infrared imaging, buttheyarenotexploited intheSEDfit.
in Cohen et al. (2000). Subsequently, two parallel spectro-
scopic projects were carried out in the GOODS area using
the Deep Imaging Multi-Object Spectrograph (DEIMOS)
on the Keck II telescope. The Team Keck Treasury Sur-
vey (TKTS) of the GOODS-N field (Wirth et al. 2004)
has focused on an R < 24.4 magnitude selected sam-
AB
ple while the survey by Cowie et al. (2004) was embedded
withinobservationstargetedathigh-redshiftgalaxiesandX-
ray and radio selected galaxies. More recently, Steidel and
collaborators have conducted a spectroscopic survey in the
GOODS-N with the blue arm of LRIS (LRIS-B) targeting
severalhundredsofstar-forming galaxies andAGNsat red-
shifts1.4 < z < 3.0.Thesourcecandidatesforspectroscopic
∼ ∼
follow-up were pre-selected using different criteria based on
theiropticalcolours(Steideletal.2003,2004;Adelbergeret
al. 2004). The resulting spectroscopic catalogue, presented
byReddyet al. (2006) includes342 objects and providefor
eachofthembothopticalphotometryandinfraredphotom-
etryat3.6,4.5,5.8,8.0and24µmfromSpitzer.Weusehere
the compilation4 of redshifts assembled by the Keck Team
combiningallthespectroscopicsurveyswithintheGOODS-
N field produced up to the 2004, and we add to it the
spectroscopic sample of Reddy et al. (2006). Note that the
TKTSisbasedprimarlyonobservationswiththeDEIMOS
instrument on Keck II, whose spectral range only allows
the detection of emission and absorption features from ob-
jects at z < 1.2. Conversely, the selection criteria used in
∼
the spectroscopic survey of Reddy et al. is better at identi-
fyinggalaxies at z > 1.4. Wefoundindeedthat only59 out
∼
of the 342 spectroscopic sources listed in the Reddy et al.
catalogue are already included in the TKTS compendium
redshift catalogue.
3.5 Optical data
Optical imaging of the GOODS-N field was obtained with
theAvancedCameraforSurvey(ACS)onboardtheHubble
SpaceTelescope(HST)atB,V,i′,z′bands.TheACSimages
and the source catalogues extracted by the GOODS Team
are of publicdomain5.
Optical/near-infrared ground-based images are also
available for the GOODS-N field (Capak et al. 2004). An
intensive multi-color imaging survey of 0.2 deg2 centered
∼
on theHDF-Nhavebeen carried out using different instru-
ments:theKittPeakNationalObservatory(KPNO)4meter
telescopewiththeMOSAICprimefocuscamera,theSubaru
8.2 metertelescope with theSuprime-Cam instrument,and
theQUIRCcamera on theHawaii 2.2 meter telescope. The
surveyed area is referred to as the Hawaii HDF-N (Capak
et al. 2004). Data were collected in U, B, V, R, I and z′
4 http://www2.keck.hawaii.edu/tksurvey/
5 http://archive.stsci.edu/prepds/goods/ 6 http://www.ifa.hawaii.edu/capak/hdf/index.html
(cid:13)c 2008RAS,MNRAS000,1–??
6
Table2. ListofinfraredsourceswithspectroscopicredshiftdrawnfromtheGOODS-Nfield,bycross-matchingthesourcelistsofAusseletal.(1999)andofTeplitzetal.(2005)with
theSpitzerandAKARIcatalogues inthesamefield.Thesourcepositionsreportedherearethoseofthespectroscopiccounterpart.
M
.
