Ultraviolet-to-Far Infrared Properties of Lyman Break Galaxies and Luminous Infrared Galaxies at z ~ 1

0.32 MB·English

by  D. Burgarella

Astronomy & Astrophysics manuscript no. 4309dbur (cid:13)c ESO 2008
February 5, 2008
Ultraviolet-to-Far Infrared Properties of Lyman Break Galaxies
∼ 1
and Luminous Infrared Galaxies at z
1 2 2 2 1
D. Burgarella , Pablo G. P´erez-Gonz´alez , Krystal D. Tyler , George H. Rieke , V´eronique Buat ,
Tsutomu T. Takeuchi1,10, S´ebastien Lauger1, St´ephane Arnouts1, Olivier Ilbert3, Tom A. Barlow4, Luciana
Bianchi5, Young-Wook Lee6, Barry F. Madore7,8, Roger F. Malina1, Alex S. Szalay9 and Sukyoung K. Yi6
6 1 ObservatoireAstronomiqueMarseilleProvence,Laboratoired’AstrophysiquedeMarseille,traversedusiphon,13376Marseille
0
cedex 12, France
0
2 e-mail: denis.burgarella@; veronique.buat@;tsutomu.takeuchi@; [email protected]
2 Steward Observatory,University of Arizona, 933 North Cherry Avenue,Tucson, AZ 85721, USA
n
e-mail: pgperez@; ktyler@; [email protected]
a 3 Osservatorio Astronomico diBologna, via Ranzani, 1 - 47 Bologna - Italy e-mail: [email protected]
J
4 California Institute of Technology, MC 405-47, 1200 E. California Boulevard, Pasadena, CA 91125, USA e-mail:
6
[email protected]

5 CenterforAstrophysicalSciences,TheJohnsHopkinsUniversity,3400NorthCharlesSt.,Baltimore,MD21218,USAe-mail:
1
v [email protected]
3 6 Center for SpaceAstrophysics, Yonsei University,Seoul 120-749, Korea e-mail: ywlee@;[email protected]
2 7 Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena 91101, USA e-mail:
1 [email protected]
1 8 NASA/IPACExtragalactic Database, California Institute of Technology, Mail Code 100-22, 770 S. Wilson Ave., Pasadena,
0 CA 91125, USA
6 9 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
0
e-mail: [email protected]
h/ 10 Present address: Astronomical Institute, Tohoku University Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan e-mail:
p [email protected]
-
o
Received ; accepted
r
t
s
a ABSTRACT
:
v
Aims. Wepresentthefirst large, unbiased sample of LymanBreak Galaxies (LBGs) at z∼1. Farultraviolet-dropout (1530 ˚A)
i
X galaxies intheChandraDeepField Southhavebeenselected usingGALEX data.Thisfirstlargesampleinthez∼1universe
r providesus with a high quality reference sample of LBGs.
a Methods. Weanalyzed the sample from theUV tothe IR usingGALEX, SPITZER,ESO and HST data.
Results. The morphology (obtained from GOODS data) of 75 % of our LBGs is consistent with a disk. The vast majority of
LBGs with an IR detection are also Luminous Infrared Galaxies (LIRGs). As a class, the galaxies not detected at 24µm are
an order of magnitudefainter relative tothe UVcompared with those detected individually,suggesting that there may be two
types of behavior within the sample. For the IR-bright galaxies, there is an apparent upper limit for the UV dust attenuation
and this upper limit is anti-correlated with the observed UV luminosity. Previous estimates of dust attenuations based on the
ultraviolet slopeare compared tonew onesbased on theFIR/UVratio(for LBGs detected at 24µm),which is usually amore
reliable estimator. Depending on the calibration we use to estimate the total IR luminosity, β-based attenuations AFUV are
larger by 0.2 to 0.6 mag. than the ones estimated from FIR/UV ratio. Finally, for IR-bright LBGs, median estimated β-based
SFRsare2-3timeslargerthanthetotalSFRsestimatedasSFRTOT =SFRUV +SFRIR whileIR-basedSFRsprovidevalues
below SFRTOT by 15 - 20 %. We use a stacking method to statistically constrain the 24µm flux of LBGs non individually
detected. Theresults suggest that theseLBGs do not contain large amounts of dust.
Key words.cosmology: observations – galaxies : starburst – ultraviolet : galaxies – infrared : galaxies – galaxies : extinction
1. Introduction
LymanBreakGalaxies(LBGs)arethemostnumerousob-
Send offprint requests to: D. Burgarella
jectsobservedathighredshift(z >2−3)intherest-frame
2 Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1
ultraviolet (UV). The discovery of a large population of forthedustattenuationtorecoverthefullStarFormation
LBGs beginning with the work of Madau et al. (1996), Density (e.g. P´erez-Gonza´lezet al. 2005).
followed by the spectral confirmation of their redshifts, The usual way of detecting and identifying LBGs is
provided the astronomicalcommunity with the first large through the so-called dropout technique, i.e. the absence
sample of confirmed high redshift galaxies (Steidel et al. of emission in the bluest of a series of bands due to the
1996;Lowenthalet al. 1997).The spectra of bright LBGs Lyman break feature moving redwards with the redshift
(e.g. cB58 by Pettini et al. 2000; Teplitz et al. 2000) are (e.g.Steidel&Hamilton1993,Giavalisco2002).However,
remarkably similar to those of local starbursts, indicat- it is well known that selection effects can have a very
ing that these objects are forming stars at a high rate. strong influence on the deduced characterics of an ob-
The observed colors of LBGs are redder than expected served galaxy sample (e.g. Buat et al. 2005; Burgarella
fordust-freestar-formingobjects.Thisreddeningsuggests et al. 2005). Until now, there has been no way to detect
thatsomedustispresentinthispopulation.However,the a general, unbiased sample of LBGs at low redshift (i.e.
