The Discovery of a Very Narrow-Line Star Forming Obat a Redshift of 5.66ject

6 Pages·0.2 MB·English

by  Y. Taniguchi

Accepted forpublicationintheAstrophysicalJournal,Letters
PreprinttypesetusingLATEXstyleemulateapjv.04/03/99
THE DISCOVERY OF A VERY NARROW-LINE STAR FORMING OBJECT AT A REDSHIFT OF
5.661,2
Yoshiaki Taniguchi 3, Masaru Ajiki 3, Takashi Murayama 3, Tohru Nagao 3, Sylvain
Veilleux 4, David B. Sanders 5,6, Yutaka Komiyama 7, Yasuhiro Shioya 3, Shinobu S.
Fujita 3, Yuko Kakazu 5, Sadanori Okamura 8,9, Hiroyasu Ando 7, Tetsuo Nishimura 7,
Masahiko Hayashi 7, Ryusuke Ogasawara 7, & Shin-ichi Ichikawa 10
Accepted for publication in the Astrophysical Journal, Letters
ABSTRACT
3 We report on the discovery of a very narrow-line star forming object beyond redshift of 5. Using the
0 prime-focuscamera,Suprime-Cam,onthe8.2mSubarutelescopetogetherwithanarrow-passbandfilter
0 centered atλ = 8150˚A with passbandof ∆λ = 120 ˚A, we haveobtained a very deep image of the field
c
2
surrounding the quasar SDSSp J104433.04 012502.2at a redshift of 5.74. Comparing this image with
−
n optical broad-band images, we have found an object with a very strong emission line. Our follow-up
a optical spectroscopy has revealed that this source is at a redshift of z =5.655 0.002, forming stars at
1 J aSinracteea∼bl1u3ehh−0a.l27fMof⊙thyerL−y1α. Rememisasiroknabcloyu,ltdhbeevealboscoirtybeddisbpyernseiountraolfhLyydαr-oegmenittgian±sg,gpaesrhiasposnilnyt2h2eksmystse−m1.,
3 a modest estimate of the velocity dispersion may be > 44 km s−1. Together with a linear size of 7.7
1 h−0.17 kpc, we estimate a lower limit of the dynamical m∼ass of this object to be ∼ 2×109M⊙. It is thus
suggested that LAE J1044 0123is a star-forming dwarf galaxy (i.e., a subgalactic object or a building
v
−
5 block) beyond redshift 5 although we cannot exclude a possibility that most Lyα emission is absorbed
3 by the red damping wing of neutral intergalactic matter.
6 Subject headings: galaxies: individual (LAE J1044 0123)—galaxies: starburst—galaxies: formation
1 −
0
3 1. INTRODUCTION erences therein). Therefore, careful investigations of Lyα
0 emission-line properties of galaxies with z > 5 provides
SearchesforLyαemitters (hereafterLAEs)athighred-
/ veryimportantcluessimultaneouslybothontheearlystar
h shift are very useful in investigating the early star forma-
formationhistoryofgalaxiesandonthe physicalstatusof
p tionhistoryofgalaxies(Partridge&Peebles1967). Recent
IGM at high redshift.
