Spitzer Imaging of Herschel-ATLAS Gravitationally Lensed Submillimeter Sources

0.43 MB·English

by  R. Hopwood

Draftversion January17,2011
PreprinttypesetusingLATEXstyleemulateapjv.08/22/09
SPITZER IMAGING OF HERSCHEL-ATLAS GRAVITATIONALLY LENSED SUBMILLIMETER SOURCES
R. Hopwood1, J. Wardlow2, A. Cooray2, A. A. Khostovan2, S. Kim2, M. Negrello1, E. da Cunha3, D.
Burgarella4, I. Aretxaga5, R. Auld6, M. Baes7, E. Barton2, F. Bertoldi8, D. G. Bonfield9, R. Blundell10, S.
Buttiglione11, A. Cava12,13, D. L. Clements14, J. Cooke3,15, H. Dannerbauer16, A. Dariush6,35, G. de Zotti11,17, J.
Dunlop18, L. Dunne19, S. Dye6, S. Eales6, J. Fritz7, D. Frayer20, M. A. Gurwell10, D. H. Hughes5, E. Ibar21, R. J.
Ivison18,21, M. J. Jarvis9, G. Lagache23,24, L. Leeuw25,26, S. Maddox19, M. J. Micha lowski18, A. Omont27, E.
Pascale6, M. Pohlen6, E. Rigby19, G. Rodighiero28, D. Scott29, S. Serjeant1, I. Smail30, D. J. B. Smith19, P.
Temi31, M. A.Thompson9, I. Valtchanov32, P. van der Werf18,23, A. Verma34, J. D. Vieira15
1
1 Draft version January 17, 2011
0
2 ABSTRACT
We present physical properties of two submillimeter selected gravitationally lensed sources, identi-
n
fiedintheHerschelAstrophysicalTerahertzLargeAreaSurvey. Thesesubmillimetergalaxies(SMGs)
a
J have flux densities > 100mJy at 500µm, but are not visible in existing optical imaging. We fit light
profiles to each component of the lensing systems in Spitzer IRAC 3.6 and 4.5µm data and success-
3
fully disentangle the foreground lens from the background source in each case, providing important
1
constraintsonthe spectralenergydistributions(SEDs)ofthe backgroundSMGatrest-frameoptical-

] near-infraredwavelengths. The SED fits show that these two SMGs have high dust obscurationwith
O AV ∼4−5andstarformationratesof∼100M⊙yr−1. Theyhavelowgasfractionsandlowdynamical
C masses compared to 850µm selected galaxies.
Subject headings: galaxies: individual (SDP.81: H-ATLAS J090311.6+003906, SDP.130: H-ATLAS
.
h J091305.0-005343)— galaxies: starburst — gravitationallensing: strong
p
-
o
1. INTRODUCTION
tr 1Department ofPhysics andAstronomy, TheOpenUniversity, Gravitational lensing is an invaluable astrophysical
s MiltonKeynes,MK76AA,UK
a 2Department of Physics and Astronomy, University of Califor- tool. Itcanbeexploitedtostudygalaxiesbeyondinstru-
[ nia,Irvine,CA92697,USA mentalblankfieldsensitivitiesandtoconstrainthetotal
3DepartmentofPhysics,UniversityofCrete,Heraklion,Greece mass of galaxy systems, without regard for its dark or
2 4Laboratoired’AstrophysiquedeMarseille,OAMP,CNRS,Aix-
luminous nature (see review by Treu 2010). The magni-
v MarseilleUniversit´e,France
0 5InstitutoNacionaldeAstrof´ısica,O´pticayElectr´onica,Aptdo. fication of distant galaxies through gravitationallensing
4 Postal51y216,72000Puebla,Mexico enables the detailed study of sources that would other-
9 6School of Physics and Astronomy, Cardiff University, Cardiff, wise be undetectable. In the submillimeter regime this
CF243AA,UK
4 7Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan includes members of the population of intrinsically faint
. 281S9,B-9000Gent, Belgium galaxiesthatcompriseasignificantfractionofthecosmic
1 8ArgenlanderInstitute forAstronomy,UniversityofBonn,Auf far-infraredbackground.
