Stellar velocity dispersions and emission line properties of SDSS-III/BOSS galaxies

2.3 MB·English

by  D. Thomas

Mon.Not.R.Astron.Soc.000,1–14(2012) Printed27July2012 (MNLATEXstylefilev2.2)
Stellar velocity dispersions and emission line properties of
SDSS-III/BOSS galaxies
D. Thomas1,2, O. Steele1, C. Maraston1,2, J. Johansson1,3, A. Beifiori1,4, J. Pforr1,5,
2 G. Stro¨mba¨ck1, C. A. Tremonti6, D. Wake7, D. Bizyaev8, A. Bolton9, H. Brewington8,
1 J. R. Browstein9, J. Comparat10, J.-P. Kneib10, E. Malanushenko8, V. Malanushenko8,
0
2 D. Oravetz8, K. Pan8, J. K. Parejko7, D. P. Schneider11,12, A. Shelden8, A. Simmons8,

ul S. Snedden8, M. Tanaka13, B. A. Weaver14, R. Yan14
J
1InstituteofCosmologyandGravitation,UniversityofPortsmouth,DennisSciamaBuilding,BurnabyRoad,Portsmouth,PO13FX,UK
5
2SEPnet,SouthEastPhysicsNetwork,(www.sepnet.ac.uk)
2
3Max-Planck-Institutfu¨rAstrophysik,D-85748Garching,Germany

4Max-Planck-Institutfu¨rextraterrestrischePhysik,Giessenbachstrae,D-85748Garching,Germany
]
O 5NOAO,950NorthCherryAve.,Tucson,AZ85719,USA
6DepartmentofAstronomy,UniversityofWisconsin-Madison,1150UniversityAve,Madison,WI53706,USA
C 7YaleCenterforAstronomyandAstrophysics,YaleUniversity,NewHaven,CT,USA
. 8ApachePointObservatory,P.O.Box59,Sunspot,NM88349-0059,USA
h
9DepartmentofPhysicsandAstronomy,UniversityofUtah,115South1400East,SaltLakeCity,UT84112,USA
p
10LaboratoiredAstrophysiquedeMarseille,CNRS/Aix-MarseilleUniversite´,38rueFre´de´ricJoliot-Curie,13388,Marseillecedex13,France
-
o 11DepartmentofAstronomyandAstrophysics,ThePennsylvaniaStateUniversity,UniversityPark,PA16802
r 12InstituteforGravitationandtheCosmos,ThePennsylvaniaStateUniversity,UniversityPark,PA16802
t 13InstituteforthePhysicsandMathematicsoftheUniverse,TheUniversityofTokyo,5-1-5Kashiwanoha,Kashiwa-shi,Chiba277-8583,Japan
s
a 14CenterforCosmologyandParticlePhysics,DepartmentofPhysics,NewYorkUniversity,NewYork,NY,10003
[

1
Accepted...Received...;inoriginalform20July2012
v
5
1
ABSTRACT
1
We perform a spectroscopic analysis of 492,450 galaxy spectra from the first two years of
6
. observationsoftheSloanDigitalSkySurvey-III/BaryonicOscillationSpectroscopicSurvey
7
(BOSS)collaboration.ThisdatasetisreleasedintheninthSDSSdatareleaseinJuly2012,
0
thefirstpublicdatareleaseofBOSSspectra.BOSSgalaxieshaveredshiftsuptoz ∼0.7.We
2
showthatthetypicalsignal-to-noiseratioofBOSSspectra,despitebeinglow,issufficientto
1
measurestellarvelocitydispersionandemissionlinefluxesforindividualobjects.Weverify
:
v thereliabilityofvelocitydispersionmeasurementsonindividualBOSSspectrathroughcom-
i parisonwithhighsignal-to-noisespectrafromrepeat-plateobservationsinBOSS.Weshow
X
thatthetypicalvelocitydispersionofaBOSSgalaxyis∼240kms−1.Thetypicalerrorinthe
r
a velocitydispersionmeasurement14percent,and93percentofBOSSgalaxieshavevelocity
dispersions with an accuracy of better than 30 per cent. The distribution in velocity disper-
sionisredshiftindependentbetweenredshifts0.15and0.7,whichreflectsthesurveydesign
targeting massive galaxies with an approximately uniform mass distribution in this redshift
interval.WeshowthatemissionlinescanbemeasuredonBOSSspectra.However,themajor-
ityofBOSSgalaxieslackdetectableemissionlines,asistobeexpectedbecauseofthetarget
