Differential Evolution of the UV Luminosity Function of Lyman Break Galaxies from z~5 to 3

3.1 MB·English

by  I. Iwata

Mon.Not.R.Astron.Soc.000,1–23(2006) Printed5February2008 (MNLATEXstylefilev2.2)
Differential Evolution of the UV Luminosity Function of
z ⋆
Lyman Break Galaxies from ∼ 5 to 3
I. Iwata1,2†, K. Ohta2, N. Tamura3,4, M. Akiyama3, K. Aoki3, M. Ando2,
G. Kiuchi2 and M. Sawicki2,5,6
7
0 1Okayama Astrophysical Observatory,National Astronomical Observatory of Japan, Kamogata, Asakuchi, Okayama, Japan 719-0232
0 2Department of Astronomy, Graduate School of Science, Kyoto University,Sakyo-ku, Kyoto, Japan 606-8502
2 3Subaru Telescope, National Astronomical Observatory of Japan, 650 North A‘ohoku Place, Hilo, HI 96720
4Department of Physics, University of Durham, South Road, Durham DH13LE, UK
n
5Physics Department, Universityof California, Santa Barbara, CA 93106, USA
a
J 6Department of Astronomy and Physics, St. Mary’sUniversity,923 Robie St., Halifax, Nova Scotia, B3H 3C3 Canada

9
2
Accepted 2007January29.Received2007January22;inoriginalform2006November14

1
v
1 ABSTRACT
4 We reportthe UV luminosityfunction (LF) ofLymanbreakgalaxiesatz ∼5 derived
8 fromadeepandwidesurveyusingtheprimefocuscameraofthe8.2mSubarutelescope
1 (Suprime-Cam). Target fields consist of two blank regions of the sky, namely, the
0
region including the Hubble Deep Field-North and the J0053+1234 region, and the
7 total effective surveyed area is 1290 arcmin2. Applications of carefully determined
/0 colourselectioncriteriainV −Ic andIc−z′ yielda detectionof853z ∼5 candidates
h with z′ <26.5 mag. The UVLF at z ∼5 based on this sample shows no significant
AB
p change in the number density of bright (L & L∗ ) LBGs from that at z ∼ 3, while
z=3
-
there is a significant decline in the LF’s faint end with increasing lookback time.
o
r This result means that the evolution of the number densities is differential with UV
t luminosity:thenumberdensityofUVluminousobjectsremainsalmostconstantfrom
s
a z ∼ 5 to 3 (the cosmic age is about 1.2 Gyr to 2.1 Gyr) while the number density
: of fainter objects gradually increases with cosmic time. This trend becomes apparent
v
thanks to the small uncertainties in number densities both in the bright and faint
i
X parts of LFs at different epochs that are made possible by the deep and wide surveys
r we use. We discuss the origins of this differential evolution of the UVLF along the
a
cosmictimeandsuggestthatourobservationalfindingsareconsistentwiththebiased
galaxy evolution scenario: a galaxy population hosted by massive dark haloes starts
active star formation preferentially at early cosmic time, while less massive galaxies
increase their number density later. We also calculated the UV luminosity density
by integrating the UVLF and at z ∼ 5 found it to be 38.8+6.7% of that at z ∼ 3
−4.1
for the luminosity range L > 0.1L∗ . By combining our results with those from the
z=3
literature, we find that the cosmic UV luminosity density marks its peak at z = 2–3
and then slowly declines toward higher redshift.
Key words: cosmology:observations– galaxies:high-redshift – galaxies:luminosity
function – galaxies: evolution – galaxies: formation.
1 INTRODUCTION erties of galaxy populations and explore the evolutionary
process of galaxies. It represents the number densities of
The galaxy luminosity function (LF) is one of the funda-
objects with specific luminosity at specific redshift ranges.
mental quantities frequently used to investigate the prop-
Recentdevelopmentoflargeimagingandspectroscopicsur-
veys improved the accuracy of LFs in the local universe in
optical and near-infrared wavelengths (Norberg et al. 2002;
⋆ Based on data collected at Subaru Telescope and partly ob-
Blanton et al. 2003; Kochanek et al. 2001). Good quality
tained from the SMOKA science archive at Astronomical Data
LFs at UV wavelengths have also been obtained by com-
AnalysisCenter,whichareoperatedbytheNationalAstronomi-
calObservatoryofJapan. binations of balloon or spaceborne missions with ground-
† E-mail:[email protected](II)
2 I. Iwata et al.
basedsurveydata(Sullivan et al.2000;Wyderet al.2005). ples of LBGs at z & 6 have also been fruitful
For LFs at earlier cosmic times, in principle a complete (e.g., Bouwens et al. 2003; Bouwens and Illingeorth 2006;
and volume-limited sample of galaxies at the target red- Bouwens et al. 2006; Stanway et al. 2003; Dickinson et al.
shift is required. Great efforts have been made by spectro- 2004; Bunkeret al. 2006). As we accumulate knowledge of
scopicsurveystoobtainlargegalaxysamplescapableofde- the physical properties of high-redshift (z > 2) galaxies
riving LFs up to z . 2 (e.g., Lilly et al. 1995; Lin et al. suchtheirassizes(e.g.,Steidelet al.1996a;Giavalisco et al.
1999; Ilbert et al. 2005). However, a construction of a large 1996; Ravindranath et al. 2006), stellar populations (e.g.,
spectroscopic sample of galaxies at higher redshift, z > 4, Sawicki and Yee 1998; Shapley et al. 2001; Papovich et al.
would require enormous observing time due to the faint- 2001;Eyles et al.2005;Yan et al.2006)andclusteringprop-
ness of the target objects and, consequently, it remains a erties (e.g., Giavalisco et al. 1998; Adelberger et al. 1998;
big issue to be addressed by future observing facilities. In- Ouchiet al. 2004b; Hamana et al. 2004; Kashikawa et al.
stead of determining redshifts of large numbers of objects, 2006), we improve our chances of constructing a compre-
we are able to derive a luminosity function statistically by hensive view of galaxy evolutionary processes in the high
using a sample of galaxies with redshift estimates based redshift universeand eventually through cosmic time.
on their spectral energy distributions traced by multi-band One serious limitation of past ground-based LBG sur-
photometry. In this way, extensive multi-wavelength sur- veys is that only luminous objects have been studied. The
veys conducted by the Hubble Space Telescope (HST) and statistical nature of galaxies fainter than the typical lumi-
large ground-based telescopes have enabled us to measure nosity(L∗ intheLFparameterizedwiththeSchechter1976
LFs at different epochs up to z . 2–3 (e.g., Sawicki et al. form) has been left unexplored, although these numerous
1997; Drory et al. 2003; Kashikawa et al. 2003; Chen et al. faint galaxies are estimated to dominate therest-frame UV
2003; Pozzetti et al. 2003; Wolf et al. 2003; Dahlen et al. luminosity density. Thus, a comparison between the evolu-
2005)andearlier(e.g.,Lanzetta et al.2002;Poli et al.2003; tion of luminous objects and that of faint objects has not
Gabasch et al. 2004a; Gabasch et al. 2006; Paltani et al. been made.