amount of dust in LBGs (Baker et al. 2001; Chapman z ≤ 2), with the same dropout method very successfully
et al. 2000), and therefore the reddening-corrrected star used at z ≥ 2 because we lacked an efficient observing
formation rate (SFRc), is poorly known. Meurer et al. facility in the UV range. This was quite unfortunate be-
(1999), Adelberger & Steidel (2000) and subsequent pa- cause the simple fact that the galaxies are closer to us
perstriedtoestimatetheamountofdustattenuationfrom means that we can access much more information on the
the β method (Calzetti et al. 1994).However,it has been morphology, detect fainter LBGs in the UV and in the
shownrecentlythatthis approachonlyprovidesroughes- IR, and therefore harvest larger samples. GALEX and
timates of the total UV attenuation in localgalaxies (e.g. SPITZERchangedthis situationandhaveallowedus to
Buatetal.2005;Burgarellaetal.2005;Seibertetal.2005, define a large sample of LBGs at z ∼ 1 in the Chandra
Goldader et al. 2002 and Bell 2002). Deep Field South (CDFS).
In this study, we combine the detection in the UV of
High redshift LBGs are mainly undetected at sub-
true (i.e. with a detected Lyman break) LBGs at z ∼ 1
millimeter (sub-mm) wavelengths, where the emission of
withGALEX andat24µmwithSPITZER/MIPS.These
galaxies is dominated by the dust heated by young stars
dataletusestimatethetotaldustemissionandtherefore,
(Kennicutt 1998). Only the most extinguished LBGs are
the dust-to-UV flux ratio, which provides a good tracer
detected by SCUBA (Chapman et al. 2000, Ivison et al.
of the dust attenuation in the UV. We also use high spa-
2005) and we have no idea of the dust attenuation for a
tialresolutionimagestoanalysetheirmorphology.Weare
representative sample. Very recently, Huang et al. (2005)
therefore able to perform a complete analysis for the first
observed a population of LBGs at 2 < z < 3 detected
time on a large LBG sample.
withSPITZER.Unfortunately,duetotheveryhighred- −1 −1
shift, the SPITZER/MIPS observations were not deep A cosmology with H0 =70 km.s .Mpc , ΩM =0.3
and ΩΛ =0.7 is assumed in this paper.
enough to detect the thermally reradiated emission from
very many LBGs at z ∼ 3 and the SPITZER/IRAC
data, although deep enough to detect many LBGs, only 2. The Galaxy Sample
probe the rest-frame NIR (i.e. no information about the
GALEX (Martin et al. 2005) observed the CDFS field
dustenshroudedstarformationcanbeinferred).Themor-
for 44668 sec (Deep Imaging Survey = DIS) in both the
phology of LBGs is also a matter of debate: early works
far ultraviolet (FUV) and the near ultraviolet (NUV).
(e.g. Giavalisco et al. 1996) suggested that LBGs could
The GALEX field is centered at α = 03h32m30.7s, δ
be ellipsoidal,andthereforeperhapsthe progenitorsofel-
= -27deg52’16.9” (J2000.0). The GALEX IR1.1 pipeline
lipticals or of the bulges of spiral galaxies. The problem
identified 34610objects within the 1.25o-diameter field of
is that we can hardly detect low surface brightness areas
view. The GALEX resolution (image full width at half
athighredshiftbecauseofthe cosmologicaldimming.For
Maximum = FWHM) is about 4.5 arcsec in FUV and 6
instance, Burgarella et al. (2001) suggest that only com-
arcsec in NUV.
pactstar forming regionscould be easily detected in deep
This field has also been observed by SPITZER using
HST observations.
MIPS (Rieke et al. 2004) in the guaranteed time observ-
On the other hand, observations in the sub-mm range ing program. The MIPS observations provide about 7 - 8
have revealed a population of FIR-bright galaxies that sources arcmin−2 at 24 µm centered on a ∼ 1.45×0.4=
might be similar to local (Ultra) Luminous IR galaxies 0.6 deg2 field of view. The SPITZER image FWHM is
((U)LIRGs) (Blain et al. 1999) with 1011L⊙ < LIR = about 6 arcsec and almost perfectly matches that of
L(8−1000µm)<1012L⊙forLIRGsand1012L⊙ <LIR < GALEX.
1013L⊙ forULIRGs.Theseobjectsarelikelytodominate Redshifts from GOODS (Vanzella et al. 2005) and
the Cosmic InfraredBackground(CIB; Elbaz& Cesarsky VVDS (Le F`evre et al. 2004) are available at the cen-
2003)athighredshift.ThelinkbetweenLBGsandLIRGs ter of the GALEX field. Part of the GALEX CDFS
is still an open question: are there two classes of objects field was observed by COMBO 17 (Wolf et al. 2004)
or are they related? If they are two facets of the same over 0.5 × 0.5 deg2. We made use of COMBO 17 red-
population,thenwecould,forinstance,correctUVfluxes shifts for objects with r<24.5. In this range, the quality
Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1 3
δz/(1+z) remains within σ < 0.03 for 53 % of the ob-
z
jects.Finally,weobtainedphotometryfromtheEuropean
Southern Observatory Imaging Survey (EIS) in U, B, V,
R and I.