selectiondesigntowardmassivegalaxies.Weanalysetheemissionlinepropertiesforasub-
samplebelowz =0.45andpresenttheclassicaldiagnosticdiagramswiththeemissionlines
Hβ,[OIII],Hα,and[NII](detectedin5percentofthegalaxies)toseparatestar-formingob-
jectsandAGN.Forthissubsetweshowthattheemissionlinepropertiesarestronglyredshift
dependentandthatthereisaclearcorrelationbetweenobservedframecoloursandemission
lineproperties.Ingeneral,thefractionofstarforminggalaxiesdecreasesandthefractionof
AGNincreaseswithincreasingredshift,mostlyowingtoselectioneffects.Withininthelow-
z sample (LOWZ), the majority of emission-line galaxies have some AGN component, the
fractionofpurelystarforminggalaxiesatz >0.15onlybeingafewpercent.Thefractionof
star-forminggalaxiesamongtheemission-linegalaxieswithinthehigh-z sample(CMASS),
instead, is ∼ 20 per cent. We show that these objects typically have blue observed g − r
coloursandarewellseparatedintheg−rvsr−itargetselectiondiagram.
Key words: surveys – galaxies: general – galaxies: evolution – galaxies: kinematics and
dynamics–galaxies:ISM–galaxies:activeevolution
(cid:13)c 2012RAS
2 Thomasetal.
1 INTRODUCTION detail in Padmanabhan et al. (2012), we refer the reader to these
papers for a comprehensive description. Here we summarise the
Within the framework of hierarchical galaxy formation theory
major aspects of galaxy target selection that are relevant for the
(White&Rees1978;Frenketal.1985),theformationandevolu-
presentpaper.TargetsareselectedfromSDSSimaging(Fukugita
tionofgalaxiesisdrivenbytheinterplaybetweendarkmatterand
et al. 1996; Gunn et al. 1998) from the eighth data release (DR8
baryonphysicsinvolvingcomplexprocessessuchasgasaccretion,
Aihara et al. 2011). Galaxy target selection is based on the two
star formation, black hole accretion, chemical enrichment, galac-
colours g − r and r − i, which have been shown to provide a
tic winds etc. Progress in our understanding of galaxy formation
powerfulcolour-colourspacetoselectluminousgalaxiesuptored-
cruciallyreliesontheobservationalconstraintsthatcanbeseton
shiftz ∼0.7intheconstructionoftheSDSSluminousredgalax-
theoretical models. Traditionally, this can be done either through
ies (LRG) sample (Eisenstein et al. 2001) and in the 2dF-SDSS
detailed studies of local galaxies (e.g. Kuntschner 2000; Davies
LRGandQSOsurvey(Cannonetal.2006,2SLAQ).BOSSreaches
et al. 2001; Thomas et al. 2005; Kormendy & Kennicutt 2004;
significantlydeeperthantheSDSSLRGsampleandsignificantly
Kuntschner et al. 2010; Kormendy et al. 2009, 2010), or through
widerthan2SLAQ.
theanalysisofremotegalaxiesatlargedistancesusingtheuniverse
The galaxy sample is split in a high redshift sample called
asatimemachine(e.g.Bundyetal.2006;Cimatti&others.2004;
CMASS and a low redshift sample LOWZ. The limiting appar-
Genzeletal.2003).
ent magnitude for galaxy targets is i = 19.9 mag AB (Oke &
Recentlargegalaxysurveyshaveopenedanewavenuetoap-
Gunn1983)intheCMASSandr =19.6magintheLOWZsam-
proachthisproblem,byprovidinglargedatasetsthatallowstatis-
ples.WhiletheSDSSMAINsampleispurelymagnitudelimited,
ticalstudiesoflargegalaxysamplesinthenearbyuniverse.Inthe
CMASSandLOWZemploycolourcutsinordertotargethigher-
lastdecadeanoverwhelmingnumberofstudiesbasedondatafrom
z samples without being overwhelmed by low-z contamination.