2006), and to discuss the evolution of galaxies through the The Keck Deep Fields (KDF) (Sawicki and Thompson
comparison of LFs at different cosmic times. 2005) made a very deep imaging survey reaching R(AB)∼
One of the most powerful observational methods to 27 mag and constructed samples of LBGs at z ∼2 to 4 in-
probe galaxies at high redshift (z & 2) is the detection cludingobjectsfarfainterthanL∗.TheyfindthattheUVLF
of the spectral discontinuity due to the redshifted Lyman at z ∼ 4 shows a significantly smaller number density in
limit (912˚A) or Lyman α (caused by absorption by inter- the fainter part than that at z ∼ 3, while in the bright
galactic hydrogen gas) through multi-wavelength broad- end no difference between z ∼ 4 and 3 has been found.
band imaging. This method is called the Lyman break SuchdifferentialevolutionoftheUVLFwasnotdetectedin
method, and since the pioneering work by Steidel and thepreceding LBG work with brighter limiting magnitudes
his co-workers (Steideland Hamilton 1992; Steidel et al. (Steidelet al. 1999). The KDF indicated that deep surveys
1996a), large number of Lyman break galaxies (LBGs) reaching fainter than L∗ (with sufficient survey area) are
at z ≈ 2–4 have been detected (e.g., Madau et al. 1996; required to closely inspect the nature of high-z galaxy evo-
Steidel et al. 2003; Steidel et al. 2004; Foucaud et al. 2003; lution. It is an interesting subject to examine whether such
Sawicki and Thompson 2005). differential evolution started at earlier epoch, z & 5. For
There are several obstacles in searching for higher red- thatsubjecttheconstructionofdeepsampleofLBGsdown
shift (z > 4) LBGs. First, the larger distance makes the to 1 magnitudefainter than L∗ at z&5 is indispensable.
apparent brightness of target objects dimmer so that, for HerewereporttheUVLFofLBGsatz∼5basedonour
the same luminosity, an object at z = 5 is about 1 magni- updated sample that builds on thework of I03. The survey
tudefainterthanthatatz=3.Next,thesensitivityofCCD now consists of two independent fields and has an effective
chips declines at λ > 8000˚A. And finally, for ground-based survey area of 1290 arcmin2 after masking bright objects.
instruments, the increase of background sky brightness due The limiting magnitudes of the sample LBGs are z′ =26.5
to the night sky lines emitted from the Earth’s atmosphere magforonefield(thefieldincludingtheHubbleDeepField
makes the S/N much worse than in the bluer wavelength - North) and 25.5 mag for the other (J0053+1234). The
ranges. Despite these difficulties, it is important to push to sample limiting magnitude is now ∼ 0.5 mag deeper than
these earlier epochs in the history of galaxy formation. in our previous work (I03), which was 26.0 mag in Ic. The
In Iwata et al. (2003; hereafter I03) we created a sam- surveyareaismorethantwotimeslargerthanourprevious
ple of ≈ 300 LBGs with Ic(AB) < 26.0. It was the first survey, which enables us to examine the degree of field-to-
large sample of LBGs at z ∼ 5, and was made possi- field variance and to estimate how representative are our
ble by the unique combination of the large mirror aper- measured properties of z ∼ 5 galaxies. Through the com-
ture of the 8.2m Subaru telescope and the wide field parison of the z ∼ 5 UVLF obtained with this deeper and
of view of its prime focus camera named Suprime-Cam wider sample with the deep UVLFs at z ∼ 3 and 4 by
(Miyazaki et al. 2002). It was also in this work that the Sawicki and Thompson (2006a), we attempt to disclose the
UVLF of LBGs at z ∼ 5 was derived statistically for evolution in theUVLFs of LBGs.
the first time. We found no significant change in the In section 2 we describe the observations for the two
bright luminosity range (MUV . −21 mag) between z = target fields. Procedures of data processing, catalog con-
5 and z = 3. After I03 several studies have success- struction and sample selection are described in section 3.
fully created LBG samples at z ∼ 5 (Lehnert and Bremer In section 4 we present our z∼5 UVLFsand also compare
2003; Ouchiet al. 2004a; Giavalisco et al. 2004b; Lee et al. them with previous results. In section 5 we make a com-
2006). Attempts to obtain statistically meaningful sam- parison of our z ∼ 5 UVLF with those at z = 4 and 3 in
Differential Evolution of UVLF of LBGs 3
Sawicki and Thompson (2006a) and then discuss thediffer-
ential evolution of UVLF. Then in section 6 we derive the
UV luminosity density at z ∼ 5 and discuss the evolution
of theUVluminosity densitythroughthecosmic time.Our
findings are summarized in section 7. Throughout this pa-
per we adopt a cosmology with the parameters ΩM = 0.3,
ΩΛ = 0.7, H0 = 70 km/s/Mpc. Magnitudes are on the AB
system.