1 demHugel71,53121Bonn,Germany The Herschel35 (Pilbratt et al. 2010) Astrophysical
0 9CentreforAstrophysicsResearch,ScienceandTechnologyRe-
Large Area Survey (H-ATLAS; Eales et al. 2010) is the
1 searchCentre,UniversityofHertfordshire,HertsAL109AB,UK
: 10Harvard-Smithsonian Center for Astrophysics, Cambridge, largestopen-timekeyprojectcurrentlybeingundertaken
v MA02138,USA
Xi tor1i1oI5N,AI-F3,51O2s2sePravadtoovrai,oIAtasltyronomico di Padova, Vicolo Osserva- 23Institut dAstrophysique Spatiale (IAS), Batiment 121, Uni-
r 12Instituto de Astrofisica de Canarias, C/Va Lactea s/n, E- ver2s4itC´eNPRarSis(USuMdR18161971)4,0951O40r5saOy,rFsarya,nFcerance
a 3821300DLepaaLrtaagmunenat,oSpdaeinAstrofisica, Universidad de La Laguna 25Physics Department, University of Johannesburg, P.O. Box
(ULL),E-38205LaLaguna, Tenerife,Spain 524,AucklandPark2006,SouthAfrica
14Astrophysics Group, Physics Department, Imperial College 2267SInEsTtiItuItnsdt’iAtustter,opMhoyusinqtuaeindVeiPewar,isC,AU,n9iv4e0r4s3it,´eUPSiAerreetMarie
London,PrinceConsortRoad,LondonSW72AZ,UK
15California Institute of Technology, MS 249-17, 1216 E. Cali- Cu2r8ieDaipnadrtCimNeRnSt,o9d8iAbisstrboonuolmeviaar,dUAnirvaegros,it7a5d0i1P4aPdaorvias,,VFricaonlcoeOs-
forniaBlvd.,Pasadena,CA91125,USA
16Laboratoire AIM Paris Saclay, CEA-CNRS-Universit´e, ser2v9aDtoerpioar2t,mIe-3n5t1o2f2PPhaydsoicvsa,aIntdalAystronomy, Universityof British
Irfu/Service d’Astrophysique, CEA Saclay, Orme de Merisiers,
Columbia,Vancouver,BCV6T1Z1,Canada
9111791ScGuiofl-asurI-nYtevrenttaeziConedaleex,SFurapnecrieore di Studi Avanzati, Via 30Institute for Computational Cosmology, Durham University,
SouthRoad,Durham,DH13LE,UK
Bo1n8oSmUePaA2,65I,nIs-t3it4u1t3e6TforriesAtest,rIotnaolymy, University of Edinburgh, 31Astrophysics Branch, NASA Ames Research Center, Mail
Stop245-6,MoffettField,CA94035, USA
Ro1y9aSlcOhbosoelrovfatPohryy,siEcsdiannbdurAghst,rEonHo9m3yH, UJ,nUivKersityofNottingham, 32Herschel Science Centre, European Space Agency, P.O. Box
78,28691VillanuevadalaCan˜ada,Madrid,Spain
Un2i0veNrasittiyonPaalrRk,aNdioottAinsgthroanmomNyG7Ob2RseDrv,aUtoKry P.O. Box 2, Green 33Leiden Observatory, Leiden University, P.O. Box 9513, NL
2300Leiden,TheNetherlands
Ba2n1kU,KWVAs2t4ro9n4o4m, UySTAechnology Center, Royal ObservatoryEdin- 34Oxford Astrophysics, Denys Wilkinson Building, University
ofOxford,KebleRoad,Oxford,OX13RH,UK
bu2r2ghS,coEtdtiisnhbuUrgnhi,veErHsit9ie3sHPJ,hUysKics Alliance, University of Edin- 35School of Astronomy, Institute for Research in Fundamental
Sciences(IPM),POBox19395-5746, Tehran,Iran
burgh,RoyalObservatory,Edinburgh,EH93HJ,UK
2 Hopwood et al.
by Herschel and aims to survey 550 deg2 and detect 2, respectively.