WebuiltasampleofLBGsasfollows.Fromthesources
with redshifts, we selected objects in the rest-frame FUV
(i.e.intheobservedNUV)thatwecross-correlated(r =1
arcsec)firstwiththe ground-basedopticalEISdata,then
(r =4arcsecs)withtheMIPS24µmdata.Intheresulting
catalogue, we down-selected to objects that COMBO 17
puts in the ”GALAXY” class. Then, we extracted the
sources with redshift 0.9 ≤ z ≤ 1.3. Since we wish to
study Lyman break galaxies, we omitted objects without
observed NUV and U flux : we kept only objects down
toGALEXNUV magnitudes=24.5correspondingtothe
GALEX 80% completeness level and the U-band limiting Fig.2. The distribution in observedluminosity presented
magnitude at U=25.1. We did not use a color-color se- herecorrespondstoUVluminositiesinblue(solid)andIR
lection as performed by Steidel & Hamilton (1993). Since luminosities in red (dashed). Heavy lines are drawn from
we are studying galaxies with known redshifts, the extra the populationofLBGswithdetectedcounterpartsinthe
color is not needed to screen out interlopers. The selec- 24µm MIPS image and thin lines to upper limits only.
tiononthex-axis(i.e.G−Rcolor)mightbiasthesample Note that the cut at low LIR is not sharp because the
toward low-reddening LBGs that we wish to avoid so we 83µJy limit used here is not the detection limit but the
can achieve a more general understanding of the sample. 80 % completeness limit. UV luminosities cover the same
Infact,wefindthatthemembersofoursamplefallwithin rangeforthetwosamplesandreachuncorrectedUVlumi-
the traditional color-color LBG range, or very close to it, nosities of LogLUV =11(inL⊙). This upper limit is con-
asdiscussedinSection3.5.Forgalaxiesatz ∼1,GALEX sistentwithAdelberger&SteidelrangeforLBGsatz ∼3
FUV corresponds to a rest-frame wavelength of ∼ 765 but seems inconsistent with the Balmer-break sample at
˚A and GALEX NUV corresponds to ∼ 1155 ˚A. The ob- z ∼ 1. Two areas are shaded. Starting from the left, the
servedFUVandNUVfiltersarethereforeinthesamerest- first corresponds to the range covered by UV Luminous
framewavelengthrangesasU andGfiltersusedtoidentify Galaxies (open right ended) and the second one corre-
Lyman breaks at high redshift (e.g. Giavalisco 2002 for a sponds to Luminous IR Galaxies. A few Ultra Luminous
review). The observed FUV-NUV color thus gives a clear IR Galaxies are also detected.
indicationoftheLymanbreak.Wepickedtheobjectswith
the strongest indication of a break : FUV −NUV > 2;
theirmorphologiesandtheirstarformationasrevealedby
the final sample contains 297 LBGs. Of this list, 49 ob-
the UV and IR data and finally discuss the implications
jects (16.5 %) have a measured flux above the 80 % com-
they have for studies centered on higher redshifts.
pleteness limit ofthe SPITZER24 µm data (e.g. P´erez-
Gonz´alez et al. 2005) and thus at a high enough ratio of
signaltonoisetobetreatedindividually.Fig.1showstwo 3.1. Ultraviolet and Infrared Luminosities
examples of these LBGs at several wavelengths including
the two GALEX bands, the B and R EIS bands and the We find a large range of observed (i.e. un-corrected for
24µm SPITZER/MIPS band. Many additional galax- dust attenuation) FUV luminosities (λfλ in rest-frame
ies were detected by SPITZER but at a weaker level; FUV) with 9.3 ≤ LogLUV[L⊙] ≤ 11.0. The lowest lu-
below we will describe how we used a stacking technique minosity is set by the limiting magnitude in the U-band
to probe their properties. Our sample of 297 UV-selected at U = 25.1. Below the break, the limiting magnitude
LBGs in the redshift range 0.9 ≤ z ≤ 1.3 constitutes amounts to FUV = 26.0. Although fainter objects are
the database that we will study in this work (except for detected in the NUV (down to NUV = 25.9), we use
Sect. 3.5). The common GALEX - SPITZER/MIPS - a limiting magnitude of NUV = 24.5 to compute safer
COMBO17 field ofview (mainly limited by COMBO17) FUV −NUV colorsandthereforeperformasaferLyman
isabout0.25deg2,whichtranslatesto∼1180LBGsdeg−2 Break selection. The averagevalue is <LogLUV[L⊙]>=
and ∼ 200 LBGs deg−2 for which we have individual IR 10.2±0.3forthesamplewithindividualIRdetections,and
detections. <LogLUV[L⊙]>=10.1±0.3 for the rest of the sample.
Total IR luminosities (L ) are estimated following
IR
the procedure described in P´erez-Gonza´lez et al. (2005).
3. Lyman Break Galaxies at z ∼ 1
Briefly, rest-frame 12 µm fluxes are calculated by com-
Our LBGs provide the first opportunity to study an un- paring the observed mid-IR SEDs (including IRAC and
biased sample of LBGs systematically at z ∼ 1. We ana- MIPS fluxes) of each individual galaxy with models of
lyzetheUVandIRluminositiesofthesegalaxies,measure dust emission (e.g. Chary & Elbaz 2001 or Dale et al.
4 Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1
Fig.1. Two galaxies from our LBG sample are shown here, from left to right in GALEX FUV and NUV, then EIS
B, HST GOODS B, EIS I, HST GOODS I and finally SPITZER/MIPS 24µm band. For the two galaxies, the
leftmost image (blue frame) is below the Lyman break at z ∼ 1 and the galaxy is not visible. The size of GOODS
images is 4 × 4 arcsec2. The corresponding GOODS field (green frame) is plotted in the large (35 × 35 arcsec2)
EIS images. a) a LBG classified as a disk-dominated galaxy; the more compact object is at z = 0.546 from VVDS,
it appears reddish and should not contribute in ultraviolet and in far infrared; b) a LBG classified as a merger /
interacting galaxy.
2002). This procedure is meant to cope with the strong to Log(LIR) ≥ 11L⊙, that is, nearly all the IR-detected
K-corrections observed in the mid-IR due to the emis- LBGs are LIRGs (95.9 %) and there are 2 ULIRGs (4.1
sion from aromatic molecules. We use the formulations of %). An association between UVLGs and LIRGs was pro-
Takeuchiet al. (2005)and Chary & Elbaz (2001)for con- posed by Burgarella et al. (2005). We confirm here this
versionfrom12µmflux density to L .Inadditionto the association and extend it to LBGs.