2 OBSERVATIONS
We chose two target fields for our survey of LBGs at
z ∼ 5. One is in the area of the Hubble Deep Field
- North (HDF-N; Williams et al. 1996), and the other is
J0053+1234 (Cohen et al. 1996; Cohen et al. 1999a). Both
fields are target fields of existing extensive redshift surveys
(e.g., Cohen et al. 2000; Dawson et al. 2001; Wirth et al. Figure1. Effectivetransmissions(includingquantumefficiency
2004; Cowie et al. 2004; Cohen et al. 1999b), and there are oftheCCDandatmosphericabsorption)forthethreefiltersused
manygalaxieswithspectroscopicidentificationsatinterme- in our observations (solid lines with blue (V), green (Ic) and
red(z′)).Thetransmissionofi′ filterusedinOuchietal.(2004a)
diate redshifts (z < 4). Such information is valuable to de-
isalsoshownasadottedline.Modelspectraofstar-forminggalax-
finereliable colour selection criteria for LBGs at our target
ies at z =4.8 areshown as black lines. The solidlinerepresents
redshift range, z=5.
the case without dust attenuation and the dashed lineis forthe
We use the V, Ic and z′ filters to select our z ∼ 5 casewithE(B−V)=0.4.TheseSEDsarenormalizedat10,000
LBGs. In Fig. 1 we show the transmissions of these filters ˚A. See text for details of the models. Upper-side ticks indicate
as a function of wavelength. In the figure we also indicate rest-framewavelengths foranobjectatz=4.8.
modelSEDsofstar-forminggalaxieswithandwithoutdust
attenuationthatwegeneratedusingversion2ofthepopula-
tion synthesis code PEGASE (Fioc and Rocca-Volmerange fromeachotherinseveralaspects.First,thecentralfieldpo-
1997).AconstantstarformationratewiththeSalpeterIMF sitions are ∼3′ apart, and in our observations the position
(Salpeter 1955) is assumed, and the age from the onset of angle was set to 10◦(east of north), while in the observa-
◦ ◦
star formation is 100 Myr. Dust attenuation we show here tions by the UH group position angles were 0 or 180 . The
is E(B−V) = 0.4 using the attenuation law proposed by position and the position angle in our observation were ad-
Calzetti et al.(2000)andabsorptionbyintergalacticneutral justed so that the HDF-N taken by the HST/WFPC2 was
hydrogenisappliedusingtheanalyticformulainInoueet al. located at the centre of one of Suprime-Cam’s CCD chips.
(2005).Theredshiftweassumedhereis4.8,whichisanav- Theditheringlengthsarealso different.Inourobservations
erage redshift of our sample LBGs estimated with simula- ditheringscalesareonly10′′–15′′,whichwerechosensothat
tions (see section 4.1). In this figure the model SEDs are the area with deepest integration time became as large as
normalized at 10,000 ˚A in the observer frame. The VIcz′ possible.Wecouldchoosesuchsmallditherlengthsbecause
filter combination efficiently detects the break at Lyα for weconcentratedonthedetectionofgalaxiesatz∼5,which
star-forminggalaxiesat4.3.z.5.3,asverifiedbysimula- should be apparently small (.2′′). In the UH observations
′
tions (described in section 4.1) and follow-up spectroscopy by Capak et al. (2004) the dither scales were around 1and
(Andoet al. 2004; Andoet al. 2007). In addition to our theeffectiveexposuretimewithintheobservingfieldisfairly
VIcz′filters,Fig.1alsoshowsthei′-bandfilter(dottedline) closetouniform.Inthispapertheareacommontothesetwo
and we note that Ouchiet al. (2004a) use the combination differentsetsofobservationsisused,sincethatareaachieves
of V, i′ and z′ filters to select their LBGs at z ∼ 5. The the deep image depths in all of three observing bands (V,
i′-band filter is shifted slightly shortward compared to the Ic and z′). In Table 1 we summarize observations for the
Ic-bandfilter used in ourobservations. HDF-Nregion.
2.1 The Hubble Deep Field - North 2.2 J0053+1234
We made deep imaging observations of the field that sur- Inordertoincreasethesurveyareaandexaminetheeffectof
roundstheHDF-Nusing theSuprime-Cam attached to the field-to-fieldvariance,wecarriedouttheimagingofanother
8.2m Subaru Telescope in February 2001. The results pre- blank field. Specifically, we imaged the J0053+1234 region,
sented in I03 are based on these data alone. For the new whichisoneofthetargetfieldsoftheCaltechFaintGalaxy
resultsweshowinthepresentpaperwealsousedtheimag- Redshift Survey (Cohen et al. 1999a; Cohen et al. 1999b).
ing data taken by the University of Hawaii (UH) group Using extensive Keck spectroscopy, these authors identified
(Capak et al.2004)usingthesameinstrumentandthesame 163 galaxies in the redshift range 0.173 < z < 1.44 within
filters during February 28 – April 23, 2001. We retrieved an 2′ ×7.′3 field. Steidel et al. (1999, 2003) also selected
these UH data from the SMOKA data archive (Baba et al. this region as one of the target fields of their searches for
2002). LBGs at z ∼ 3 and 4, and they identified 19 galaxies with
Propertiesofthesetwosetsofobservationsaredifferent spectroscopic redshifts at 3<z<5.
4 I. Iwata et al.
Our imaging observations of this field using Subaru / USNO-B1 catalog (Monet et al. 2003). In total 1,000 to
Suprime-Cam have been executed during the summer sea- 1,300 stars in the catalog were identified in the mosaiced
sonsfrom2002to2004.Thecentralcoordinateofthepoint- imagein eachband,andpolynomial coefficientsuptothird
ing is 00h53m23.0s, +12◦33′56′′(J2000.0). In Table 2 we orderwerecalculated.Internalposition errorsincoordinate
′′
summarize information of our observations and final lim- conversions are less than 0.2 rms all over the images. Be-
′
itingmagnitudesinV,Ic andz .Althoughthepointspread cause these accuracies are almost identical to that of the
function of the final images are smaller than that in our USNO-B1 catalog itself, we expect that positional uncer-
images of the HDF-N region, the total limiting magnitudes tainty dueto the errors in the corrections for optical image
achievedforthisfieldare0.3–0.5magshallowerthanthose distortionsandthoseinthemosaicingprocesstobesmaller
fortheHDF-Nregion sincetheeffectivenetexposuretimes than 0.2′′.
are shorter due to the poor weather conditions we experi-
enced. Nevertheless, these data more than double the total
3.3 Photometric Catalog
surveyarea and so are veryvaluable for populatingtherel-
atively rare, bright part of the sample and for examining ThedeterminationofphotometriczeropointsfortheHDF-
field-to-field variations. N region is described in I03. Briefly, we determined photo-
metriczeropointsbasedonimaging dataofLandolt (1992)
standardstarsforV andIcbands,whileforz′-bandthezero
3 DATA REDUCTION AND SELECTION OF point was determined through the distribution of Ic −z′
colours of field stars and galaxies exposed on our images.