> 300,000 galaxies. A new methodology for selecting
gravitational lenses using wide-area submillimeter (sub-
mm) surveys (Blain 1996, Perrotta et al. 2002; Negrello
etal.2007)hasbeentestedforthefirsttimeduringHer-
schel’s science demonstration phase (SDP; Negrello et
al. 2010, N10 hereafter). Candidates are selected us-
ing a flux cut of 100mJy at 500µm, a limit based on
the steep number counts slope in the sub-mm (Negrello
et al. 2007). Above this flux limit only gravitationally
lensed objects and easily identifiable ‘contaminants’ re-
maine.g. blazarsandlocalspiralgalaxies. Initialresults
show that this selection method has ∼100% efficiency
and should deliver a sample of hundreds of new grav-
itational lenses in planned wide area Herschel surveys,
probing galaxieswith intrinsic fluxes below the Herschel
confusion limit.
Five lens candidates in the SDP H-ATLAS data (Pas-
cale et al. 2010; Rigby et al. 2010; N10) were confirmed
withspectroscopicredshiftsobtainedviathedetectionof
carbon monoxide (CO) emission lines (Lupu et al. 2010;
Frayer et al. 2011) of the background galaxies and opti-
cal spectra (Negrello et al. 2010)of the lens galaxies. In
this Letter we study two of these gravitational lenses,
H-ATLAS J090311.6+003906 (SDP.81) and H-ATLAS
J091305.0-005343 (SDP.130), which are submillimeter
galaxy (SMG) at redshifts of z = 3.04 and z = 2.63
being lensed by ellipticals at z = 0.30 and z = 0.22,
respectively. Submillimeter Array (SMA) imaging re-
veals the sub-mm morphology, consistent with a lensing
event, with multiple peaks distributed around the po-
sition of the foreground elliptical galaxy (N10). While
Fig.1.—Kecki-band(toprow),IRACChannel1(middlerow),
no lensed background images were detectable in optical and Channel 2 (bottom row) postage stamp images for SDP.81.
imaging, the spectral energy distribution (SED) models The corresponding residual (right column) is presented after sub-
suggestthatflux fromthe backgroundsourcesshouldbe traction of single de Vaucouleurs profiles modeled on the Keck i-
band data. The Keck residuals show no signs of structure asso-
detectableabove∼10µJyatnear-IRwavelengths,from2
ciable with the overlaid 880 µm SMA signal-to-noise ratio (S/N)
to5 µm(N10). Atthesewavelengths,the emissionfrom contours. For the IRAC residual there is structure that can be
the foreground lenses, at z ∼ 0.25, and the background associated with the SMA contours, which suggests this is lensed
structureoftherespectivebackgroundgalaxy. TheSMAcontours
SMGs becomes comparable.
are overlaid in steps of 3σ, 8σ, 13σ etc. SMA peaks with S/N >
In this Letter we present Spitzer/IRAC imaging, light
8(four for ID81 and two for ID130) have amaximum rms on the
profile models, photometry and physical characteristics positionofapoint sourceof 0.1′′. Allpostage stamps areplotted
forSDP.81andSDP.130derivedfromSEDfitting. Inthe with the same pixel intensity scaling after normalizing the Keck
next section, we present a summary of the Spitzer data, i-banddatatothedynamicpixelvaluerangeoftheIRACdata.
while in Section 3 we discuss modeling of the light pro-
file of the foreground lenses. We perform SED modeling Corrected basic calibrated data pre-processed by the
of background SMGs and present results related to the SpitzerScienceCenter(SSC),usingthestandardpipeline
properties of these two sources in Section 4. Through- version S18.18.0, were spatially aligned, resampled, and
out the Letter we assume flat ΛCDM cosmology with combined into a mosaic image using version 18.3.1 of
Ω =0.3 and H = 70 kms−1Mpc−1. the SSC’s MOPEX software suite (Makovoz & Marleau
M 0
2005). The IRAC mosaics havea resampledpixelsize of
2. SPITZERIRANDKECKOPTICALIMAGINGDATA 0.6′′ and angular resolution of 2 to 2.5′′. For the work
presentedhere, we also use the IRAC point spreadfunc-
The IR imaging data are part of Spitzer program 548
tion (PSF; version 2010 April) file as provided by the
(PI: A.Cooray), released for analysis on 2010 July 30.
SSC 36.