IR
intrinsic differences due to the two calibrations (the for-
mer provides L lower by 0.2 dex), the conversion from
IR 3.2. The Morphology
L12µm to LIR can introduce errors up to a factor of 2 for
individual normal galaxies and 4 for galaxies with SED The Great Observatories Origins Deep Survey (GOODS)
variations over the full IRAS sample range (Takeuchi et provides high resolution and high signal-to-noise images
al. 2005; Dale et al. 2005). The effects of these errors are of some of our LBGs, which can be used to study their
greatly reduced in this work because we discuss average morphology in the rest-frame B band. An advantage of
properties, not those of individual galaxies; in this case, our low redshift sample of LBGs is that the images ex-
theuncertaintiesarelikelytobecommensuratewiththose tend to low surface brightness and hence morphologies
in the overall conversions to LIR (i.e., still ∼ 0.2 dex). can be determined well. We compute the asymmetry and
There are also uncertainties in the luminosity values due concentration (Fig. 3) as in Lauger et al. (2005a) from
to the distance (since we use photometric redshifts with the objects within the GOODS field which have a signal-
δz/(z+1) ≤ 0.03), but they are less than 10% for LUV to-noise ratio larger than S/N ≈ 1 per pixel and whose
and LIR. coordinates are within 2 arcsecs from the GALEX detec-
Itisdifficulttocompareoursampletopreviouslypub- tion. We were able to obtain the morphology for only 36
lished ones since none was available in this redshift range LBGS out of our 300 LBGs (about 1/4 of our GALEX
before GALEX. However, Adelberger & Steidel (2000) + SPITZER + COMBO 17 field is covered by GOODS).
have discussed a Balmer-break sample at z ∼ 1. The dis- Fig. 3 leads us to two conclusions: i) all but one LBG in
tributions in L and L for our sample are shown in our sample are located on the disk side of the line sepa-
UV IR
Fig. 2. The lower limit is set by the flux limits but the ratingdisk-dominatedandbulge-dominatedgalaxies,and
upper limit of our LBG sample is about the same as the ii) part of them (22 %) are in the top part of the dia-
z >3 one in Adelberger & Steidel’s (2000). However, the gram, i.e. with an Asymmetry larger than 0.25 and could
upper limit of our sample is higher by a factor of ∼ 4 be interpretedas mergers.This kind of quantitative anal-
(assuming H0 = 70) than Adelberger & Steidel’s (2000) ysis is also applied to higher redshift LBGs, however, we
sample. mustbecarefulintheinterpretationbecause,evenifdisks
Heckman et al. (2005)defined UV Luminous Galaxies are present, it would be very difficult to detect them due
(UVLGs) as galaxies with UV luminosities above to the cosmologicaldimming (e.g. Burgarellaet al. 2001).
Log(LUV)=10.3L⊙.TheyfoundthattheseUVLGsbear Indeed,indeepspectroscopicobservations(e.g.Moorwood
similarities to LBGs, especially a sub-sample of compact et al. 2000,Pettini et al. 2001), the profiles of the optical
ones.Inoursample,22.2%ofthe LBGsareUVLGs,30.6 nebular lines suggestthe presence of disks in some LBGs.
% of the LBGs with an IR counterpart are UVLGs. The Anumberofstudieshavebeendevotedtogalaxymor-
83µJydetectionlimitat24µmapproximatelycorresponds phology in the redshift range 0.6 ≤ z ≤ 1.2. At z ∼ 0.7,
Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1 5
most of the works seem to agree that about 60 - 70%
ofthe objectscanbe classifiedasdisk-dominatedgalaxies
(mainlyspiralsandMagellanicirregulars)and10-20%as
mergers/interactinggalaxies.Herespiralsareasub-group
of disks which exhibit a more symmetric (spiral) struc-
ture than irregular-like objects similar to the Magellanic
clouds. Their asymmetry is therefore lower. Lauger et al.
(2005b) found about 70 - 80% of disk-dominated galaxies
atz ∼1.Zhengetal.(2004)studiedtheHST morphology
of a sample of LIRGs and also found that a large major-
ity (∼ 85 %) of them are associated with disk-dominated
galaxies.This conclusion is reached whether the selection
is in the rest-frame ultraviolet(Wolf etal. 2005)orin the
infrared (Bell et al. 2005). However, some dispersion due
tothe cosmicvariancemightexist(Conselice,Blackburne
& Papovich 2005). Fig.3. The morphology of the LBG sample is quantita-
tively estimated from the asymmetry and the concentra-
Therefore, overall it appears that the majority of the
tion (sub-sample drawn from a larger CDFS analysis by
star formation at 0.6 < z < 1 resides in disks and about
Lauger et al. 2005b). The smaller number of objects pre-
half of it in spirals. The numbers that we draw for our
sented here is due to the smaller field of view of GOODS
LBG sample at z ∼ 1 are globally consistent: we find
as compared to ours. Black circless are LBGs without
that ∼22% of the LBGs are likely mergers (e.g. Fig. 1b),
IR counterpartswhile reddiamonds representLBGs with
∼ 75% are disks (e.g. Fig. 1a) and only ∼ 3% (i.e. 1
a 24µm MIPS detection. The line corresponding to the
galaxy) is possibly a spheroid. In the cases where our
limitbetweendisk-dominatedandbulge-dominatedgalax-
LBGs canalso be classifiedasLIRGS fromtheir IR lumi-
ies (Lauger et al. 2005a).The location in the diagram re-
nosity,ourmeasuredmorphologiesareconsistentwiththe
flects the morphological type of the galaxies: more asym-
LIRG morphology measurements of Zheng et al. (2004).
metrical LBGs (e.g. mergers) are in the top part of the
diagram (A > 0.25) while early-type spirals would have
3.3. Ultraviolet Dust Attenuations A<0.1.TheLBGsampleismainlydominated(75%)by
disk-dominated galaxies and the contribution from merg-
Until now, it has been difficult to estimate the validity ers amounts to ∼21 %.