LBG CANDIDATES
Systematic errors in the determination of zero points are
3.1 Image Processing estimated to be 0.02, 0.08 and 0.1 mag for V, Ic and z′-
bands,respectively.Thephotometriczeropointsofthefinal
The basic image processing was done using nekosoft —
mosaiced images were determined by comparison with the
the software developed for the data reduction of mosaiced
images used in I03. For the J0053+1234 region, the same
CCD cameras (Yagi et al. 2002) — as well as with IRAF.
Bias was subtracted using overscan regions. Next, flat-field p′rocedure was followed for the V and Ic bands, while for
z -band images of spectrophotometric standard stars taken
framesweregeneratedbycombiningmedian-normalizedob-
duringthesame observingnights were used.Systematicer-
ject frames after masking objects. Then, after division by
rorsareestimated tobe0.04magfor V-bandand0.03mag
theflat-fieldframe and cosmic-ray removal (usingCRUTIL ′
in IRAF),a distortion correction was madefor everyframe forIc andz -bands.CorrectionsforGalacticextinctionwere
made based on the dust map by Schlegel et al. (1998). The
withcorrectionfactorswhichweredeterminedforeachCCD
amount of reddening for the HDF-N region is assumed to
position.
be E(B−V) = 0.012, and for the J0053+1234 region it is
Image mosaicing in each band was made by identify-
E(B−V)=0.066.
ing 40–180 stars common in object frames. The correction
Object detection and photometry were made by us-
fordifferencesinsensitivityandfluctuationsofpointsource
ingSExtractor(Bertin and Arnouts1996)ver.2.3insingle-
fluxwas also madebased onthesestardata. IntheHDF-N
image mode. A Gaussian filter with FWHM=3 pixels was
region,thedatatakenbyushavesmall ditherscales andso
used, and at least 4 contiguous pixels with counts higher
wecouldnot adoptamosaicing procedureusuallyexecuted
than1.3σabovebackgroundcountswererequiredtobede-
innekosoft,whichdeterminesrelativepositionsofframesby
tected as an object. The cross-identifications of objects in
commonstarsindifferentCCDchips.Thus,wefirstderived
our three bands were made in equatorial coordinates. Ob-
relativepositionsofframestakenbytheUHgroupandgen-
jects were included in the object catalog only when they
erated amosaiced image usingtheUHdataalone. Wethen
werewithintheareawithsufficientimagedepth,determined
calculated the positions of the frames taken by us relative
by checking the exposure maps which take account of the
tothatmosaiced image.Thefinalmosaiced imagewasthen
number of exposures and variations in sensitivity between
madeusingboththeUHandourdata.Amongthe10CCD
individual CCD chips. This procedure guarantees that the
chipsoftheSuprime-Cam,onechipwasnotworkingduring
image quality is fairly close to uniform and that the vari-
part of the observations of the HDF-N region. We also ex-
anceofphotometric errorsdoes notaffect theuniformityof
cludeddataofanotherCCD chipin ourdatabecause there
the galaxy sample. For V-band dropout LBGs we require
isnotenoughoverlappingareabetweenthepositionsofthis ′
chipandtheUHdatatoallowustodeterminereliablerela- objects to be detected both in Ic and z -bands. We used
MAG_AUTO as a total magnitude in all bands, and derived
tcihviepspowseitrieonuss.eIdn. tThheeJe0ff05ec3t+iv1e23s4urrveegyioanredaasta(afrftoemr mallasCkCinDg V −Ic andIc−z′ coloursusing1.6′′diameteraperturemag-
nitudes.Sinceobjectcentroidsaredeterminedseparatelyin
bright objects) are 508.5 arcmin2 (the HDF-N region) and
each band, there is a possibility that the positions of aper-
781.4arcmin2 (theJ0053+1234 region).TheFWHMvalues
′′ tures in different bands have offsets (e.g., star-forming re-
ofpointsourcesinthemosaicedimagesareabout1.1 forthe
HDF-Nregionand0.9′′fortheJ0053+1234 region(Tables1 gions are prominent in Ic-band and their distribution is ir-
regular).Wefeel thatsuchproblemsarerare,sinceFWHM
and 2).
sizes of LBG candidates are mostly smaller than the aper-
′′
turesizeusedtoderivecolours(1.6 ).Wealsofindthrough
visualinspectionthatmorphologiesofV-dropoutobjectsin
3.2 Astrometry ′
Ic-band and in z -band are similar in most cases. The pho-
The conversions from pixel coordinates in mosaiced CCD tometric catalogs of both fields were constructed based on
′
images to equatorial coordinates were made using the the z -band. For objects undetected by SExtractor in the
Differential Evolution of UVLF of LBGs 5
V-band,V-bandaperture photometry was made at the ob-
′
jects’ positions in the z -band. In these cases, if the flux
′′
density within 1.6 diameter exceeded the 3-σ sky fluctu-
ation then the measured aperture magnitude was used to
calculate V −Ic colour. Otherwise the 3-σ upper limit was
adopted.
3.4 Selection of LBG Candidates
Thecolour criteriawe adoptedtoselect LBGcandidatesat
z ∼5 are the same as those used in I03. They were defined
to efficiently select star-forming galaxies at z ∼ 5 without
heavy contamination by objects from lower redshifts, and
werechosenbyconsideringthecoloursofbothmodelgalax-
ies and real galaxies within our images which havespectro-
scopic redshifts identified by previous surveys available at
that time (see below for the cross-identifications of galax-
ieswithspectroscopic redshiftswithourobjectcatalogues).
Ourcolourcriteriaareexpressedbythefollowing equations
in theABmagnitude system:
V −Ic >1.55
and
V −Ic >7.0(Ic−z′)+0.15.