ID Spec.position zspec F3.6µm F4.5µm F5.8µm F8µm F24µm F6.5µm F15µm F11µm F18µm F16µm ISOname N
αJ2000 δJ2000 e
(hms) (◦′ ′′) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) g
r
e
ID1 123648.30 +621426.86 0.1389 86.3+−22..22 60.1+−11..77 54.8+−33..22 386.6+−44..00 460.0+−66..11 254+−7713 307+−6627 364+−1155 291+−3322 283+−2200 HDFPM324 llo
ID2 123723.79 +621046.35 0.1133 92.9+2.9 62.1+2.3 54.9+4.2 242.3+4.2 198+6.6 - - 184+16 129+33 141+24 -
−2.9 −2.3 −4.2 −4.2 −6.6 −16 −33 −24 e
ID3 123612.48 +621140.79 0.2759 136.9+4.1 127.5+3.5 99.9+6.4 401.9+6.1 1240+13 - - 900+15 1069+32 973+27 - t
−4.1 −3.5 −6.4 −6.1 −13 −15 −32 −27
ID4 123634.47 +621213.45 0.4573 316.3+5.7 244.5+4.5 196.0+7.9 338.4+5.0 1290.0+8.8 - 448+68 858+18 897+32 853+32 HDFPM32 a
ID5 123622.94 +621526.97 2.5920 46.4−+25..27 51.1+−14..95 72.8−+47..89 130.1+−35..00 529.0−+68..28 - −59- 159+−1158 348+−3322 335+−2392 - l.
−2.2 −1.9 −4.8 −3.0 −6.2 −15 −32 −29
ID6 123708.32 +621056.41 0.4225 61.9+1.9 64.5+1.9 50.0+3.2 183.3+3.0 648.0+7.4 - - 576+15 494+33 423+26 -
−1.9 −1.9 −3.2 −3.0 −7.4 −15 −33 −26
ID7 123719.14 +621131.58 0.5560 74.1+2.1 52.3+1.7 48.5+3.2 50.7+1.7 190.0+5.9 - - 171+15 200+32 213+24 -
−2.1 −1.7 −3.2 −1.7 −5.9 −15 −32 −24
ID8 123639.71 +621526.68 0.3765 46.6+1.7 40.3+1.5 27.9+2.4 66.7+1.7 161.0+5.7 - - 121+15 136+32 121+19 -
−1.7 −1.5 −2.4 −1.7 −5.7 −15 −32 −19
ID9 123651.12 +621031.23 0.4099 90.7+3.3 91.6+2.9 74.0+5.0 261.1+4.6 984+9.1 - 341+40 713+15 657+33 745+29 HDFPM328
−3.3 −2.9 −5.0 −4.6 −9.1 −65 −15 −33 −29
ID10 123603.26 +621111.27 0.6382 135.2+4.1 83.6+2.8 97.8+6.2 92.2+3.0 1210.0+9.5 - - 608+22 733+144 655+33 -
−4.1 −2.8 −6.2 −3.0 −9.5 −22 −144 −33
ID11 123622.50 +621544.78 0.6393 71.4+2.6 48.5+1.8 57.1+4.1 67.3+2.1 721.0+6.6 - - 294+15 346+32 390+28 -
−2.6 −1.8 −4.1 −2.1 −6.6 −15 −32 −28
ID12 123650.20 +620845.09 0.4335 101.3+3.3 86.3+2.9 79.8+5.5 129.9+3.6 585.0+7.8 - - 323+15 293+33 348+20 -
−3.3 −2.9 −5.5 −3.6 −7.8 −15 −33 −20
ID13 123641.56 +620948.54 0.5186 204.8+4.9 151.7+3.8 128.6+7.3 152.5+4.1 433.0+6.9 - - 355+15 322+34 327+23 -
−4.9 −3.8 −7.3 −4.1 −6.9 −15 −34 −23
ID14 123655.75 +620917.80 0.4191 72.9+2.8 71.7+2.6 67.4+4.9 150.6+3.7 846.0+9.9 - - 411+15 426+32 408+21 -
−2.8 −2.6 −4.9 −3.7 −9.9 −15 −32 −21
ID15 123643.98 +621250.44 0.5560 58.9+2.1 46.9+1.8 52.6+3.7 64.4+2.1 424+4.6 <50 282+60 281+15 317+34 343+22 HDFPM317