- progress in the observational capability of 10-m class op-
o The firststepisto lookforalargenumberofLyαemit-
tical telescopes has enabled us to discover more than a
r tercandidatesathighredshiftthroughdirectimagingsur-
t dozen of Lyα emitting galaxies beyond redshift 5 (Dey et
s veys. The detectability of high-redshift objects is signifi-
a al. 1998; Weymann et al. 1998; Hu et al. 1999, 2001,
cantly increased if they are hosts to recent bursts of star
: 2002; Dawson et al. 2001, 2002; Ellis et al. 2001; Ajiki et
v formation which ionize the surrounding gas and result in
al. 2002;seealsoSpinradetal. 1998;Rhoads&Malhotra
i strongemissionlines like the hydrogenrecombinationline
X 2001). ThemostdistantobjectknowntodateisHCM-6A
Lyα. These emission-line objects can be found in prin-
r at z = 6.56 (Hu et al. 2002). These high-z Lyα emit-
ciple through deep optical imaging with narrow-passband
a ters can be also utilized to investigate physical properties
filters customized to the appropriate redshift. Indeed, re-
of the intergalactic medium (IGM) because the epoch of
centattempts withthe Keck10mtelescopehaverevealed
cosmic reionization (z ) is considered to be close to the
r the presence of Lyα emitters in blank fields at high red-
redshifts of high-z Lyα emitters; i.e., z 6 – 7 (Djorgov-
r ∼ shift (e.g., Cowie & Hu 1998; Hu et al. 2002). These
ski et al. 2001; Becker et al. 2001; Fan et al. 2002). In
recentsuccesseshaveshownthegreatpotentialofnarrow-
otherwords,emission-linefluxesoftheLyαemissionfrom
bandimagingsurveyswith8-10mtelescopesinthesearch
such high-z galaxies could be absorbed by neutral hydro-
for high-z Lyα emitters. It is also worthwhile noting that
gen in the IGM if neutral gas clouds are located between
subgalacticpopulationsathighredshifthavebeenrecently
thesourceandus(Gunn&Peterson1965;Miralda-Escud´e
foundthankstothegravitationallensing(Ellisetal. 2001;
1998;Miralda-Escud´e& Rees 1998;Haiman2002andref-
1BasedondatacollectedatSubaruTelescope,whichisoperatedbytheNationalAstronomicalObservatoryofJapan.
2Based on data collected at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of
Technology, theUniversityofCalifornia,andtheNationalAeronauticsandSpaceAdministration.
3AstronomicalInstitute, GraduateSchool ofScience, TohokuUniversity,Aramaki,Aoba,Sendai 980-8578,Japan
4DepartmentofAstronomy,UniversityofMaryland,CollegePark,MD20742
5Institute forAstronomy,UniversityofHawaii,2680WoodlawnDrive,Honolulu,HI96822
6Max-PlanckInstitutfurExtraterrestrichePhysik,D-85740,Garching,Germany
7SubaruTelescope,NationalAstronomicalObservatory,650N.A’ohokuPlace,Hilo,HI96720
8DepartmentofAstronomy,GraduateSchoolofScience,UniversityofTokyo,Tokyo113-0033, Japan
9ResearchCenterfortheEarlyUniverse,SchoolofScience,UniversityofTokyo, Tokyo113-0033, Japan
10National AstronomicalObservatory,2-21-1Osawa,Mitaka,Tokyo181-8588,Japan
1
2
Hu et al. 2002). of LAE J1044 0123are given in Fig. 1. As shown in this
−
Inanattempttofindstar-formingobjectsatz 5.7,we figure,LAE J1044 0123isseenclearlyonly inthe NB816
≈ −
havecarriedoutaverydeepopticalimagingsurveyinthe image. Although it is seen in the I image, its flux is
C
field surroundingthe quasarSDSSp J104433.04 012502.2 below the 3σ noise level. The observed equivalent width
at redshift of 5.7411 (Fan et al. 2000; Djo−rgovski et is EW > 238 ˚A. The NB816 image reveals that LAE
obs
al. 2001; Goodrich et al. 2001), using Suprime-Cam J1044 0123is spatially extended; its angular diameter is
(Miyazaki et al. 1998), the wide-field (34′ 27′ with a 1.6arc−sec(abovethe 2σ noiselevel). Thesizeofthepoint
×
0.2 arcsec/pixelresolution)prime-focus cameraonthe 8.2 spread function in the NB816 image is 0.90 arcsec. Cor-
mSubarutelescope(Kaifu1998). InthisLetter,wereport recting for this spread, we obtain an angular diameter of
onourdiscoveryofaverynarrow-linestar-formingsystem 1.3 arcsec.
at z 5.7.