In Fig. 2 we show our z ∼ 5 colour selection criteria
along with redshift colour tracks of several types of galax-
ies. The colour tracks of model star-forming galaxies are
calculated based on the same SEDs as used in Fig. 1 (see
section 2). Colour tracks for spiral (Sbc and Scd) and el-
lipticalgalaxiesarecalculated usingtemplateSEDsoflocal
galaxies by Coleman et al. (1980). Figure 2. A V − Ic and Ic − z′ two colour diagram. The
shaded area stands for the colour selection criteria adopted for
In Fig. 3 we show colours of detected objects within a
the selection of Lyman break galaxies at z ∼ 5. Lines are loci
′
magnitude range 23.0 <z < 24.5. The numbers of objects
of colour changes for model galaxies along redshift. Solid blue:
which satisfy the colour criteria are 617 and 236 for the a star-forming galaxy without dust attenuation. Solid orange: a
HDF-N region (z′ <26.5 mag) and the J0053+1234 region star-forming galaxy with dust attenuation of E(B−V) = 0.4.
′
(z <25.5 mag), respectively. Solidcirclesareplotted forz>4.1withanintervalof0.1.Sym-
AmongtheHDF-NfieldLBGcandidatesinI03thatare bolsatz=4.5andz=5.0areenlarged.Colourtrackscalculated
within theregion common tothenew,deeperandnarrower using template SEDs by Colemanetal. (1980) are also shown.
imagingdata,alloftheobjectswithIc 624.5magand82% Cyan (dot-dashed): Sbc, light blue (dotted): Scd, red (dashed):
elliptical.Symbolsareplottedwitharedshiftintervalof0.5.Star
of objects with Ic 6 26.0 mag are included in our updated
symbolsindicatethecoloursofA0–M9starscalculatedbasedon
source catalog. The number of objects that were listed as
thelibrarybyPickles(1998).
LBGcandidatesinI03butareoutsidethecolourcriteriain
thenewcatalogis20–30%largerthantheestimatednumber
of low-redshift contaminants (see section 4.1 and Table 5).
Thisfactisnotsurprising,becausethesignal-to-noiseratios ≈22 and their redshifts are 0.32 (ID=7066 by Wirth et al.
ofourpreviousdataareworsethantheHDF-Nregion data 2004) and 0.64 (ID=3249). For the J0053+1234 region, we
presented in thispaper. show objects with published spectroscopic redshift smaller
Both of our target fields have been observed with than4.5withmagentasquaresinFig.3(b).Onesensibleex-
other intensive spectroscopic surveys. We identified objects ceptiontothisisCDFb-G5atz=4.486,whichsatisfiesour
withspectroscopic redshiftsinourimagesandplottedtheir colour selection criteria (V −Ic =2.70, Ic−z′ =0.29) and
coloursin Fig.3.FortheHDF-Nregion weusedtheresults isshownwithafilledgreencircleasoneofthespectroscopi-
of two surveys (Cohen et al. 2000 and Wirth et al. 2004). callyconfirmedLBGsatz∼5(seesection3.5).Inaddition
The numbers of objects identified are 644 for Cohen et al. to this object, two objects at z <4.5 are within the colour
(2000)and1,803forWirth et al.(2004)(mostoftheobjects selection criteria for z ∼ 5 LBGs. One of them is object id
in Cohen et al. 2000 are included in Wirth et al. 2004). In D0K 163 in Cohen et al. (1999a). The redshift provided in
Fig. 3(a) we only show objects with Ic > 22.0 mag and Cohen et al.(1999a)isz=0.580andistaggedwith’absorp-
V >22.0mag,becausephotometryofobjectsbrighterthan tiononly;faint;iduncertain’.IthasabluecolourinIc−z′
theselimits would beunreliable dueto saturation or detec- (0.07) in ourphotometry.Anotherobject is CDFa-GD12in
tornon-linearity.Outof1237objectswithspectroscopicred- Steidelet al. (1999).Theobject is selected with their z∼4
shiftssmallerthan4.5,onlytwofallwithinourz∼5colour LBG criteria (using G, R, I filters) and the spectroscopic
′
selection criteria. Both of them have z -band magnitude redshiftisz =3.469.AlthoughtheI-bandmagnitudelisted
6 I. Iwata et al.
in Steidel et al. (1999) is similar to our z′-band magnitude more than half of the observed spectra have too low a
′
(z =24.76mag),thedistancebetweenthecoordinatelisted signal-to-noise ratio to determine their redshifts. Four ob-
inSteidel et al.(1999) andoursarefairly large (1.74′′),un- jects which have slightly redder Ic − z′ colours than the
like otherobjects (mostly <1′′). colour selection criteria havebeen identified as Galactic M-
In I03 we defined the colour criteria for selection of typestars (Andoet al. 2004).
z∼5LBGsusingthecoloursofgalaxieswithspectroscopic In addition, there are 8 objects in the HDF-N region
and photometric redshifts in the HDF-N region, as well as and 2 objects in J0053+1234 which have been spectroscop-
modelcolourtracks.SincethepublicationofI03thenumber ically identified to beat 4.5<z <5.5 by other researchers.