theSloanDigitalSkySurvey(Yorketal.2000,SDSS)haveledto
Thesecutsaredesignedtotargetmassivepassivelyevolvinggalax-
significantprogressinourunderstandingofthelocalgalaxypopu-
ies,butalsoincludesomestar-forminggalaxiesasexploredinthe
lation(e.g.Blantonetal.2003;Kauffmannetal.2003a,b;Tremonti
presentpaper.Ther−icolourservesasapowerfulandsimplered-
etal.2004;Brinchmannetal.2004;Wakeetal.2006;Panteretal.
shiftestimator.Atredshiftz ∼ 0.4the4000 A˚ breakmovesfrom
2007;Schawinskietal.2007;Thomasetal.2010).TheBaryonOs-
thegintother-band,whichresultsinasteepriseofr−iasafunc-
cillationSpectroscopicSurvey(hereafterBOSS),partoftheSloan
tionofredshiftbeyondz=0.4.Thetwocuts,LOWZandCMASS,
DigitalSkySurveyIIIcollaboration(Eisensteinetal.2011,SDSS-
reflectthisdividinglineatz =0.4.Themodeltrajectoryofapas-
III), will now extend this database to higher redshifts obtaining
sively evolving massive galaxy with M ∼ 1011 M (Maraston
spectra of 1.5 million luminous galaxies up to redshifts z ∼ 0.7 (cid:12)
et al. 2009b) shows that this galaxy class hits the detection limit
by2014.Thiswillallowlarge-scalestatisticalstudiesofthegalaxy
of i = 19.9 mag at a redshift of z ∼ 0.7. As a consequence,
populationatanepochwhentheuniversehadonlyhalfofitscur-
theCMASSsamplecontainsmassivegalaxiesintheredshiftinter-
rentage. val0.4 (cid:54) z (cid:54) 0.7.ThenumberdensityofBOSSgalaxiesdrops
Inthispaperwepresentthespectroscopicanalysisof492,450
sharply beyond z = 0.7 (Padmanabhan et al. 2010; White et al.
galaxy spectra observed in the first 1.5 years of BOSS operation
2011).AboutonethirdoftheLOWZsampleiscoveredwithex-
coveringaboutonethirdofthetotalsurveyarea.Weintroducethe
isting SDSS-I/II spectroscopy, while most of the CMASS galaxy
spectroscopicanalysistoolsdevelopedforthispurposeandpresent
spectraareobtainedforthefirsttimewithBOSSobservations.
stellar velocity dispersions, emission line fractions, and emission
AsubsamplecalledSPARSEhasbeendefinedwithinCMASS
lineclassifications.TheaimofthisworkistocharacteriseBOSS
inodertoallowustostudygalaxiesbeyondthenominalCMASS
galaxiesandtodiscusstheirbasicspectroscopicpropertiesinthe
selectioncuts.TheSPARSEsamplecontainsgalaxieswithslightly
BOSStargetingcolour-colourandcolour-magnitudespace.
bluerr−icoloursfortargetswithani-bandmagnitudefainterthan
The paper is organised as follows. We briefly introduce the
19.5 (for details see Padmanabhan et al. 2012). Furthermore, the
BOSSprojectinSection2.Section3presentsthemethodologyand
BOSS dataset includes a sample obtained during commissioning
thecalibrationoftheanalysistool.ResultsareshowninSections4,
calledCOMMthatispartoftheCMASSsample.Thecurrentstudy
5and6.ThepaperconcludeswithSection7.
includestheSPARSEandtheCOMMsamplesaspartofCMASS.