of dust attenuation estimates for distant LBGs, because
we had no clear idea of their L . Adelberger & Steidel
IR
(2000) tried to estimate the 800µm fluxes of their LBG There are now studies of high-redshift LBGs (z > 2)
sample from the β method and compared the results to with SPITZER (e.g. Labb´e et al. 2005; Huang et al.
observations. However, only the most extreme LBGs can 2005). However,the results are so far inconsistent: Labb´e
be detected either directly in the sub-millimeter range or et al. (2005) found that LBGs are consistent with low-
intheradiorangeat1.4GHz andthereforecouldbeused reddening models while Huang et al. (2005) found more
in this comparison. reddened LBGs. Further observations will resolve these
In this paper, we use total IR luminosities, L , and differences and provide a firm basis for comparison with
IR
L to compute the FIR/UV ratio, which is calibrated our sample.
UV
into FUV dust attenuation A (e.g. Burgarella et al. With the 24µm SPITZER flux for the individually
FUV
2005). This method has been shown to provide more ac- detectedz ∼1objects,wecangoastepfurtherandcheck
curatedustattenuationsthanthose fromthe UV slopeβ. how UV dust attenuation estimations carried out from
Fig. 4 shows an apparent anti-correlation of A with the β method compare with the better IR/UV-based es-
FUV
the UV luminosity. It is not clear, however, whether this timates. This comparisonhas already been performed for
relationship is real or only observational. Indeed, in ad- nearby galaxies (Buat et al. 2005, Burgarella et al. 2005,
dition to the observational cut at low L , the 24µm Seibertetal.2005andreferencestherein).Giventhewide
FUV
lower limiting flux means that we cannot detect individ- use ofthe β method on highredshift LBGs, it is useful to
ually low-luminosity galaxies with low dust attenuations. compare with our lower-redshift sample.
It is very interesting to note that we do not detect LBGs Using the equations in Adelberger & Steidel (2000)
withbothahighUVluminosityandahighUVdustatten- (deduced from Meurer et al. (1999)), we estimate the IR
uation,andthis cannotbe causedbyobservationallimits. luminosity that is used to compute A and the total
FUV
In other words, we seem to observe a population of high luminosity for each LBG. We observe a small overesti-
L LBGs (which qualify as UVLGs) with dust attenua- mation of the dust attenuation as compared to the ones
UV
tions similar to UV-selected galaxies in the local universe estimated from SPITZER/MIPS data and the dust-to-
(e.g. Buat et al. 2005). UVLG galaxies are LIRGs with UV flux ratio. The β-based mean dust attenuation esti-
the lowest A . mated for our z ∼ 1 LBG sample is A = 2.76±0.13
FUV FUV
6 Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1
3.4. Star Formation Rates and Implications for the
Cosmic Star Formation Density
We estimate SFRs for our LBG sample from the IR lu-
minosities and after applying dust corrections estimated
fromβ andwe comparethem, inFig.5,to the totalSFR:
SFR =SFR +SFR where SFR is not cor-
TOT UV IR UV
rected for dust attenuation. SFR is assumed to be
TOT
the best SFR estimate and we use it as a reference. The
first conclusion is that the dispersion is much larger for
UV SFRcs computed with β dust correctionsthanfor IR
SFRs.Butthemedianvaluesarealsodifferent:SFRc =
UV
112.3 ± 33.8 (σ = 260.9) M⊙.yr−1 while SFRIR = 49.9
± 13.1 (σ = 100.9) M⊙.yr−1 if we use Chary & Elbaz
(2001) and SFRIR = 30.5 ± 6.0 (σ = 46.6) M⊙.yr−1
Fig.4. Blue and red symbols are the same objects but if we use Takeuchi, Buat & Burgarella (2005). Median
luminosities are LUV for the former and LIR for the lat- SFRTOT for the two above calibrations are, respectiv-
ter. The dust attenuation strongly decreases while LUV elly, SFRTOT = 62.8 ± 13.2 (σ = 102.1) M⊙.yr−1 and
increases. Part of this apparent correlation might be due SFRTOT =41.1±6.3(σ =48.6)M⊙.yr−1.Asexpected,
to the fact that observational limits prevent us from de- for LBGs detected in IR, SFRIR is therefore a better es-
tecting low-luminosity LBGs with low dust attenuation. timate. About 22 % of our LBGs with an IR detection
The clear cut on the upper parts of the box cloud cannot have SFRTOT > 100 M⊙.yr−1 (using Chary & Elbaz’s
be due to observational biases. We do not seem to ob- calibration)ascomparedtolessthan1%inFloresetal.’s
serve UVLGs with high dust attenuations. On the other (1999) galaxy sample which confirms that our LBGs are
hand, the more dispersed but well-known increase of the forming stars very actively. However, none of the LBGs
dust attenuation with LIR is observed here. Larger sym- undetected at 24µm is above SFRTOT >100 M⊙.yr−1.
bols correspond to UVLGs (LogLUV >10.3L⊙). Most of The higher SFRs reached when dust attenuations are
them havelowdust attenuationsbut one is a ULIRGand computed with the UV slope β (depending on the L
IR
has A ∼4. calibration,+79to+173%)leadtoanoverestimatedcon-
FUV
tributionofLBGstotheCosmicStarFormationDensityif
the same quantitative difference exists at higher redshift.