−2.1 −1.8 −3.7 −2.1 −4.6 −64 −15 −34 −22
ID16 123648.63 +620932.55 0.5174 31.6+1.9 25.7+1.6 20.1+2.7 29.8+1.8 127.0+7.3 - - 86+15 131+32 104+16 -
−1.9 −1.6 −2.7 −1.8 −7.3 −15 −32 −16
ID17 123653.89 +621254.40 0.6419 58.4+1.9 37.3+1.4 36.5+2.7 25.3+1.2 200.0+6.3 <36 179+60 92+15 181+32 207+22 HDFPM333
−1.9 −1.4 −2.7 −1.2 −6.3 −43 −15 −32 −22
ID18 123636.80 +621213.46 0.8477 126.4+3.5 83.7+2.7 68.9+4.8 50.4+2.1 379.0+5.2 <113 202+58 101+15 261+32 300+22 HDFPM38
−3.5 −2.7 −4.8 −2.1 −5.2 −50 −15 −32 −22
ID19 123651.79 +621354.19 0.5561 46.7+1.6 34.8+1.3 36.2+2.6 42.2+1.4 203.0+6.3 <39 151+74 195+15 176+32 185+30 HDFPM330
−1.6 −1.3 −2.6 −1.4 −6.3 −68 −15 −32 −30
ID20 123617.44 +621551.58 0.3758 22.7+1.6 25.9+1.5 17.5+2.7 54.9+2.2 145.0+9.7 - - 119+25 116+32 95+23 -
−1.6 −1.5 −2.7 −2.2 −9.7 −25 −32 −23
ID21 123646.18 +621142.41 1.0164 108.2+3.4 79.4+2.7 49.4+4.2 41.5+2.0 290.0+4.9 88+45 170+59 61+15 212+37 275+18 HDFPM319
−3.4 −2.7 −4.2 −2.0 −4.9 −80 −42 −15 −37 −18
ID22 123646.89 +621447.78 0.5560 44.1+1.6 32.2+1.3 30.2+2.4 33.9+1.3 277+11 <179 144+72 66+15 266+32 193+17 HDFPM323
−1.6 −1.3 −2.4 −1.3 −11 −47 −15 −32 −17
ID23 123631.65 +621604.41 0.7840 44.9+1.7 30.0+1.3 30.9+2.6 23.8+1.2 301+5.3 - - 87+15 239+32 245+22 -
−1.7 −1.3 −2.6 −1.2 −5.3 −15 −32 −22
ID24 123701.49 +620842.37 0.7038 37.7+2.3 20.7+1.6 23.1+3.5 21.3+1.9 185.0+7.2 - - 70+15 123+33 130+24 -
−2.3 −1.6 −3.5 −1.9 −7.2 −15 −33 −24
ID25 123649.72 +621313.39 0.4745 53.5+1.8 49.0+1.6 47.4+3.1 113.0+2.4 371+10 136+68 320+39 356+15 410+33 370+27 HDFPM327
−1.8 −1.6 −3.1 −2.4 −10 −57 −62 −15 −33 −27
ID26 123639.93 +621250.38 0.8462 64.1+2.5 38.9+1.7 40.0+3.5 30.9+1.6 493.0+6.1 <64 302+67 120+15 288+32 425+20 HDFPM311
−2.5 −1.7 −3.5 −1.6 −6.1 −55 −15 −32 −20
ID27 123617.33 +621529.95 0.8497 79.4+3.1 55.8+2.2 52.9+4.6 31.6+1.7 499.0+7.2 - - 93+15 354+32 448+27 -
−3.1 −2.2 −4.6 −1.7 −7.2 −15 −32 −27
ID28 123633.59 +621320.24 0.8446 49.9+2.3 32.9+1.7 34.6+3.6 23.1+1.5 323.0+6.8 - 122+54 68+15 192+35 320+29 HDFPS33
−2.3 −1.7 −3.6 −1.5 −6.8 −40 −15 −35 −29
ID29 123649.50 +621407.13 0.7517 32.6+1.3 21.4+1.0 21.7+2.0 17.6+1.0 186.0+6.4 <40 150+74 69+15 143+33 130+20 HDFPM326
−1.3 −1.0 −2.0 −1.0 −6.4 −48 −15 −33 −20
ID30 123633.66 +621006.20 1.0156 72.1+3.2 56.2+2.4 44.0+4.1 45.4+2.2 581.0+9.0 - - 94+15 398+32 438+25 -