Galaxy target selection is designed to construct a sample of
massivegalaxieswithconstantco-movingnumberdensitybetween
2 DATA redshifts 0.1 and 0.7 with a uniform mass distribution at all red-
shifts.Theanalysesofnumberdensity(Padmanabhanetal.2010;
BOSSisoneoffoursurveysoftheSDSS-IIIcollaborationusingan
White et al. 2011) and galaxy masses (Chen et al. 2012; Maras-
upgradeofthemulti-objectspectrographonthe2.5mSDSStele-
tonetal.2012)fromearlyBOSSdatashowthatthisgoalhasbeen
scope(Gunnetal.2006)tocollectspectraofgalaxiesandquasars
achieved. Both these constraints are critical for the proper mea-
over 10,000 deg2 on the sky. BOSS has started operation in Fall
surementoflarge-scalestructureasafunctionofredshiftandthe
2009andby2014itwillhaveobservedabout1.5millionluminous
cosmologysciencegoalsofBOSS(Andersonetal.2012).Theuni-
galaxiesuptoredshiftsz∼0.7.Acomprehensiveoverviewofthe
form mass distribution is crucial for galaxy evolution studies, as
project is presented in Eisenstein et al. (2011) and Dawson et al.
it provides a sample that is homogeneous in mass, thus minimis-
(2012).
ingthemassbiaswhenstudyinggalaxypropertiesasafunctionof
redshift.
2.1 Targetselection
2.2 Spectroscopy
Galaxiesandquasarsaredealtwithseparatelyinthetargetselection
forBOSSspectroscopy.QuasarselectionisdescribedinRossetal. BOSS multi-object spectroscopy is being performed with an up-
(2012),whilethegalaxytargetselectionalgorithmisexplainedin gradeoftheSDSSfibre-fedspectrograph.Thehardwareupgrade
(cid:13)c 2012RAS,MNRAS000,1–14
SpectroscopicpropertiesofSDSS-III/BOSSgalaxies 3
is introduced in Bhardwaj et al. (2010), Roe et al. (2010), and areevaluatedbypPXF.Thesubtractionoftheemissionlinespec-
Smeeetal.(2012),whiledetailsaboutthespectroscopicobserva- trumfromtheobservedoneproducesacleanabsorptionlinespec-
tions, data reduction, and the spectroscopic pipeline are provided trumfreefromemissionlinecontamination.Thisspectrumwillbe
inSchlegeletal.(2012)andBoltonetal.(2012).Themostimpor- usedforanyfurtheranalysisofabsorptionfeatures(Thomasetal.
tantfeaturesoftheupgradeincludetheincreaseofthenumberof 2012).Thefitsaccountfortheimpactofdiffusedustinthegalaxy
fibresto1,000perplate,thedecreaseoffibrediameterto2”,and onthespectralshapeadoptingaCalzetti(2001)obscurationcurve.
theemploymentofanewCCDwithextendedwavelengthcoverage Thecodefurtherdeterminesthekinematicsofthegas(velocityand
tospan3600to10,000 A˚ andmuchimprovedthroughput.Large velocitydispersion)andmeasuresemissionlinefluxesandequiva-
format VPH gratings and thick, fully-depleted CCDs are special lentwidths(EWs)ontheresultingGaussianemissionlinetemplate.
features of the upgrade (Roe et al. 2010). The SDSS pipeline is
used for data reduction and simple measurements. Standard stars
aretargetedoneachplate,sothattheBOSSspectrahavebeenflux- 3.1 Stellarpopulationtemplates
calibrated.
Thestellarpopulationtemplateisacriticalingredientinthispro-
Theupgradeimpliesthatspectraofreasonablesignal-to-noise
cedure.WeadoptthenewstellarpopulationmodelsfromMaraston
ratiocanbeobtainedin1-hourexposuresdowntothelimitingmag-
&Stro¨mba¨ck(2011)basedontheMILESstellarlibrary(Sa´nchez-
nitudeinthei-bandof19.9mag.Thetypicalsignal-to-noiseratios
Bla´zquez et al. 2006). The wavelength range covered by these
ofaBOSSgalaxyspectrumareabout5perA˚,whichissufficient
templates is 3,500 − 7,500 A˚. As this blue limit corresponds
foranaccurateredshiftdetermination.Infactthedesiredredshift
to a relatively red rest-frame wavelength of ∼ 5,200 A˚ around
efficiencyof95percenthasbeenaccomplished(Rossetal.2011;
z ∼ 0.5, Maraston & Stro¨mba¨ck (2011) have augmented their
Whiteetal.2011;Mastersetal.2011).Thequalityofthespectrais
templates with theoretical spectra at wavelengths λ < 3,500 A˚
sufficientforthedetectionofemissionlinesandthemeasurement
usingthemodelbyMarastonetal.(2009a)basedonthetheoreti-
ofsimpledynamicalquantitiessuchasstellarvelocitydispersion,
callibraryUVBLUE(Rodr´ıguez-Merinoetal.2005).Weusethis
which are subject of the present paper. The spectral resolution is
hybridmodelinthepresentstudy.