≈ 2.2. Optical Spectroscopy
2. OBSERVATIONS Our optical spectroscopy was made by using the Keck
2.1. Optical Imaging II Echelle Spectrograph and Imager (ESI: Sheinis et al.
2000)on 2002March 15 (UT). We used the Echelle mode
In this survey, we used the narrow-passband fil-
withthe slitwidthof1arcsec,resultinginaspectralreso-
ter, NB816, centered on 8150 ˚A with a passband of lutionR 3400at8000˚A.Theintegrationtimewas1800
∆λ(FWHM) =120˚A;thecentralwavelengthcorresponds ≃
seconds. ThespectrumofLAEJ1044 0123,showninFig.
toaredshiftof5.70forLyαemission. Wealsousedbroad- 2, presents a narrow emission line at−λ = 8090 ˚A. This is
passband filters, B, R , I , and z′. A summary of the
C C the only emission line that was detected within the ESI
imaging observations is given in Table 1. All of the ob- wavelengthrange (from 4000˚A to 9500˚A). This line may
servationsweredoneunderphotometricconditionandthe
beeitherLyαor[Oii]λ3727. Theemission-lineprofileap-
seeing size was between 0.7 arcsec and 1.3 arcsec during
pears to show a sharper cutoff at wavelengths shortward
the run. Note that we analyzed only two CCD chips,
of the line peak, providing some evidence that this line is
in which quasar SDSSp J104433.04 012502.2 is present,
− Lyα. A stronger argument in favor of this line identifica-
to avoid delays for follow-up spectroscopy. The CCD
tioncomesfromthe lackofstructureinthe profile. Ifthis
data were reduced and combined using IRAF and the
line were [O ii] emission, the redshift would be z 1.17.
mosaic-CCD data reduction software developed by Yagi ≈
Sincethe[Oii]featureisadoubletlineof[Oii]λ3726.0and
et al. (2002). Photometric and spectrophotometric stan-
[O ii]λ3728.8,the line separationwouldbe largerthan6.1
dardstarsusedinthefluxcalibrationareSA101fortheB,
˚Aandthelineswouldbe resolvedintheESIobservations.
R ,andI data,andGD108,GD58(Oke1990),andPG
C C Further, if the line were Hβ, [O iii]λ4959, [O iii]λ5007, or
1034+001 (Massey et al. 1988) for the NB816 data. The
z′ data were calibrated by using the magnitude of SDSSp Hα line, we would detect some other emission lines in our
spectrum. Therefore, we conclude that the emission line
J104433.04 012502.2(Fan et al. 2000).
The total−size ofthe field is 11′.67by 11′.67,correspond- at 8090 ˚A is Lyα, giving a redshift of 5.655±0.002.
ing to a solidangleof 136arcmin2. The volumeprobed 3. RESULTSANDDISCUSSION
≈
by the NB816imaginghas(co–moving)transversedimen-
sions of 27.56 h−0.17×27.56h−0.17 Mpc2, and the half–power 3.1. Star Formation Activity in LAE J1044−0123
points ofthe filter correspondto aco–movingdepthalong The rest-frame equivalent width of Lyα emission be-
the line of sight of 44.34 h−1 Mpc (z 5.653 and comesEW >36˚A.OurKeck/ESIspectrumgivestheob-
z 5.752; note that th0e.7transmissmioinn ≈curve of our servedLyα0fluxoff(Lyα)=(1.3 0.1) 10−17ergscm−2
max
NB816≈filter has a Gaussian-like shape). Therefore, a to- s−1 and the rest-frame equivalen±t widt×h of Lyα emission
timalavgoel.uHmeereo,fw3.e4a×do1p0t4ha−0.fl37aMt upnc3iveisrsperowbitehdΩin our N=B08.136, EofWLA0 E>J31604˚A4. O01n2t3hgeivoetshfer(Lhyaαn)d, o4u.r1NB108−1617meraggsncitmu−d2e