For this program, Infrared Array Camera (IRAC; Fazio
We also make use of optical images of SDP.81 and
et al. 2004) images of SDP.81 and SDP.130 were taken
SDP.130that wereacquiredon 2010March10 using the
at 3.6µm (Channel 1) and 4.5µm (Channel 2). Both
dual-arm Low Resolution Imaging Spectrometer (LRIS;
lensing systems were imaged with a 36-position dither
Oke et al. 1995, McCarthy et al. 1998) on the Keck I
pattern and an exposure time of 30 s for each frame to
telescope and reported in N10 (see Figures 1 and 2).
achieve an effective total exposure time of 1080 s. The
Eachtarget received simultaneous 3×110 s integrations
rms depth reached is 0.3 and 0.4 µJy in Channels 1 and
with the g-filter and 3×60 integrations with the i-filter
35 Herschel is an ESA space observatory with science instru- using the blue and red arms of LRIS, respectively. A
ments provided by European-led Principal Investigator consortia
andwithimportantparticipationfromNASA. 36http://ssc.spitzer.caltech.edu/irac/calibrationfiles/psfprf/
Spitzer imaging of Herschel-ATLAS lenses 3
20′′ dither patternwasemployedtogenerateon-skyflat-
field frames. We performed photometric calibration us-
ing 1 s g- andi-band observations of brightstars in each
field. The data were reduced using IDL routines and
combined and analyzed using standard IRAF tasks; the
seeing FWHM of the final science exposures is ∼0.8′′.
Fig.2.—Kecki-band(toprow),IRACChannel1(middlerow)
and Channel 2 (bottom row) postage stamp images for SDP.130. Fig.3.—Multi-componentlightprofilemodelsforSDP.81(top)
Thecorrespondingresidualsarepresentedaftersubtractionofsin- andSDP.130(bottom), forthe IRAC Channel 1data. Shownare
gledeVaucouleurs profilesmodeledontheKeck i-banddata. Al- the un-subtracted data, the lens model (a single S´ersic profile for
though the Keck residuals show no signs of significant structure, SDP.81 and a S´ersic plus exponential disk profile for SDP.130 ),
thesubtracted IRAC data haveresidualstructurethat canbeas- the residual after subtraction of the lens model profile only and
sociated with the SMA contours, which suggests lensed structure the total model residual (full residual). The SMA contours are
oftherespectivebackgroundgalaxyispresent. TheSMAcontours presentedasinFigure1. Alldataareplottedwiththesamepixel
arepresented asinFigure1. Allpostage stamps areplotted with intensityscaling.
thesamepixelintensityscalingafternormalizingtheKecki-band
datatothedynamicpixelvaluerangeoftheIRACdata.
IRAC band residuals strongly suggest that a more com-
plex structure, associated with the background SMG, is
presentforbothSDP.81andSDP.130(see Figures1and
3. MODELINGTHELENSES 2).
To construct models of the light profiles for each lens- We verify that these residuals are significant, and not
ing system we use GALFIT (Peng et al. 2002), which anartifactofimperfectionsintheIRACPSF,bycompar-
allows multiple profiles per object and performs a si- ing them with residuals derived for three (non-lensing)
multaneousnonlinearminimization. Priortofitting pro- elliptical-likegalaxiesinthesamefield,afterfittingthem
files to SDP.81 and SDP.130 in the IRAC data, de Vau- with single S´ersic profiles. For both the lensing systems
couleurs profile (de Vaucouleurs 1948) models are con- and the comparison ellipticals aperture flux ratios were
structed for the Keck i-band imaging,to take advantage determinedfortheresidualimageandthecorresponding
of the comparatively higher resolution of these images, un-subtracted data. To consider only positive structure,
and the presence of only one component per system i.e. pixel values > 2σ below the local background were re-
the lens galaxy. placedwith the medianlocalsky value. The SMG resid-
To look for any potentially lensed structure in the uals were found to have flux ratios ∼ 3−5 greater than
IRAC data, the Keck models are used to fit the IRAC those for the random elliptical galaxies.