R∼2,000,hencehighenoughforanaccuratederivationofthese
Aproperstellarpopulationfitrequiresalargesetoftemplates
basicquantities.
spanning a wide range in the parameter space of formation ages,
The typical signal-to-noise ratio clearly does not allow for
starformationhistories,metallicities,stellarinitialmassfunctions
more detailed spectroscopic studies of individual objects. How-
and dust attenuations (Maraston et al. 2006; Tojeiro et al. 2009;
ever,veryhighsignal-to-noiseratiospectracanbeobtainedthrough
Marastonetal.2010).Suchananalysisiscomputationallyexpen-
stacking(Thomasetal.2012;Munaetal.2011).Thisapproachis
sive. However, for the purpose of this work, we merely need the
necessaryforabsorptionlinemeasurementsandstudiesofelement
stellarpopulationfitstoseparatetheabsorptionspectrumfromthe
abundance ratios and other stellar population parameters, which
emissionspectrum,andtoperformasimpleanalysisofstellarkine-
willbepresentedinfuturework(Thomasetal.2012).
matics.Tothisenditismostcriticaltoobtainagoodrepresenta-
tionoftheabsorptionspectrum.Henceinthiscasewecanbenefit
from the age-metallicity degeneracy, and exclude redundant age-
2.3 Thepresentsample
metallicitycombinationsandcomplexstarformationhistoriesfrom
BOSS began in December 2009, and has taken 534,474 galaxy ourtemplates.Thefinaltemplatesetweuseconsistsofsimplestel-
spectraintotalbyJuly2011,observing813platesover3,275deg2 lar populations with fixed solar metallicity and a Salpeter (1955)
onthesky,whichcorrespondstoaboutonethirdofthefinalsurvey initialmassfunction,butspanningawiderangeinagesfrom1Myr
footprint.Wewillusethisdatasetinthepresentwork,whichcor- to15Gyr.Wehaveverifiedthatthequantitiesderivedherearenot
respondstothedatavolumethatwillbereleasedintheninthSDSS affectedbythischoice,whilethegainincomputingtimeissignifi-
datarelease(DR9Ahnetal.2012),thefirstpublicdatareleaseof cant.
BOSSspectra.Weselectthoseobjectsthatareclassifiedasgalax-
iesandforwhichtheZWARNING NQSOflagissetzero(reliable
redshiftsandexcludingQSOs)resultingin492,450uniqueobjects 3.2 Stellarvelocitydispersion
(excluding repeat observations). This includes all BOSS galaxies
StellarkinematicsareevaluatedbypPXF.Theline-of-sightveloc-
fromDR9thathavebeenselectedthroughtheLOWZandCMASS
itydistributionisfitteddirectlyinpixelspace,whichisalternative
selectioncuts.TheSPARSEsampleisincludedwithintheCMASS
to but consistent with Fourier methods (e.g. Bender 1990; Rix &
selection.WedonotconsiderLOWZorCMASStargetswithspec-
White1992).
trafromSDSSI/II.