−3.2 −2.4 −4.1 −2.2 −9.0 −15 −32 −25
ID31 123633.14 +621514.01 0.5196 16.7+1.1 13.9+0.9 13.5+1.9 17.8+1.1 142.0+7.0 - - 112+15 182+32 41+17 -
−1.1 −0.9 −1.9 −1.1 −7.0 −15 −32 −17
c(cid:13) ID32 123654.81 +620847.67 0.7913 64.1+2.7 44.1+2.1 41.9+4.0 42.9+2.1 246.0+5.6 - - 75+15 102+33 134+17 -
−2.7 −2.1 −4.0 −2.1 −5.6 −15 −33 −17
20 ID33 123646.23 +621527.64 0.8510 84.52−.22.2 56.7+−11..77 54.2+−33..33 42.3+−11..55 544.0+−77..66 - 418+−9914 119+−1155 350+−3322 433+−2277 HDFPM321
08 ID34 123655.94 +620808.58 0.7920 108.5+−33..99 76.0+−33..22 73.2+−66..11 73.9+−33..33 832.0+−77..55 - - 213+−1155 384+−3322 568+−2200 -
R ID35 123632.48 +621513.57 0.6827 34.4+1.6 23.0+1.2 22.8+2.3 18.8+1.1 142.0+6.7 - - 112+15 182+32 143+17 -
A −1.6 −1.2 −2.3 −1.1 −6.7 −15 −32 −17
S,M IIDD3376 112233663371..8500 ++662211114194..7502 01..80318204 3472..38+−+−2222....0101 3312..63+−+−1111....6767 2297..29+−+−3333....2222 2349..67+−+−1212....8181 147830..00+−+−3535....6969 <61- 231525+−+−54568050 817.45+−+−11115555 223887+−+−33333434 144320+−+−22220000 HHDDFFPPMM33110
N ID38 123638.30 +621151.16 0.8410 36.9+1.9 23.3+1.4 29.3+3.2 16.9+1.5 230.0+3.3 <61 212+58 81.4+15 238+33 189+19 HDFPM310
RA ID39 123634.53 +621241.34 1.2190 66.1−+−212...696 71.9−+−212...444 58.1−+−434...424 72.6−+−212...454 446.0−+−535...131 - 363+−−735995 104+−−111555 705+−−333223 923+−−331229 HDFPM33
S ID40 123634.86 +621628.62 0.8470 58.9+1.9 40.1+1.5 38.5+2.8 34.8+1.3 482.0+4.6 - - 104+15 300+32 368+21 -
,1–000 IIIDDD444123 111222333665345516...541930 +++666222111412241429...385217 200...059045588005 758190...694+−−+−+223212......451495 966910...925+−−+−+222212......534553 1645363...478+−−+−+644624......307380 2837299...674+−−+−+422412......009030 24231504..8000−+−++−564511....001060 127+−9691-- 441<+−487322- 31615995−++−+−111111555555 217435976−++−+−333333432232 216931545−++−+−213213552152 -HHDDFFPPMM3251
? −3.1 −2.4 −4.7 −2.9 −6.1 −15 −34 −25
? ID44 123644.35 +621453.34 1.4865 13.8+0.9 14.1+0.9 16.8+1.9 21.7+1.1 81.7+6.4 <329 105+94 47+15 133+32 37+16 HDFPM318
−0.9 −0.9 −1.9 −1.1 −6.4 −21 −15 −32 −16
continuedonnextpage
c(cid:13) Table 2.Continued
2
0
0
8
R ID Spec.position zspec F3.6µm F4.5µm F5.8µm F8µm F24µm F6.5µm F15µm F11µm F18µm F16µm ISOname
A αJ2000 δJ2000