However,Takeuchi,Buat&Burgarella(2005)showedthat
the current assumption of a constant dust attenuation
(σ = 1.02) while the dust-to-UV dust attenuation gives does not seem to be verified. The increase of the mean
AFUV = 2.16±0.11 (σ = 0.84) with LIR from Takeuchi AFUV from 1.3 to 2.3 from z = 0 to z = 1 means
etal.(2005)andAFUV =2.53±0.12(σ =0.94)withLIR that, for a given observed FUV luminosity density, the
fromChary& Elbaz(2001).The averagevalue ofthe two dust-corrected star formation density would vary. Note
dust-to-UVestimatesisconsistentwithTakeuchi,Buat& that those mean dust attenuations cannot be compared
Burgarella(2005).Theneteffectisthattotalluminosities with the numbers given for the Kaplan-Meier estimates
basedon the β method are slightly larger than the actual which are biased toward large M LBGs while fainter
FUV
values and the deduced SFRs are therefore overestimated LBGs seem to have larger dust attenuations in our sam-
(see next section). The mean of the ratios of the β vs. ple.AlthoughFIRdataarenotalwaysavailable,itisvery
FIR/UV AFUV = 1.31±0.07 (σ = 0.52) for Takeuchi et important that one is aware of these uncertainties when
al. (2005) and AFUV = 1.09±0.05 (σ = 0.40) for Chary using Star Formation Densities derived from UV values
& Elbaz’s calibration. corrected from the β method for LBGs with high dust
Applying the Kaplan-Meier estimator (and using attenuations, especially at high redshift where we have a
Chary & Elbaz’s calibrations), we can take upper lim- very poor knowledge of actual attenuations. These ambi-
its into account to estimate mean dust attenuations. guities may be reduced by further study of Spitzer data,
We find moderate values < A >= 1.36 ± 0.07 for and with Herschel.
FUV
M ≤ −22 (5 data points of which 1 is an upper
FUV
limit) and < A >= 1.08± 0.11 for M ≤ −21
FUV FUV 3.5. Extension to Galaxies Faint at 24µm
i.e. L at z=3 (35 data points of which 65 % are upper
⋆
limits). Using only detections, we reached, respectivelly, So far, most of our arguments have been based on the
< A >= 1.54±0.09 and < A >= 1.59±0.18. 49 galaxies individually detected at 24µm well above the
FUV FUV
Since we only use a small number of bright LBGs, this is completeness limit. To put the behavior of these galaxies
hardlycomparableto the numbers quotedin the previous inabroadercontext,wehavedeterminedaverageinfrared
paragraph. However, it suggests that lower L LBGs fluxdensitiesforgroupsofgalaxiesbystacking.Imagesat
FUV
have higher dust attenuations (see Fig. 4). 24µm are shifted to a common center on the basis of the
Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1 7
and 1.1×1044 ergs/s, respectively. However, the average
infrared flux densities differ by a factor of ten: 13µJy for
the undetected group and 143µJy for the detected one.
There is no significant difference in average infrared flux
density among the subgroups in the detected group.
These results indicate that the LBGs divide into two
classes. About 40% of them are infrared bright. The av-
erage NUV flux density for this group is 1.63µJy, so as
measured in νF , the NUV and 24µm luminosities are
ν
similar. Since there is a substantial correctionto total far
infraredluminosity,theinfraredcomponenttotheoutput
fromyoungstarsissignificant,probablyaccountingforthe
majority of the luminosity for these objects. The remain-
ing 60%are infraredfaint: νF is about ten times greater
ν
in the NUV than at 24µm, indicating that their outputs
Fig.5. Blue open circles compare SFRUcV to SFRTOT aredominatedby the UV.The resultsofthis paperapply
whileredfilledboxescompareSFRFIRtoSFRTOT.Both to galaxies like those in the detected group only.
SFRs are computed from Kennicutt (1998). The median To explore other possible differences between these
SFRFIR is underestimated by about 80 % and the un- classes,wecomputed the averageB (i.e.rest-frameNUV)
derestimationincreasesatlowerSFRs,whichisconsistent flux densities for the same two groups and three sub-
withthefactthatthe UVcontribution(notaccountedfor groups.Althoughitisinfluencedbyotherfactors,wetake
from LIR) is usually higher at low SFRs than at high the ratio of B to NUV flux densities (or equivalently the
SFRs. For this sample of LBG detected at 24 µm, the NUV - B color) to be an indicator of the level of redden-
median value of SFRUcV is overestimated by a factor of ing, and the ratio of 24µm to B flux density to measure
2 - 3 in average with a possible trend for the difference the relative portion of the luminosity from young stars
to increase at high SFRs. The dispersion of β-based UV emerging in the infrared compared with the UV. The re-
SFRcs (σSFR ∼ 200M⊙.yr−1) is much larger than IR- sults are in Table 1. First, they demonstrate that all the
based SFR (σSFR ∼ 30−40M⊙.yr−1 depending on the galaxiesinour selectionfallin, or close,to the color-color
calibration into LIR). LBGzoneasadjustedfromhighztoz∼1(seeGiavalisco
2002). There is a trend for LBGs with a high 24/B ratio
to present a high B/NUV, which is consistent with the
24µm coordinates if the object was well enough detected relation for the UV slope β and the FIR/UV ratio found
in that band, or the coordinates of the optical identifi- by Meurer et al. (2000) on a sample of local starburst
cation otherwise. We use sigma-clipping to eliminate sur- galaxies. An analysis based on detections is required to
roundingsources;the levelatwhichclipping occursis ad- check whether Meurer et al.’s law can be applied safely
justed empirically to provide the smoothest possible sky to those LBGs while we showed in the previous section
image.Thequotedresultsareforalevelof5-σ forsources that it provides dispersed SFRs for the detected sam-
detected at 24µm and 4-σ for undetected ones (see be- ple. Finally, the amount of dust attenuation for the un-
low), but they are not sensitive to modest adjustments in detected group is very low (A ∼0.5−0.6 for a mean
FUV
this level.Ingeneral,the resultingbackgroundshaveonly LogL ≈ 10.5) which corresponds to LBGs with the
TOT
weak structure and, where the sources are fairly bright lowestreddening found by Adelberger andSteidel (2000).
in the infrared, the 24µm stacked image is similar to the This very low reddening is consistent with the very blue
point spread function of the instrument. We confirmed UV slope β ≈ −2.4. If confirmed, this would mean that
that stacking sources of known flux density gave consis- about half of the LBGs do not contain large amounts of
tent results. These behaviors validate the procedure. dust.