Apreciseknowledgeofthespectralresolutionoftheunderly-
ingstellarlibraryisessentialfortheaccuratederivationofstellar
velocitydispersion.Forthisreason,Beifiorietal.(2011)haveper-
formedacarefulanalysisofthespectralresolutionoftheMaras-
3 ANALYSISTOOLS
ton&Stro¨mba¨ck(2011)stellarpopulationmodels.Theyhavede-
FollowingtheapproachesofSchawinskietal.(2007)andThomas terminedresolutionsforstellarspectrafromtheMILESlibraryas
et al. (2010), we use the publicly available codes pPXF (Cappel- wellastheMaraston&Stro¨mba¨ck(2011)stellarpopulationmod-
lari&Emsellem2004)andGANDALFv1.5(Sarzietal.2006)to elsthroughtemplatefitswithindependent,high-resolutionempir-
calculatestellarkinematicsandtoderiveemissionlineproperties. icalandtheoreticalspectra.ThespectralresolutionoftheMILES
Werefertothosepapersformoreinformation.Inbrief,GANDALF librarycorrespondstoawavelengthindependentdispersion∆λof
fitsstellarpopulationandGaussianemissionlinetemplatestothe 2.54 A˚ (FWHM). This is slightly larger than the ∆λ = 2.3 A˚
galaxyspectrumsimultaneouslytoseparatestellarcontinuumand indicated in Sa´nchez-Bla´zquez et al. (2006) resulting in a lower
absorptionlinesfromtheionizedgasemission.Stellarkinematics spectralresolution.Thisnewresolutionvaluehasalsobeenfound
(cid:13)c 2012RAS,MNRAS000,1–14
4 Thomasetal.
120 Table1.Emissionlinesmeasured.
40
Index Name λ(A˚) free/tied
7) 20 1 HeII 3203.15 free
R
D 2 [NeV] 3345.81 free
S 3 [NeV] 3425.81 free
S
D 0 60 4 [OII] 3726.03 free
S
ort - 56 [N[OeIIII]I] 33782688..7639 ffrreeee
P
σ ( -20 7 [NeIII] 3967.40 free
8 H5 3889.05 free
9 H(cid:15) 3970.07 free
-40 10 Hδ 4101.73 free
0 11 Hγ 4340.46 free
0 100 200 300 400 12 [OIII] 4363.15 free
σ (SDSS DR7) 13 HeII 4685.74 free
14 [ArIV] 4711.30 free
Figure1.VelocitydispersionsmeasuredinthisworkforasubsetofSDSS
15 [ArIV] 4740.10 free
galaxiesincomparisonwiththemeasurementspublishedintheDataRe-
16 Hβ 4861.32 free
lease7.Thereisgoodagreementbetweenthemeasurements.Themedian
17 [OIII] 4958.83 tiedto18
offsetinσis2kms−1withadispersionof30kms−1.
18 [OIII] 5006.77 free
19 [NI] 5197.90 free
20 [NI] 5200.39 free
byFalco´n-Barrosoetal.(2011)andPrugnieletal.(2011)ininde-
21 HeI 5875.60 free
pendentre-analysesoftheMILESlibrary. 22 [OI] 6300.20 free
WehaveusedtheresolutionmeasurementfromBeifiorietal. 23 [OI] 6363.67 free
(2011)todowngradethestellarpopulationtemplatestotheBOSS 24 [NII] 6547.96 tiedto26
resolution.Notethata∆λof2.54 A˚ correspondstoaresolution 25 Hα 6562.80 free
ofR ∼ 2,000at5,000 A˚ (or∼ 65 km s−1),whichisveryclose 26 [NII] 6583.34 free
tothenominalSDSSandBOSSresolutionsatthesewavelengths. 27 [SII] 6716.31 free
28 [SII] 6730.68 free
The correction is therefore minimal. As a result, the Maraston &
Stro¨mba¨ck(2011)templatesonlyrequireamodestdowngradingat
longerwavelengthstothecorrectBOSSresolution.