S (hms) (◦′ ′′) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy) (µJy)
,
M
N ID45 123629.16 +621046.46 1.0130 98.1+3.5 88.3+3.1 76.0+5.3 70.6+2.7 724.0+12 - - 75+15 434+33 465+25 -
RA ID46 123646.60 +621049.36 0.9399 32.6−+−131...959 24.3−+−131...515 24.3−+−353...030 18.2−+−121...474 354+−−661..552 - 327+−3693 71+−−111555 259+−−333223 315+−−222995 HDFPM322
S ID47 123636.86 +621135.17 0.07860 170.6+4.1 114.9+3.1 113.5+6.0 545.2+6.6 732.0+9.1 <135 300+62 370+15 377+32 - HDFPM37
−4.1 −3.1 −6.0 −6.6 −9.1 −67 −15 −32
00 ID48 123636.64 +621347.12 0.9590 78.4+−22..66 77.3+−22..33 91.1+−55..00 110.6+−22..77 474.0+−55..99 - 353+−4606 188+−1155 346+−3333 - HDFPM36
,0 ID49 123705.87 +621154.03 0.9032 83.5+−22..33 55.8+−11..77 50.4+−33..11 39.7+−11..44 655.0+−88..11 - 431+−4830 100+−1155 397+−3333 - HDFPM345
1–?? IIDD5501 112233665593..9221 ++662211415107..2182 00..79631500 5803..54+−+121...787 3538..64+−+121...323 3591..35+−+242...717 3437..01+−+121...222 436667..00+−+565...545 -- 219754+−[phone]+−[phone]+−+333434 -- HHDDFFPPMM334301
−2.8 −2.2 −4.1 −2.2 −6.4 −43 −15 −33
ID52 123653.37 +621139.97 1.2698 32.8+1.6 34.6+1.6 26.4+2.7 40.3+1.7 322.0+6.2 - 180+60 57+15 393+33 - HDFPM332
−1.6 −1.6 −2.7 −1.7 −6.2 −43 −15 −33
ID53 123658.99 +621209.20 0.8517 62.7+2.0 42.6+1.6 35.1+2.7 24.9+1.2 269.0+6.0 <66 157+75 85+15 179+33 - HDFPM339
−2.0 −1.6 −2.7 −1.2 −6.0 −49 −15 −33
ID54 123702.74 +621402.02 1.2463 62.6+1.9 60.0+1.7 40.3+2.8 49.5+1.6 334.0+7.6 - 144+73 61+15 417+33 - HDFPM344
−1.9 −1.7 −2.8 −1.6 −7.6 −47 −15 −33
ID55 123657.79 +621455.28 0.8493 45.9+1.6 31.8+1.2 33.4+2.5 31.0+1.2 366.0+7.8 - 225+60 79+15 258+32 - HDFPM337
−1.6 −1.2 −2.5 −1.2 −7.8 −56 −15 −32
ID56 123638.13 +621116.44 1.0174 52.2+2.3 39.6+1.9 32.7+3.4 31.9+1.9 291.0+4.9 - 212+58 56+15 287+39 - HDFPM39
−2.3 −1.9 −3.4 −1.9 −4.9 −55 −15 −39
ID57 123654.64 +621127.43 0.2542 50.1+2.0 43.7+1.8 27.1+2.8 118.2+2.9 173.0+6.0 - 42+29 180+15 103+33 - HDFPS323
−2.0 −1.8 −2.8 −2.9 −6.0 −09 −15 −33
ID58 123642.19 +621545.77 0.8572 126.6+2.7 102.6+2.3 106.1+4.5 121.0+2.3 850.0+7.5 - 459+46 190+15 457+32 - HDFPS310
−2.7 −2.3 −4.5 −2.3 −7.5 −86 −15 −32
ID59 123704.33 +621446.58 2.2110 5.6+0.6 8.5+0.7 15.7+1.8 39.5+1.4 376.0+4.1 - 80+74 97+15 187+32 - HDFPS336
−0.6 −0.7 −1.8 −1.4 −4.1 −19 −15 −32
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8 M. Negrello et al.