of low-redshift galaxies with spectroscopic identificationsin Amongthem,6objectsareselectedasV-dropoutcandidates
the HDF-N region has greatly increased, but, as described in our images. Tables 3 and 4 provide summaries of the
above,thecross-identificationofthesespectroscopicsamples cross-identifications of these objects with previously pub-
withourLBGcatalogshowsthatthenumberoflow-redshift lished spectroscopic redshifts. In Fig. 3, the objects that
galaxiesthatcontaminateourcolourselectioncriteriaisvery have been selected as V-dropout candidates in our images
small. and that have spectroscopic confirmations are shown with
Since the magnitude range of our LBG candidates is filled green circles (or upper arrows for objects not de-
fainter than most of these spectroscopically identified low- tected in V-band). There are four objects in the HDF-N
redshiftobjects, theobjectsidentifiedascontaminantscan- region which have been previously reported to have spec-
not beused to estimate thefraction of contaminantsin our troscopic redshifts larger than 4.5 and are detected in our
sample.Consequently,weneedresamplingteststoestimate images but do not satisfy our colour selection criteria (see
the contamination fraction as a function of magnitude, as table 3). Their positions in the V − Ic and Ic − z′ di-
described in section 4.1. agram are shown as open circles in Fig. 3. The first of
OurHDF-NregioncontainstheGOODS-Narea,where these, HDF-N 4-439.0 at z = 4.54, has V − Ic = 1.48
deepHST/ACSandSpitzer/IRACandMIPSimaginghave and is slightly off from our colour selection window. An-
been made (Giavalisco et al. 2004a). The ACS images are other object, F36219−1516, which has been reported to be
in the B435, V606, I775 and z850 filters and using V606, I775 at z=4.89 byDawson et al.(2001),hasaveryblue V −Ic
and z850, a V-dropout LBG sample has been constructed colourinourimage(V−Ic=0.67).Sincetheredshiftdeter-
(Ravindranathet al. 2006). However, it is not straightfor- minationwasmadewithsolelyasingleemissionline(identi-
ward to compare our sample with those based on the ACS fiedasLyα),wesuggestthatthismaybeamisidentification
images. Since the filters used are different, especially V606 of a lower redshift object. F36376−1453 is another object
which has a very broad bandpass (width is ≃ 1500˚A), the with a single emission linein thespectrum which was iden-
colour tracks of the same galaxy SEDs with GOODS/ACS tified as Lyα at z = 4.886 by Dawson et al. (2001). This
filters in the two-colour diagram are quite different from object is very bright (z′ = 21.97) and its colours in both
ours. V −Ic andIc−z′ arered(V −Ic =3.25,Ic−z′ =0.78and
In Fig. 4 the sky distributions of LBG candidates thisobjectisoutsideofplottedareaofFig.3).Ithasaround
in our two fields are shown. There is a clear fluctua- shapeinourimagesinIcandz′-bands(FWHM∼1.0′′).The
tion in surface density of LBG candidates; some areas fourthobject,GOODSJ123721.03+621502.1, wasidentified
have many LBGs while others don’t, and these irregu- asan object at z =4.76 byCowie et al. (2004)(no.1173 in
larity in the surface density is not caused by the masks theirtable1).ThisobjectalsohasredcoloursinV −Ic and
used to avoid effect of foreground bright objects such as Ic−z′.Noinformation aboutthequalityofthespectrumis
galactic stars and nearby galaxies. Fairly strong cluster- providedinCowie et al.(2004).Ontheotherhand,oneob-
ing signals have been found with past wide-field LBG jectintheJ0053+1234 regionwhichisatz=4.486(CDFb-
searches(e.g.,Giavalisco et al.1998;Adelberger et al.1998; G5inSteidel et al.1999)andiswithinourselectionwindow
Foucaud et al. 2003; Ouchiet al. 2004b; Hamana et al. (V−Ic =2.70,Ic−z′=0.29;seetable4),anditisshownas
2006). These figures also tell us that in small-area surveys anfilledgreencircleinFig.3(b).Insummary,spectroscopic
such as theHDF-N,the HubbleUltra Deep Field and even observationsforourLBGcandidatesstillcoveronlyasmall
GOODS, the effect of field-to-field variance would be much fraction of the sample, but the results obtained so far are
more serious than for our data. Detailed analyses of clus- encouraging. Both the existence of objects at z ∼ 5 within
teringpropertiesofourLBGsampleswill bepresentedina theselectionwindowandthesmallnumberofcontaminants
forthcoming paper. described in section 3.4 strongly support thevalidityof our
selection criteria.
Capak et al. (2004) analysed multicolour data includ-
3.5 Spectroscopic Identifications of Galaxies at
ing some of the same Suprime-Cam data that we now re-
z ∼5
processedandincludedinthepresentworktoconstructour
We carried out optical spectroscopic observations us- multi-band object catalog in the HDF-N region. They also
ing a multi-object spectroscopy mode of the FOCAS made a selection of V-dropout galaxies, derived their num-
(Kashikawa et al. 2002) attached to the Subaru telescope. bercounts,andcomparedthemwiththoseinI03.Thenum-
Until now, 8 objects in the HDF-N region and 4 ob- ber counts in Capak et al. (2004) are just 13–21% of those
jects in the J0053+1234 region have been identified to in this paper. They claimed that our sample may be con-
be in the redshift range between z = 4.2 and 5.2 taminated by z ≃1 galaxies since our selection criteria are
(Andoet al. 2004; Andoet al. 2007; including one AGN redder than theirs in Ic −z′ colour. However, their argu-
GOODSJ123647.96+620941.7). Noobjectwithsmallerred- mentsareincorrect.ThecolourcriteriausedinCapak et al.
shift has been found in our observations so far, although (2004)areV −Ic >2.4andV −Ic>7(Ic−z′)−0.2andthe
Differential Evolution of UVLF of LBGs 7
Figure 3. V −Ic andIc−z′ twocolourdiagrams.(a): datapointsareforobjects intheHDF-Nregion.Blackcirclesindicatecolours
of objects with 23.0<z′ 624.5. For objects not detected in V-band, arrows are used to show the 3-σ lower limits of V −Ic colours.
Redandbluesquaresrepresentgalaxies withspectroscopic redshifts(z<4.0)publishedinWirthetal.(2004)andCohenetal.(2000)
respectively. Objects in Cohenetal. (2000) which are listed in the catalog by Wirthetal. (2004) are omitted. Colour track lines and
star symbols are the same as in Fig. 2. Filled green circles indicate colours of galaxies at z > 4.5 confirmed by spectroscopy. Three
open green circles show colours of objects which are not included in our LBG candidates catalog but are reported to be at z >4.5 in
previous spectroscopic observations (seetable 3). (b): sameas (a)but fortheJ0053+1234 region. Squares inmagenta areobjects with
spectroscopic redshiftssmallerthan4.0inthe region,mostlytaken fromCohenetal.(1999a),and greencirclesaregalaxies atz>4.2
identifiedbyspectroscopybyusandSteideletal.(1999)(seetable4).Seetextformoredetailsofthesefigures.
lattercriterion isslightly redderthanours(inboth I03and E(B−V)=0.2).BecauseupperlimitsonIc−z′ colourval-
thepresent paper we useV −Ic >7.0(Ic−z′)+0.15 when uesareimposedduringtheselection ofLBGs,therearefew
on the AB normalization), contrary to their argument. In- objectswithIc−z′>0.4.Thisdoesnotmeanthatallstar-
stead,themaincauseofthesmallnumbercountswiththeir forminggalaxiesatz ∼5havesuchblueUVcolours.Wedo
selection is that their first criterion sets too strong a lower notknowwhatfractionofstar-forminggalaxieswithredUV
limitinV−Iccolourandthusmissesmanyz≃4.5galaxies. colours (probably dueto heavy attenuation by dust) at the
As is shown in Fig. 3, their criterion, V −Ic > 2.4, misses target redshift are missed in oursearch; at least in the red-
many LBGs with high-z spectroscopic identifications. shift range z = 2–3 significant populations of star-forming
galaxies with redder rest-frame UV colour have been de-
tected(e.g,F¨orster Schreiberet al.2004;Daddi et al.2004).