For this study, we useda total of336 sources,selected
similarly to those discussed above (but without screening
4. Conclusions
for COMBO-17 type classification). We divided the sam-
ple into two groups. The first, hereafter the undetected We use multi-wavelength data in the CDFS to define the
group, includes 201 UV objects (60% of the total) for first large sample of Lyman Break Galaxies at z ∼ 1;
which visual inspection indicated no reliable 24µm detec- GALEX is used to observe the Lyman break. Redshifts
tion. The second (40%), hereafter the detected group, is aretakenfromspectraandfromCOMBO17.Quantitative
the objects withevidence foraninfrareddetection;it was morphologies (Lauger et al. 2005a) are estimated from
in turn divided into three equal subgroups according to high spatial resolution images. Finally, dust attenuations
NUV luminosity. As shown in Table 1, the two groups and total luminosities are computed from SPITZER
have very similar average NUV flux density, 1.89µJy for measurements at 24µm extrapolated to get the total IR
the undetected and 1.63µJy for the detected group. The luminosity.
averageNUVluminositiesarealsosimilar,1.3×1044erg/s The main results of this analysis are:
8 Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1
Table 1. Stacking Analysis Results
number LUV F(24µm) F(B) F(NUV) F(B)/F(NUV) F(24)/F(B)
1044ergs/sec mJy mJy mJy
All detected 135 1.11 0.14 0.0018 0.0016 1.11 80
low UV 45 0.67 0.16 0.0014 0.0012 1.21 113
middle UV 45 0.99 0.16 0.0017 0.0014 1.19 95
high UV 45 1.67 0.14 0.0023 0.0024 0.98 59
Undetected 201 1.33 0.013 0.0015 0.0019 0.79 8.7
1. We detect LBGs in the range 9.3 ≤ LogLFUV[L⊙] ≤ support for construction, operation and science analysis for
11.0, i.e. well into the UVLG class as defined theGALEXmissiondevelopedincooperation withtheCentre
by Heckman et al. (2005). For the same objects, Nationald’EtudesSpatialesofFranceandtheKoreanMinistry
LogLIR ≥ 11L⊙, which means that almost all the ofScienceandTechnology.Finally,wethankEmericLeFloc’h
for his help and discussions during thiswork.
LBGs with a SPITZER confirmed detection are
LIRGs (and 1 ULIRG) at z ∼1.
2. LBGsatz ∼1aremainlydisk-dominatedgalaxies(75 References
%) with a small contribution of interacting/merging
Adelberger K.L., Steidel C.C. 2000, ApJ, 544, 218
galaxies (22 %) and a negligible (3 %) fraction are
BakerA.J.,LutzD.,GenzelR.,TacconiL.J.,LehnertM.D.,
spheroids. The morphologies of our LBG sample are
2001 A&A,372, 37
consistent with star-forming galaxies.
Bell E. F. 2002 ApJ, 577, 150
3. The FUV dust attenuation appears to be anti-
Bell E. F., Papovich C., Wolf C., LeFloc’h E., Caldwell J. A.
correlated with the observed FUV luminosity. Part of R., Barden M., Egami E., McIntosh D. H., Meisenheimer
this correlation might be due to observational limits K., Prez-Gonzlez P.G. et al. 2005 ApJ, 625, 23
at 24 µm. However, non-detection of LBG with high BlainA.W.,SmailI.,IvisonR.J.,KneibJ.-P.,1999MNRAS,
L and high A cannot be explained by obser- 302, 632
FUV FUV
vational biases. BuatV.,Iglesias-PramoJ.,SeibertM.,BurgarellaD.,Charlot
4. About40%ofoursampleofLBGsisdetectedat24µm. S., Martin D. C., Xu C. K., Heckman T. M., Boissier S.,
Boselli A.et al., 2005 ApJ,619, L51
Theseobjectsalsoshowevidenceforreddeningintheir
Burgarella D., Buat V., Donas J., Milliard B., Chapelon S.
UVcontinua.Theremaining60%ofthesampleareon
2001 AJ, 369, 421
averageanorderofmagnitude lessluminous inthe in-
Burgarella D., Buat V., Small T., Barlow T, A., Boissier S.,
frared compared with the rest frame near UV. Their
Gil de Paz A., Heckman T. M., Madore B. F., Martin D.
UV continua also appear to be significantly less red-
C., Rich R.M. et al., 2005 MNRAS,360, 1413
dened. Calzetti,D.,Kinney,A.L.,Storchi-BergmannT.1994,ApJ429,
5. Dust corrections of our IR-bright LBG sample com- 582
puted via the β method are over-estimated by ∼ 0.2 ChapmanS.C.,ScottD.,SteidelC.C.,BorysC.,HalpernM.,
mag. if we use Chary & Elbaz (2001) to compute Morris S. L., Adelberger K. L., Dickinson M., Giavalisco
L and by ∼ 0.6 mag. if we use Takeuchi, Buat & M., Pettini M. 2000 MNRAS,319, 318
IR
Burgarella (2005). Chary R.& Elbaz D., 2001 ApJ, 556, 562
Conselice C. J., Blackburne, J. A., Papovich C, 2005 ApJ620,
6. Dustattenuationsestimatedbytheβ methodforsuch
564
galaxies lead to overestimation of the SFRs at z ∼ 1
Dale D.A. et al., 2005 ApJin press (astro-ph/0507645)
by a factor of 2 to 3, depending on the calibration of
Dale D.A., Helou G. 2002 ApJ576, 159
the 24µm flux to L while IR-based SFRs are of the
IR Elbaz D.,Cesarsky C.J. 2003 Science 300, 270
same order as SFR .