3.6 Cross-matching bands).ForthisreasonwehavedecidednottoincludeMD39
in ourfinalcatalogue and toadd instead ID24tothelist of
We have cross-matched the spectroscopic catalogues with ourinfrared sources with uncertain far-IR photometry.
each of the infrared catalogues mentioned above by using Fig. 3 shows the observed flux ratios as a function of
forthematchingradiusavalueequaltotwicetheGaussian the spectroscopic redshifts (black dots), and compare them
rms width σFWHM (=FWHM/2√2ln2) of the instrument to the predictions from the starburst reference templates
beam, where σFWHM is 6.4′′ for ISOCAM at 15µm, 0.68′′, for a representative set of SED model parameters (blue
0.72′′, 0.81′′, 0.85′′ for IRAC at 3.6, 4.5, 5.8, 8.0µm respec- curves), and from the AGN reference templates (megenta
tively, 2.55′′ for MIPS at 24µm, 1.53′′ for IRS at 16 µm, curves).TheeffectsofthePAH6.2µmemissionfeatureand
2.04′′ and 2.42′′ for the AKARI at 11 and 18µm, respec- of the 9.7µm silicate absorption are clearly manifest in the
tively.Inordertoensurefullspectralcoveragefrommid-to data. The deep “well” observed in the F /F , F /F ,
5.8 8.0 8.0 11
far-IR wavelengths we kept only spectroscopic sources with F /F and F /F diagrams at z 0.2, z 0.6, z 1.2
11 15 11 16
acounterpartin allof theSpitzer andAKARIinfrared cat- andz 1.4isduetothepassageoft∼hePAH6∼.2µmfe∼ature
aloguesconsideredhereplusacounterpartineithertheISO throug∼h the 8.0, 11, 15 and 16µm wavebands, respectively.
15µm or the IRS 16µm catalogues. The advantage of cov- The9.7µmsilicate absorption manifestsitselfasabumpin
ering the whole mid- to far-IR spectral range is made clear the 8.0/11µm flux ratios around z 0 where the feature
bylooking at themiddle panelof Fig. 1, where the Spitzer, falls into the AKARI11µm passban∼d. As the same feature
AKARIandISOfiltersareshown:allthemainPAHandsil- entersthe15and16µmwavebands,whichoccursatz 0.5,
icatefeaturesaresampleduptoz 2withthephotometric it producesa significant corresponding peak in theF∼/F
∼ 11 15
data exploited here. At z > 2 the PAH features are shifted andF /F diagrams. Ahintofabumpduetothesilicate
∼ 11 16
outsidethewavelengthrangecoveredbySpitzerandAKARI absorption is seen also in the 15/18µm and 16/18µm flux
so that they can no longer be exploited as redshift indica- ratios at z 0.7, although it is not particularly prominent
∼
tors. In this case, the rest-frame 1.6µm bump can be used dueto thewider wavelength coverage of the AKARI18µm
instead for photometric redshift estimates (Sawicki 2002). filter compared to the 11µm band. Finally, by entering the
We ended up with a sample of 59 objects; 22 of them MIPS 24µm band, the silicate absorption induces another
have flux measurements at both 15µm and 16µm. ISO significantbumpinthe18/24µmfluxratioatz 1.5.Note
∼
6.5µm flux measurements (or upper limits) have been in- thatsuchabumpcan beusedforefficiently selectingultra-
cluded,whenavailable,intheSEDfittingprocess.Thered- luminous infrared galaxies in the redshift range 1 < z < 2
∼ ∼
shifts of the sources and their infrared fluxes are listed in withAKARI(seeTakagi&Pearson2005).The 10µmsil-
∼
Table2.WealsoprovideinFig.2postagestampimagesfor icate absorption is also responsible for the behaviourin the
allthesourcesinoursampleatR,HK′,SpitzerandAKARI predicted flux ratios of the AGN templates when the torus
wavebands. is seen edge-on. For a face-on AGN the corresponding flux
For almost all of the sources in the sample we found a ratios are almost independentof redshift, implying that for
single counterpart at each infrared waveband. However few a power-law infrared spectrum the recovery of the redshift
of them appear to lie in crowded regions, possibly affect- from mid-/far-IR photometry alone is extremely challeng-
ingtheinfrared fluxatthelongest wavebands,seee.g.ID3, ing, if not impossible.