3.6 UV colour distribution Applicationofsimilarmethodsathigherredshiftswillbean
important subject of future space-based infrared missions,
If our LBG candidates are real objects at z ∼ 5 then their
Ic −z′ colours represent rest-frame UV colours. In Fig. 5 but for now we must press on with UV-selected samples.
we show the distribution of Ic−z′ colours along apparent In the median values of Ic−z′ colours there is a weak
z′-band magnitude. In this figure we also show the distri- trend in that objects bright in z′-band have redder Ic−z′
bution of Ic−z′ colours for objects with 23.0 < z′ < 26.0 colours.Inotherwords,theratioofthenumberofblue(e.g.,
asfilled circles andthemediancolour valuesindifferent z′- Ic−z′ <0) objects to thenumber of red objects is smaller
band magnitudes (in a 0.5 mag step) as squares. Typical forbrightobjectsthanforfainterones.Sincetheupperlimit
photometricerrorsinIc−z′ coloursarecalculatedfromthe on Ic −z′ colour in our selection criteria depends on V −
simulationsusingartificialobjects(describedinsection4.1), Ic colour, the selection effect is not straightforward in this
assumingIc−z′=0.07mag(fromamodelSEDatz=4.8, figure.However,sincethereisnorestrictionfortheselection
8 I. Iwata et al.
Figure 4.SkydistributionofLBGcandidates intheHDF-Nregion(a)andintheJ0053+1234 region(b). Filledmagentacirclesshow
the positions of LBG candidates. Symbol size indicates objects’ brightness ina 0.5 mag step. Filledgreen circles are for objects which
havebeenconfirmedtobeatz&4.5withopticalspectroscopy.Largecircleswithdashedlinesindicatethepositionsandareasofoptical
spectroscopybyus.Graycirclesindicatethepositionofmasksusedtoavoidtheeffectsofbrightstars.Roughboundariesofimageareas
usedforselectionofLBGcandidatesareshownwithdashedlines.Notethatnotalloftheareawithintheselinesisused:duetothegaps
between CCD chips, different position angles and dither scale lengths, some positions near boundaries have smaller effective exposure
time,andsuchareaswerenotusedforcandidateselection.LargeandsmallareasdrawnwithsolidlinesinFig.4(a)aretheHSTsurvey
areasoftheGOODS-NandtheHDF-N,respectively.
4 UV LUMINOSITY FUNCTION AT Z ∼5
4.1 Correction for Incompleteness and
Contamination
TheprocedurethatweusetoderivetheUVluminosityfunc-
tionfollowsthatdescribedinourpreviouspaper(I03)which
wasbased on shallower datafortheHDF-Nregion only.To
estimate the volumes sampled with our adopted colour cri-
teria,weexecutealargesetoftestsinwhichweuseartificial
galaxieswhichmimicthesizedistributionofrealgalaxiesin
our sample. We are able to derive the completeness of our
survey for objects with specific magnitudes and colours by
counting the number of artificial objects which have been
recovered by the object detection and colour selection pro-
cedures. In a single test run we generate 25,000 artificial
Figure 5. z′-band magnitude against Ic −z′ colour for LBG objects for each bin of 0.5 in magnitude, 0.1 in redshift,
candidates at z ∼ 5 in the HDF-N region. Open circles show and reddening by dust (five different degree of reddening
magnitudes and colours of individual galaxies. Typical errors in from E(B−V)=0.0 to 0.4 with steps of 0.1). Colours are
Ic−z′ colours aredisplayed inthe rightsideof the panel. Solid assigned based on the model spectra of star-forming galax-
squares show the median of Ic −z′ colours in a 0.5 mag step ies generated with PEGASE.2 (see Fig. 2), Calzetti et al.
in z′-band magnitude. Filled circles indicate the distribution of
(2000) extinction law, and a prescription for IGM attenua-
2fr3a.c0ti<onz′o<f I2c6−.0,za′ncdoliotsursscailneaisnsh0o.1wndeixnsthteeprifgohrtosbidjeecvtserwticitahl tion byInoue et al. (2005).These objects are then put into
′
axis. random positions in the V, Ic and z images. The meth-
ods of object detection and sample selection according to
the colour criteria are the same as those for real objects,
described in sections 3.3 and 3.4.
of objects with blue Ic −z′ colours, the absence of bright In Fig. 6 we show the completeness of our survey as a
objects withbluerest-frameUVcolourmustbereal. There functionof redshift andz′-bandmagnitude.Resultsforfive
isnoselectionbiasduetothelimitingmagnitudeinIc-band, different amounts of reddening were averaged with weights
since this is Ic = 26.86 mag in the HDF-N region. The red estimated from the UV colour distribution of real galaxies
colour of UV continua can be attributed to various origins, (section 3.6). The maximum completeness is achieved for
includingheavierdustattenuation,olderstellarpopulations, brightest (23.0–23.5 mag) objects at redshift 4.5–5.0 in the
andweakerLymanαemission (forobjectsatz>4.8theIc HDF-N region. We note that there is a significant (&10%)
filter covers the rest-frame Lyman α wavelength). We will detection rate for objects at z ∼ 4. It suggests that in
brieflydiscussthisdependenceoftheUVcolourdistribution our sample of z ∼ 5 LBG candidates there should be a
on UVluminosity in section 5.2.2. small number of objects at such lower redshift ranges. So
Differential Evolution of UVLF of LBGs 9
Veff(m) is the effective volume. The multiplication by 2 is
requiredtoobtainnumberdensitiespermagnitude,sincewe
take0.5magbinsinoursample.TheUVLFsderivedforthe
HDF-N region and the J0053+1234 region are listed in Ta-
bles5and6,respectively,andtheyareshowninFig.7with
opencircles(HDF-N)andopentriangles(J0053+1234).The
error bars indicate 1σ error estimates including statistical
(Poisson)noise,photometricerrorsanduncertaintiesincon-
taminationsandcompleteness. Inconvertingfrom apparent
z′-band magnitude to absolute magnitude we assume a lu-
minositydistanceofanobjectatz=4.8undertheadopted
cosmology (MUV = mz′ − 46.33, ΩM = 0.3, ΩΛ = 0.7,
H0=70 km/s/Mpc). InI03wemadea correction forlumi-
nosity in the conversion from Ic-band apparent magnitude
to rest-frame luminosity at 1700˚A, assuming the SED of a
modelstar-forminggalaxy.Inthepresentanalysiswedonot
Figure 6. Detection rates of the galaxies for the HDF-N re-
make such a correction in the conversion from magnitudes
gion(solidlines)andtheJ0053+1234region(dashedlines),asa
function of z′-band magnitude and redshift. The lines represent inz′-band(theeffectivewavelengthis9086˚A),andthusthe
z′-band magnitude of 23.0–23.5, 24.0–24.5, 25.0–25.5 and 26.0– LFs we present are at ≈1570˚A.