Ω = 0.7, and h = 0.7 where h = H /(1m0a0ttekrm s−1 s−1,being hig−herthanby afactor≃of3t×hanthe Keck/ESI
Λ 0
Mpc−1). flux. Since the Keck/ESI spectrum was calibrated by a
Source detection and photometry were performed us- single measurement of a spectroscopic standard star, HZ
ing SExtractor version 2.2.1 (Bertin, & Arnouts 1996). 44, the photometric accuracy may not be good. Further,
Our detection limit (a 3σ detection within a 2′′.8 diam- ourslitwidthmaynotcovertheentireLyαnebulaofLAE
eter aperture) for each band is listed in Table 1. As for J1044 0123. Therefore, we use the NB816-based flux to
−
the source detection in the NB816 image, we used a crite- estimatethestarformationrate. TheNB816fluxgivesthe
rion that a source must be a 13-pixel connection above Lyα luminosity L(Lyα) 1.4 1043 h−2 ergs s−1. Using
5σ noise level. Adopting the criterion for the NB816 the relation SFR = 9.1≃ 10×−43L(Ly0α.7) M⊙yr−1 (Ken-
estxrcoensgs,emICis−sioNn-Blin8e16sou>rce1s..0Omuargf,olwloew-huapveopftoiucanldsptewco- ynric−u1t.tT1h9i9s8i;sBarloocwkelerhliumrsitt×b19ec7a1u),sewneoocbotrarienct∼io1n3wha−0s.27mMad⊙e
troscopy of these sources reveals that one source found at for possible internal extinction by dust grains in the sys-
α(J2000)=10h 44m 27s and δ(J2000)= 01◦ 23′ 45′′ (here- tem. The lackofUV continuum fromthis objectprevents
−
after LAE J1044 0123) is a good candidate to be a sub- us from determining the importance of this effect.
galactic object a−t high redshift12. Its AB magnitude in The most intriguing property of LAE J1044 0123 is
−
the NB816 band is 24.73. The optical thumb-nail images that the observed emission-line width (full width at half
11The discovery redshift was z = 5.8 (Fan et al. 2000). Since, however, the subsequent optical spectroscopic observations suggested a bit
lowerredshift;z=5.73(Djorgovskietal. 2001)and z=5.745(Goodrichetal. 2001), weadoptz=5.74inthisLetter.
12Another sourcehasbeenidentifiedasaLyαemitteratz=5.687(Ajikietal. 2002)
3
maximum; FWHM) of redshifted Lyα is only 2.2 0.3 limit of the star formation timescale in the system; i.e.,
±
˚A. Since the instrumental spectral resolution is 1.7 0.1 τSF < τdyn. However, if the observed diameter is deter-
˚A, the intrinsic width is only 1.4 0.5 ˚A; note tha±t this mine∼dbytheso-calledStro¨mgrenspherephotoionizedbya
gives a upper limit because the lin±e is barely resolved. It centralstarcluster,itisnotnecessarytoadoptτSF τdyn.
corresponds to FWHM 52 19 km s−1 or a veloc- Itseemsmoreappropriatetoadoptashortertimesc∼alefor
obs
Tityhidsisvpaelurseioins σcoobmsp=arFabWleHt≃oMtohbso/±s(e2√of2llnu2m)in≃ou2s2gklmobsu−la1r. saudcohptaedhigfohr-zHsCtaMr--f6oArmaintgzgala6x.i5e6s,(eH.gu., eτtSFa∼l. 120070y2r),bays
≈
clusters (Djorgovski 1995). Haiman (2002). One may also derive a dynamical mass
It is interesting to compare the observational proper- Mdyn = (D/2)σ02G−1 ∼ 2×109M⊙ (neglecting possible
ties of LAE J1044 0123 with similar LAEs at z > 5. inclination effects).