Channels 1 and 2 data, keeping the effective radius and In order to more precisely disentangle the lens and
ellipticity fixed, and using the appropriate IRAC PSF background components we represent the flux from the
36. On subtraction of the results, and in comparison to lensed SMGs with S´ersic profiles. Peaks were identified
the model subtracted Keck data and SMA contours, the in the IRAC single profile fit residual images, by fitting
4 Hopwood et al.
Gaussian profiles. We add three profiles for SDP.81 and
TABLE 1
two for SDP130,which allcorrespondto significantsub- Parametersof the ForegroundLens andBackgroundSMG
mm contour peaks in the SMA data, within 0.8′′. N10 for SDP.81and SDP.130
modeled the Keck i band with a S´ersic profile plus an
exponential disk, as this combination gave a marginally
Parameters SDP.81 SDP.130
improved χ2 over single profile models, and found that
Foregroundlens
the exponential component significantly contributes to
RA 09h03m11.6s 09h13m05.0s
thetotalprofileforSDP.130. Therefore,aS´ersicplusex- Dec 00d39m06s −00d53m43s
ponentialdiskprofileisadoptedfortheSDP.130lensand Redshifta 0.299 0.220
a single S´ersic profile for the SDP.81 lens. To produce SDSSu(µJy) 3.9±2.0 1.7±1.7
the final models for the lens and background galaxies, SDSSg(µJy) 24.9±1.1 19.4±0.7
we refit the profile representing the lens galaxy and the SDSSr(µJy) 115±2 66.1±1.2
SDSSi(µJy) 198±4 109±2
additional S´ersic profiles simultaneously for each system
SDSSz(µJy) 278±8 143±7
withGALFIT.Thesefinalmodelfitsforthebackground
UKIDSSY(µJy) 320±20 ...
componentofSDP.81showapartial“Einsteinring”-like UKIDSSJ(µJy) 370±20 ...
morphologicalstructureforthelensedsource(Figure.3). UKIDSSH(µJy) 510±50 ...
ForSDP.130,thelensedcomponentismorecompactand UKIDSSK(µJy) 570±70 ...
in close proximity to the lens profile. The resulting best Spitzer3.6µm(mJy) 0.35±0.04 0.213±0.03
fit profiles for the background galaxies agree well with Spitzer4.5µm(mJy) 0.22±0.04 0.230±0.01
BackgroundSMG:observedquantities
the SMA contours, and the combined models subtract a
Redshift 3.042 2.625
cleanly, suggesting successful lens/source decoupling. Magnificationa 25±7 6±1
4. SPECTRALENERGYDISTRIBUTIONOFTHELENSED Keck/LRISg(µJy) <0.13 <0.20
Keck/LRISi(µJy) <0.20 <0.35
SMGS
UKIDSSY(µJy) <6.27 ...
The GALFIT-integrated magnitudes for each com- UKIDSSJ(µJy) <9.23 ...
ponent of the final SDP.81 and SDP.130 models were UKIDSSH(µJy) <8.52 ...
converted to flux density to extend the existing multi- UKIDSSK(µJy) <13.5 ...
waveband photometry (see table 1 and N10, and refer- Spitzer3.6µm(mJy) 0.062±0.04 0.044±0.01
Spitzer4.5µm(mJy) 0.126±0.05 0.047±0.01
ences therein) into the near-IR. We assign 1σ errors to
b
PACS70µm(mJy) <8.0 <9.0
the photometry obtained from the final GALFIT model
PACS100µm(mJy) <62 ...