TOT FloresH.,HammerF.,ThuanT.X.,CsarskyC.,DesertF.X.,
7. By using the stackingmethod, wefind thatLBGs non OmontA.,Lilly S.J.,Eales S.,Crampton D.,LeF`evreO.
detected at 24µm seem to have very low dust attenu- 1999 ApJ517, 148
ations. Giavalisco M., Steidel C. C., Macchetto F. D 1996 ApJ, 470,
189
Acknowledgements. TTT has been supported by the Japan Giavalisco M. 2002 ARA&A,40, 579
Society for the Promotion of Sciences (April2004 - December GoldaderJ.D.,MeurerG.,HeckmanT.M.,SeibertM.,Sanders
2005). Support for this work was provided by NASA through D.B., Calzetti D., Steidel C.C. 2002, ApJ568, 651
ContractNumber960785issuedbyJPL/Caltech.Thisworkis HeckmanT.M.,HoopesC.G.,SeibertM.,MartinD.C.,Salim
basedonobservationsmadewiththeSpitzer,SpaceTelescope, S., Rich R. M., Kauffmann G., Charlot S., Barlow T. A.,
whichisoperatedbytheJetPropulsionLaboratory,California Bianchi L. et al. 2005 ApJ619, L35
Institute of Technology under NASA contract 1407. Analysis HuangJ.-S. et al., 2005 ApJ in press (astro-ph/0507685)
ofthedataforthispaperwassupportedbythecontracttothe Ivison R.J., Smail I., Bentz M., Stevens J. A., Menendez-
MIPS team, 1255094. We also thank the French Programme Delmestre K., Chapman S. C., Blain A. W. 2005
National Galaxies and the Programme National Cosmologie MNRAS362, 535
fortheirfinancialsupport.WegratefullyacknowledgeNASA’s Kennicutt R.C., 1988 ARA&A36,189
Burgarella D.et al.: A Multi-λ Analysis of Lyman Break Galaxies at z ∼ 1 9
Labb´eI., HuangJ., Franx M., RudnickG., Barmby P., Daddi
E., van Dokkum P. G., Fazio G. G., Forster Schreiber N.
M., Moorwood A. F. M. et al. 2005 ApJ624, L81
Lauger S., Burgarella D., Buat V.2005a A&A434, 77
Lauger S. et al., 2005b A&A submitted
LeF`evreO.,VettolaniG., PaltaniS.,Tresse L.,Zamorani G.,
Le Brun V., Moreau C., Bottini D.,Maccagni D.,Picat J.
P., et al. 2004 A&A428, 1043
LowenthalJ.D.,KooDavidC.,GuzmanR.,GallegoJ.Phillips
A.C.,FaberS.M.,VogtN.P.,IllingworthG.D.,Gronwall
C. 1997 ApJ481, 673
Madau P., Ferguson H.C., Dickinson M.E., Giavalisco M.,
Steidel C.C., FruchterA. 1996 MNRAS,283, 1388
Martin D. C., Fanson J., Schiminovich D., Morrissey P.,
Friedman P. G., Barlow T. A., Conrow T., Grange R.,
Jelinsky P. N. et al., 2005. ApJ,619, L1
Meurer, G.R., Heckman, T.M., & Calzetti, D. 1999, ApJ 521,
64
Moorwood A.F. M., vanderWerfP.P., CubyJ. G., OlivaE.
2000. A&A,362, 9
P´erez-Gonz´alez, P.G. et al., 2005 ApJ in press,
(astro-ph/0505101)
Pettini M., Steidel C. C., Adelberger K. L., Dickinson M.,
Giavalisco M. 2000 ApJ528, 96
PettiniM.,ShapleyA.E.,SteidelC.C.,CubyJ.-G.,Dickinson
M., Moorwood A. F. M., Adelberger K. L., Giavalisco M.
2001 ApJ554, 981
Rieke G.H., Young E. T., Engelbracht C. W., Kelly D. M.,
Low F. J., Haller E. E., Beeman J. W., Gordon K. D.,
Stansberry J. A., Misselt K. A.et al. 2004 ApJS154, 25
SeibertM.,MartinD.C.,HeckmanT.M.,BuatV.HoopesC.,
Barlow T., Bianchi L., ByunY.-I.,DonasJ., Forster K.et
al. 2005 ApJ, 619, L59
Steidel, C.C. & Hamilton D., 1993 AJ, 105, 201
Steidel, C.C., Giavalisco, M., Pettini, M., Dickinson, M., &
Adelberger, K.L. 1996, ApJ 462, L17
TeplitzH.,McLeanI.S.,BecklinE.E.,FigerD.F.,GilbertA.
M., Graham J. R., Larkin J. E., Levenson N. A., Wilcox,
M. K.ApJ, 533, 65
Takeuchi T.T., Buat V., Iglesias-Paramo J., Boselli A.,
Burgarella D. 2005 A&A,432, 423
TakeuchiT.T.,BuatV.,&BurgarellaD.2005,A&A,440,L17
Vanzella E., Cristiani S., Dickinson M., Kuntschner H.,
MoustakasL.A.,NoninoM.,RosatiP.,SternD.,Cesarsky
C., Ettori S.,et al. 2005 A&A434, 53
WolfC.,MeisenheimerK.,KleinheinrichM.,BorchA.,DyeS.,
Gray M., Wisotzki L., Bell E. F., Rix H.-W., Cimatti A.,
et al. 2004 A&A421, 913
Wolf C., Bell E. F., McIntosh D. H., Rix H.-W., Barden M.,
Beckwith S. V. W., Borch A., Caldwell J. A. R., Haussler
B., HeymansC., al., 2005 ApJ630, 771
ZhengX.Z.,HammerF., FloresH.,Assmat F.,Pelat D.2004
A&A421, 847