For this compariso−n, we choose Abell 2218 a (Ell∼is et At the source redshift, z = 5.655, the mass of a dark
matter halo which could collapse is estimated as M
al. 2001), LAE J1044 0130 (Ajiki et al. 2002), and vir
J123649.2+621539 (Daw−son et al. 2002) because these 9×106rv3ir,1h−0.17M⊙wherervir,1istheVirialradiusinunit∼s
objects were also observed using KecK/ESI. A summary of 1 kpc [see equation (24) in Barkana & Loeb (2001)].
is given in Table 2. This comparison shows that LAE If we adopt rvir = D/2 = 3.85 kpc, we would obtain
J1044 0123 has the narrowest line width that may be Mvir 5 108 M⊙. However, the radius of dark mat-
roughl−y comparable to that of Abell 2218 a although the ter ha∼lo co×uld be ten times as long as D/2. If this is the
mLAasEsoJf1A04b4ell02122138,(s∼ee10n6eMxt⊙s,uibssmecutcihons)m.aAllenrotthhaernitmhaptoor-f Ccaosme,pwaerinogbttahiinsMvevloirc∼ity5d×isp10er1s0ioMn⊙wiatnhdthσe0 o∼bs7e5rvkemdos−n1e.,
tantpointa−ppearsthatthelineprofileofLAEJ1044 0123 we estimate that the majority of Lyα emission would be
doesnotshowintenseredwingemissionwhichisevi−dently absorbed by neutral hydrogen.
seeninthoseoftheotherthreeLAEs. Thediversityofthe The most important issue related to LAE J1044 0123
observationalpropertiesofthese LAEssuggestthatthe H seems how massive this source is; i.e., 109M⊙ or−more
i absorptions affect significantly the visibility of the Lyα massive than 1010M⊙. If the star form∼ation timescale is
emissionline. Further,thecontributionofsuperwindsmay as long as the dynamical one, the stellar mass assembled
be different from LAE to LAE. in LAE J1044 0123 at z = 5.655 exceeds 109 M⊙, be-
−
ing comparable to the nominal dynamical mass, M
dyn
3.2. What is LAE J1044−0123 ? 2 109M⊙. Sinceitisquiteunlikelythatmostmassisa∼s-
×
Now a question arises as; “What is LAE J1044 0123 sembledto form starsin the system,the dark matter halo
?”. There aretwo alternative ideas: (1) LAE J1044−0123 around LAE J1044 0123 would be more massive by one
is a partof a giantsystem and we observe only the−bright order of magnitude−at least than the above stellar mass.
star-forming clump, or (2) LAE J1044 0123 is a single If this is the case, we could miss the majority of the Lyα
star-formingsystem. Solelyfromourobs−ervations,wecan- emissionandtheabsorptioncloudbeattributedtothered
not judge which is the case. If this is the first case, LAE damping wing of neutral hydrogen in the IGM. Since the
J1044 0123maybe similartoAbell2218afoundbyEllis redshift of LAE J1044 0123 (z = 5.655) is close to that
et al.−(2001). One problem in this interpretation seems of SDSSp J104433.04 −012502.2 (z = 5.74), it is possible
−
thatthe spatialextension, 7.7kpc,ofLAEJ1044 0123 that these two objects are located at nearly the same cos-
is fairly large for such a le∼ss-massive system. Ther−efore, mological distance. The angular separation between LAE
adoptingthesecondcase,itseemsimportanttoinvestigate J1044 0123andSDSSpJ104433.04 012502.2,113arcsec,
possible dynamical status of LAE J1044 0123 for future corres−pondstothelinearseparation−of4.45h−1 Mpc. The
0.7
consideration. − Stro¨mgren radius of SDSSp J104433.04 012502.2 can be
Ifasourceissurroundedbyneutralhydrogen,Lyαpho- estimated to be rS 6.3(tQ/2 107 −yr)1/3 Mpc using
∼ ×
tons emitted from the source are heavily scattered. Fur- equation(1) inHaiman& Cen(2002)wheretQ is the life-
thermore, the red damping wing of the Gunn-Peterson time of the quasar (see also Cen & Haiman 2000). Even
trough could also suppress the Lyα emission line (Gunn if this quasar is amplified by a factor of 2 by the gravita-
& Peterson 1965; Miralda-Escud´e 1998; Miralda-Escud´e tional lensing (Shioya et al. 2002), we obtain rS 4.9
∼
& Rees 1998; Haiman 2002 and references therein). If Mpc. Therefore, it seems likely that the IGM around
this is the case for LAE J1044 0123, we may see only a LAE J1044 0123 may be ionized completely. If this is
part of the Lyα emission. Haim−an (2002) estimated that the case, we−cannot expect that the Lyα emission of LAE
only 8% of the Lyα emission is detected in the case of J1044 0123isseverelyabsorbedbythereddampingwing
−
HCM-6Aatz =6.56foundbyHuetal. (2002). However, emission. In order to examine which is the case, L-band
theobservedLyαemission-lineprofileofLAEJ1044 0123 spectroscopy is strongly recommended because the red-
showsthesharpcutoffatwavelengthsshortwardofth−eline shifted [O iii]λ5007 emission will be detected at 3.33 µm.