profiles, using the magnitude distributions for all the PACS160µm(mJy)b 51±5 45±8
GALFIT trials that converged. PACS re-imaging of the
SPIRE250µm(mJy) 130±20 105±17
lensed H-ATLAS sources provides new photometry at SPIRE350µm(mJy) 180±30 128±20
160µmandupperlimitsat70µm(I.Valtchanovetal.in SPIRE500µm(mJy) 170±30 108±18
preparation; Ibar et al. 2010). For the goal of deriving SMA880µm(mJy) 76±4 39±2
physical properties, the IRAC photometry adds partic- IRAM1200µm(mJy) 19.6±0.9 11.2±1.2
ularly important constraints to the SMG SEDs, which BackgroundSMG:derivedquantities
previously consisted of just upper limits at wavelengths LIR(1012L⊙)c 2.0±0.6 5.6±1.2
below 250µm for SDP.130 and 160µm for SDP.81. AV 4.4±0.6 5.0±0.5
MH −25.7±0.3 −26.01±0.17
The SEDs of the SMGs are fitted using the models SFR(M⊙ yr−1) 74±30 150±50
of da Cunha et al. (2008), calibrated to reproduce the M(H2)(1010 M⊙)d 1.4 2.7
ultraviolet-to-infraredSEDsoflocal,purelystar-forming M⋆ (1011 M⊙)e 2.5±1.7 4.5±2.5
Ultra Luminous Infrared Galaxies (ULIRGs; 1012 ≤ Mdust (108 M⊙)f 3.4±1.0 11±2
LIR/L⊙ < 1013; da Cunha et al. 2010). The SED µg 0.05±0.01 0.08±0.01
models assume a Chabrier (2003) initial mass function
(IMF) that is cutoff below 0.1 and above 100 M⊙; using Note. —Theopticalandsub-mmfluxdensitiesand3σ upperlim-
a Salpeter IMF instead gives stellar masses that are a itsaretakenfromN10,unlessotherwisenoted. Derivedquantitiesare
factor of ∼ 1.5 larger. We find that a significant atten- correctedformagnificationandtheerrorsquotedaccountforthetab-
ulateduncertaintyinthemagnification.
uation by dust (AV ∼4-5) is required to be consistent (a)Spectroscopicredshifts(N10).
with the IRAC photometry and optical/near-IR upper (b) From PACS re-imaging data (I. Valtchanov et al. in preparation
2011;Ibaretal.2010).
limits(Figure4),whichisconsistentwithotherULIRGs
(c)2to1000µmluminositybasedonthebest-fitSED;
and SMGs (e.g. Geach et al. 2007; Hainline et al. 2010; (d)Frayeretal.2011,uncertaintyisatleastafactorof∼2.
Michal owski et al. 2010; Wardlow et al. 2010). (e)Thereisanadditionalsystematicuncertaintyuptoafactorof∼10
(seetext).
Using the Chabrier (2003) IMF, with parameters de-
(f)Calculatedwithadustmassabsorptioncoefficientapproximatedas
rived from the SED fits, we find that SDP.81 and κλ ∝ λβ with β = 1.5 for warm dust and β = 2 for cold dust, and
SDP.130havestellarmasses(M⋆)of(2.5±1.7)×1011M⊙ a normalization of 0.77 g−1 cm2 at 850 µm. There is an additional
and(4.5±2.5)×1011M⊙,respectively(seetable1). How- (sgy)stGemasatfricacutniocner,tµai=ntyMo(fHa2t)/le[aMst⋆a+fMac(tHor2o)]f,∼un2c.ertaintyisafactorof
ever,wenotethatthereisanadditionalsystematicerror 2–3.
of up to a factor of 10, due to uncertainty in the appro-
priate mass-to-light ratio (see Wardlow et al. 2010 for a
discussion)andmagnificationfactors,aswellasalackof
observations at optical/near-IR wavelengths. J and Ks
photometric data on these galaxies with VLT/HAWK-I
observations(A.Vermaetal. inpreparation2011)could
Spitzer imaging of Herschel-ATLAS lenses 5
Fig.4.— Left: photometry and best-fit SEDs for the foreground elliptical (blue) and background SMG (red) for SDP.81 and SDP.130.
The photometric points and upper limits are taken from Negrello et al. (2010), with updated PACS flux densities at 160 µm and upper
limits at 70 µm. Photometry from the best fitting light profiles to the IRAC data are added for the foreground lens galaxies (turquoise
points)andforthebackgroundSMG(pinkpoints). TheSEDsarefittedusingthemodelsofdaCunhaetal.(2008). Wefindthathighlevels
ofvisualextinctionoftheSMGsarerequired(AV>4)tobeconsistentwiththeoptical-to-NIRdata. Topright: aplotof(magnification
corrected)rest-frameabsoluteH-band(MH)magnitudeagainstredshiftforSDP.81andSDP.130,showingtheapproximatecorrespondence
ofMH withstellarmass. Forcomparison,wealsoshow850-and870-µmselectedgalaxies(Hainlineetal.2010;Wardlowetal.2010). The
twolensed H-ATLASsources arebrighter than ∼95% of the comparisonsample,and correspondingly, arelikelytobeamongst the most
massive. Error bars are omitted from the comparison sample for clarity. Bottom right: ratio of CO and H-band luminosity against CO
linevelocityfortheH-ATLASlensedgalaxiesand850µmselectedSMGs. LCO/LH isindependentofthelensingmodel(tothefirstorder;
see discussioninthe text) and represents the gas-to-stellar mass ratio. σCO is indicative of dynamical mass, although italsodepends on
theinclinationangleandsizeoftheCOemittingregion,whichincreasestheobservedscatter. SDP.81andSDP.130havesmalldynamical
massesandsmallgasfractions,relativetothecomparisonsample.