26.5(thelastbinfortheHDF-Nregiononly). In the calculation of selection functions and effective
volumes we assume a dust attenuation distribution from
E(B−V)=0.0to0.4basedontheIc−z′colourdistribution.
far we have detected one object at z = 4.26 in the optical
In order to examine the robustness of the LF against this
spectroscopyoftheJ0053+1234region(table4;Andoet al.
assumption about dust attenuation, we also calculated ef-
2007). The existence of such relatively low redshift objects
fectivevolumeswithtwoextremecasesindustattenuation:
doesnothaveaseriouseffectinderivingtheUVLFatz ∼5,
one case is that using just a model without dust attenua-
since we properly treat them by calculating the survey vol- tion (E(B−V)=0.0) and the other is only using a model
ume using these selection functions. The detection rates in with relatively large dust attenuation (E(B −V) = 0.4).
the J0053+1234 region are lower than those in the HDF-N
Intheseextremecasestheshapeofselection functionalong
region at the same magnitude ranges, and they are similar
redshiftchangesfromthatshowninFig.6,becausethered-
to those for the data used in I03, which were ∼ 0.8 mag
shiftrangecoveredwithourselection criteria changeswhen
′
shallower in z -band than thepresent HDF-Ndata.
the assumed model is different. However, we find that the
A weighted average value of redshift derived from this
differences in the resulting LF — even with these extreme
completenessdistributionis4.8forboththeHDF-Nandthe
cases — are small, as shown in Fig. 8. In this figure the
J0053+1234regions.Hereafterweusethisvalueasatypical
UVLFs in the HDF-N region using effective volumes cal-
redshift in the presentation of the UV luminosity function.
culated based on these extreme assumptions are compared
Fromthesecompletenessestimatesfordifferentmagnitudes,
with the UVLF computed using the fiducial dust attenu-
redshifts and amount of dust reddening, we calculated the
ation distribution. Since the extreme assumptions in these
effective volumes (Steidel et al. 1999) of our surveys, which
tests(allobjectsarefreefromdustorallobjectshavefairly
are listed in Tables 5 and 6. large dust attenuation of E(B−V)=0.4) are unlikely, we
As we did in I03, we use a resampling method to esti-
can say that ourUVLFisrobust against theassumption of
matetherateofcontamination bygalactic starsand galax-
model colours and distribution of amount of dust attenua-
ies at intermediate redshifts. We perturbed the colours of
tion.
detected objects which lie outside of the colour selection
The modification of the selection criteria should also
criteria by adding random errors based on the error mea-
alter the effective volume. We examined the change of ef-
surements in photometry. Then we counted the number of
fective volumes and luminosity function by means of a test
objects which fell intothe colour selection criteria after the
in which the colour selection criteria were shifted in both
addition of these random errors. For each bin of 0.5 magni- V −Ic and Ic−z′ with 0.05–0.15 magnitude, and derived
tude these resampling tests were executed 1,000 times, and
luminosityfunctionsinthesamewayaswedidfortheorig-
averagenumbersofinterloperswerederived.Thesenumbers
inal selection criteria. The UVLFs obtained by these tests
are listed in Table 5 and 6 as Nint. 1 showdifferencessmallerthantheerrorbarsofourLFshown
We computetheUVLFas
inFig.7.WiththesetestsweverifiedthatourUVLFresults
Φ(m)=2×(N(m)−Nint(m))/Veff(m), arerobustagainst slight changestothecolourselection cri-
teria.
where N(m) represents the number of LBG candidates de-
tected within the magnitude bin of m ± 0.25 mag, and
4.2 Combination of the UVLFs in the two survey
fields and Schechter function fitting
1 We also tested a case in which we perturbed the colours of
SincetheJ0053+1234 sample is shallower than the HDF-N
objects within the colour selection area and count the number
of objects which get out from the area, and confirmed that the sample, the UVLF of the J0053+1234 region is limited to
′
changes intheUVLFaresmallerthanorcomparabletothesize z < 25.5 mag. As shown in Fig. 7, the number densities
ofoverallerrors.Seesection4.1ofI03formoredeitals. of these two fields agree fairly well, although in two bright
10 I. Iwata et al.
Figure 7. UV luminosity functions (UVLFs) at z ∼ 5. Open circles and open triangles represent UVLFs of LBGs at z ∼ 5 in the
HDF-NregionandtheJ0053+1234region,respectively.FilledcirclesshowtheUVLFderivedbycombiningresultsofthetwofields.The
data points of UVLFsbased onsinglefields areshifted+0.05magforclarity.The solidlinerepresents the Schechter function fitted to
the combined UVLF, and the dashed line is a fitting for the UVLF in the HDF-N. Open squares are the UVLF in the HDF-N region
determinedwithpreviousshallowerdata(I03).
Figure 8.TheUVLFatz∼5fortheHDF-Nregion(sameastheUVLFwithopencirclesinFig.7)calculated usingthedistribution
of dustattenuation estimated fromIc−z′ colours (Fig. 5) iscompared withUVLFs derivedbased on twoextreme assumptions about
thedustattenuation ofoursampleLBGs(noattenuation and E(B−V)=0.4forallobjects), shownwithopencircles.LFswithopen
circlesareshifted+0.05magforclarity.Seesection4.1fordetails.