peak. This property suggests that the H i absorption is However, we need the James Webb Space Telescope to
dominated by H i gas in the system rather than that in complete it.
theIGMTherefore,itseemsreasonabletoadoptthatblue We would like to thank T. Hayashino and the staff at
half of the Lyα emission could be absorbed in the case of both the Subaru and Keck Telescopes for their invaluable
LAE J1044 0123. Then we estimate a modest estimate help. We would like to thank Paul Shapiro and Renyue
of the veloc−ity dispersion,σ 2σ 44 km s−1. Given Cen for their encouraging discussion on high-z Lyα emit-
0 obs
the diameter of this object p∼robed by∼the Lyα emission, ters. We would also like to thank Zoltan Haiman and an
D 7.7h−1 kpc, we obtain the dynamical timescale of anonymous referee for their useful comments.
τ ≃ D/0.σ7 1.7 108 yr. This would give a upper
dyn 0
∼ ∼ ×
4
Table 1
A journal of imaging observations
Band Obs. Date (UT) Total Integ. Time (s) m (AB)a FWHM b(arcsec)
lim star
B 2002 February 17 1680 26.6 1.2
R 2002 February 15, 16 4800 26.2 1.4
C
I 2002 February 15, 16 3360 25.9 1.2
C
NB816 2002 February 15 - 17 36000 26.0 0.9
z′ 2002 February 15, 16 5160 25.3 1.2
aThe limiting magnitude (3σ) with a 2.8′′φ aperture.
bThe full width at half maximum of stellar objects in the final image
Table 2
Comparison with similar LAEs beyond redshift 5
Name Redshift f(Lyα)a FWHM(Lyα)b Ref.c
LAE J1044 0123 5.655 4.1 52 This paper
−
Abell 2218 a 5.576 6.2d 70e 1
∼
LAE J1044 0130 5.687 1.5 340 2
−
J123649.2+621539 5.190 3.0 280 3
aObserved Lyα flux in units of 10−17 ergs s−1 cm−2.
bCorrected FWHM of Lyα flux in units of km s−1.
c1. Ellis et al. (2001),2. Ajiki et al. (2002),and3. Dawsonet al. (2002).
dIn Ellis et al. (2001), the Lyα fluxes obtained with both Keck LRIS and
ESI are given. The Lyα flux given in this table is their average.
eObserved line width, 2.24 ˚A, is roughly estimated from Fig. 3 in Ellis et
al. (2002). After correcting the instrumental lien width, 1.25 ˚A, we obtain
the corrected FWHM 70 km s−1.
∼
5
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B R I NB816 z'
C C
24
25
B
A 26
m
27
28
4000 5000 6000 7000 8000 9000
Wavelength (Å)
Fig. 1.—Thumb-nailimagesofLAEJ1044−0123(upperpanel). Theangularsizeofthecircleineachpanelcorrespondsto8arcsec. Their
contours areshowninthemiddlepanel. Thelowerpanelshowsthespectral energydistributiononamagnitudescale.
6
Fig. 2.— The optical spectrum of LAE J1044-0123 obtained with the Keck II Echelle Spectrograph and Imager (ESI). We show the
spectrogram intheupperpanelandtheone-dimensionalspectrum extractedwithanapertureof1.2arcsec.