potentially improve the estimates of extinction and stel- (Hainline et al. 2010). σ is independent of L /L
CO CO H
lar mass. andisindicativeofthedynamicalmassofthesystem,al-
Rest-frameabsoluteH-bandmagnitudes(M )provide thoughsomescatterisintroducedduetothedependence
H
a guide to galaxy stellar masses that is not complicated on the inclination angle and the size of the CO emitting
by the details of the assumedstar formationhistory and region. PublishedCOluminositiesofthe850µmselected
is straightforwardto compare to other similarly selected sample (Greve et al. 2005, Coppin et al. 2008; Frayer
samples. InthetoppanelofFigure4weplotM against et al. 2008, Tacconi et al. 2008; Bothwell et al. 2010)
H
redshift for SDP.81 and SDP.130 compared to 850 and are converted to the equivalent CO(1−0) values using
870 µm selected SMGs (Hainline et al. 2010; Wardlow L /L =0.6 and L /L =0.6
CO(3−2) CO(1−0) CO(4−3) CO(1−0)
et al. 2010). The H-ATLAS lensed galaxies are brighter (Ivison et al. 2010). Galaxies with poorly defined σ
CO
than the majority of the 850–870 µm selected SMGs, are excluded. The H-ATLAS lensed galaxies have low
thus if there is no active galacticnucleus contributionto L /L (representative of gas fraction) and low σ
CO H CO
their H-band luminosities they are likely to be amongst (representative of dynamical mass) compared to 850-
the most massive. µm selected galaxies (Figure 4). There may also be a
We next consider the CO(1−0)-to-H-band luminosity weakinversetrendbetweenL /L andσ for allthe
CO H CO
ratio (LCO/LH), a representation of the gas-to-stellar SMGs,althoughalargersampleisrequiredforconfirma-
mass fraction that is mostly independent of the lensing tion.
model. However, the IRAC data trace stellar emission,
which may originate from a spatially different region of Summary: We have studied the background SMGs of
the SMG to the far-infrared emission. As such there two sub-mm brightgravitationallenses, identified in the
may be variationin the lensing amplifications acrossthe H-ATLASSDPdata. Theintrinsicsub-mmfluxdensities
source plane, potentially leading to a small effect on oftheseSMGarebelowtheHerschelconfusionnoiseand,
LCO/LH The bottom panel of Figure 4 shows LCO/LH therefore,undetectablewithoutthe fortuitouslensingby
against the CO linewidth (σCO) for the two H-ATLAS foreground ellipticals. The full 550 deg2 survey area of
lensed galaxies, compared to 850-µm selected SMGs H-ATLAS will recover a sample of > 200 lensed SMGs,
6 Hopwood et al.
andSpitzerfollow-upobservationswillenableustostudy the Science and Technology Facilities Council, grant
the physical properties of SMGs over a wide range of D/002400/1 and studentship SF/F005288/1. This work
redshiftand far-IRluminosities. Studies suchas this are is based in part on observations made with the Spitzer
necessary to further understand the nature of sources SpaceTelescope,whichisoperatedbytheJetPropulsion
that contribute to the bulk of the cosmic far-infrared Laboratory, California Institute of Technology under a
background. contractwithNASA.Supportforthisworkwasprovided
Herschel-ATLAS is a project with Herschel, which is by NASA through an award issued by JPL/Caltech.
an ESAspace observatorywith science instruments pro- US participants in H-ATLAS also acknowledge support
vided by European-led Principal Investigator consortia from NASA Herschel Science Center through a contract
and with important participation from NASA. The H- fromJPL/Caltech. IA andDHH arepartiallyfunded by
ATLAS Web site is http://www.h-atlas.org/. We thank CONACyT grants 50786 and 60878.
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