ID20,ID25,ID31,ID35,ID37,ID38andID44.Therearein Onaveragetherangeoffluxratiosspannedbythedata
particularpairsof objects“sharing” thesameAKARI(and isaccountedforbythemodelwiththesoleexceptionofthe
ISO)fluxes,i.e.ID31-ID35andID37-ID38.Inthiscasesthe 15/16µmfluxratios.Indeedweobserveforfewobjectsasig-
AKARI(andISO)photometryshouldbetakenasanupper nificant steep increase of the flux from 15 to 16µm (ID28,
limittotheintrinsicfluxofthesourceatthosewavelengths. ID39, ID42, which lie well below the theoretical expecta-
tions) or, conversely, a notable decrease of the flux when
Most of the sources in the sample lie at z < 1.5. Only
∼ movingfrom 15 to 16µm (ID44).
3 out of 59 have z > 1.5: ID5 (z = 2.5920), ID41
spec spec
(z = 2.0050) and ID59 (z = 2.2110). We found that
spec spec
just five objects in the Reddy et al. catalogue have a coun-
4 RESULTS OF SED FITTING
terpart at both 11 and 18µm and either at 15 or 16µm.
They are (according to the names they have in the Reddy Theresults of theSED fittingare listed in Table 3.A com-
etal.sourcelist):BX1321(atz=0.139),BM1156(at2.211), parison betweenthephotometricdataandthebest-fitSED
BM1299 (at z=1.595), BM1326 (at 1.268), and MD39 (at modelisshowninFig.4.Thederivedphotometricredshifts
2.583).FourofthemareincommonwiththeTKTSspectro- versus the spectroscopic redshifts are presented in Fig. 5
scopiccatalogueandfallintothefinalsampleof59infrared where sources with Pχ2 > 1% are indicated by filled cir-
sourcespresentedhere,correspondingtoID1,ID5,ID52and cles (42 in total, i.e. 70% of the whole sample), and
∼
ID59.MD39isinsteadmissingfromourfinalsamplebecause those with Pχ2 < 1% are identified by open circles. In the
it lacks a counterpart in our IRAC catalogues at both 3.6 same figure the solid line marks the ideal case in which
and 8.0µm. MD39 appears to be the source very close to zphot = zspec, while the dot-dashed lines delimit the region
ID24inthecorrespondingpostagestampimageofFig.2,at where zphot zspec/(1+zspec) < 10%. Error bars corre-
| − |
wavelength λ < 5.8µm. The pair of objects is not resolved sponding to a 99% confidence limit have been drawn, for
at longer wave∼lengths and indeed Reddy et al. do not pro- reason of clarity, only for objects with Pχ2 > 1% which lie
videa24µmfluxestimateforit,probablybecauseitcannot within the10% confidence region.
easily bedeblended from thenearby ID24 source (although For almost all of the sources with zspec < 1.5 and
∼
theyprovideforitfluxestimatesatallthefourIRACwave- Pχ2 > 1% the redshift is recovered with an accuracy
(cid:13)c 2008RAS,MNRAS000,1–??
Photo-z accuracy in AKARI Deep Surveys 9
Figure 2. 20′′×20′′ postage stamp images centered on the infrared sources listed in Table 2. From left to right: R and HK′ optical
images, IRAC images at 3.6, 4.5, 5.88.0µm, AKARIimages at 11 and18µm, andMIPS imageat 24µm. Red dots inthe Spitzer and
AKARIimagesrepresent3σ detectionsatthecorrespondingwavebandswhilethecircleisthe“error”-circleof2σFWHM-radiususedfor
cross-matching the spectroscopic catalogue with the infrared catalogues, i.e. 2σFWHM = 4.08′′, 4.84′′ and 5.10′′ at 11, 18 and 24µm,
respectively.Acircleof1.5′′ intheRbandimageindicatesthespectroscopiccounterpart.
(cid:13)c 2008RAS,MNRAS000,1–??
10 M. Negrello et al.
Figure 2.Continued.
(cid:13)c 2008RAS,MNRAS000,1–??