Atwavelengthsblueward∼5,000A˚ ourMILES-basedtem-
and[NII]6583,aswellastheBalmerlinesH andH .EWsare
β α
plateshaveslightlyworsespectralresolutionsthanourdata,which
calculatedastheratiobetweenlineandcontinuumfluxes.Thecon-
canaffectthederivationofstellarvelocitydispersion.However,we
tinuumfluxadoptedisthemedianoftheaveragefluxesinthetwo
onlyusetherest-framewavelengthrange4,500−6,500A˚ forthe blueandredcontinuumwindowsatadistanceof200kms−1from
derivation of stellar velocity dispersion, which is the wavelength thecentreofthelineandwithawidthof200kms−1.
rangemostsuitableforstellarkinematicsanalysesbecauseabsorp-
In case of a relatively low signal-to-noise ratio as provided
tion features are most pronounced (Bender 1990; Bender et al.
by SDSS and BOSS spectroscopy, it would generally be prefer-
1994).Therefore,thelossofspectralresolutionoftheMILESli-
able to tie the kinematics of most emission lines to stronger re-
brary relative to the BOSS resolution at λ (cid:46) 5,000 A˚ is not a
latedlinessuchasH 6563A˚ and[NII]6583A˚ (Brinchmannetal.
α
concernforthepresentwork.
2004; Sarzi et al. 2006; Schawinski et al. 2007). This is not pos-
Tocalibratetheprocedure,wehaveusedourtechniquetode-
sible for BOSS, however, because of the relatively large redshift
rivestellarvelocitydispersionsforasubsetofSDSSgalaxiesfrom
rangeprobed.Thesekeyemissionlinescannotbeobservedwithan
DataRelease7(DR7Abazajianetal.2009)andfoundgoodagree-
optical CCD for objects beyond redshift z ∼ 0.5. Velocities and
mentwiththevaluespublishedinDR7.ThemeasurementsinDR7
velocitydispersionsarethereforefreeparametersforallemission
arebasedonthedirectfittingmethod,thesameapproachadopted
lines, except for [OIII]λ4958.83 and [NII]λ6547.96 that are tied
bypPXF.Fig.1showsthiscomparison.Thevelocitydispersions
to the stronger nearby lines [OIII]λ5006.77 and [NII]λ6583.34,
areingoodagreementwithasmallmedianoffsetinσof2kms−1
respectively (see Column 4 in Table 1). We follow this approach
atadispersionof30kms−1.
independentlyofredshifttoguaranteehomogeneitywithinthefull
sample.
The GANDALF code considers two dust components: the
3.3 Emissionlinemeasurements
diffuse dust in the galaxy affecting the spectral shape and dust
EmissionlinefluxesandEWsofallemissionlinesavailableinthe inemissionlineregionsadditionallyaffectingemissionlinefluxes
rest-frame wavelength range covered are measured on the emis- andratios.ThelatterisestimatedthroughtheBalmerdecrementbe-
sion line template spectra. Table 1 summarises the list of emis- tweenH andH (whenavailable)(Veilleux&Osterbrock1987).
β α
sion lines measured, if accessible in the rest-frame spectrum. An However, the relatively low signal-to-noise ratios in the BOSS
emissionlinemeasurementismadewhentheamplitude-over-noise spectramakethemeasurementoftheBalmerdecrementhighlyun-
(AoN) ratio is larger than two, which implies that many of these certain. Cases in which the Hβ emission line is barely detected,
linesaretooweaktobedetectedinBOSS.ReasonableAoNratios theinclusionofthisseconddustcomponenttendstoyieldunrea-
can be obtained in a subsample of BOSS galaxies, however, for sonablyhighvaluesfordustextinction.Thus,wedonotconsider
someofthestrongerlinessuchastheforbiddenlines[OIII]5007, thisseconddustcomponentinthefitsandfocusonthediffusedust
(cid:13)c 2012RAS,MNRAS000,1–14
SpectroscopicpropertiesofSDSS-III/BOSSgalaxies 5
4 500 2.0 200
3 1.5
Port) Port)
ux) ( 2 250 W) ( 1.0 100
Hb Fl Hb E
log( log(
1 0.5
0 0 0.0 0
0 1 2 3 4 0.0 0.5 1.0 1.5 2.0
log(Hb Flux) (SDSS DR7) log(Hb EW) (SDSS DR7)
4 300 2.0 120
3 1.5
Port) Port)
ux) ( W) (
OIII] Fl 2 150 OIII] E 1.0 60
log([ 1 log([ 0.5
0 0 0.0 0
0 1 2 3 4 0.0 0.5 1.0 1.5 2.0
log([OIII] Flux) (SDSS DR7) log([OIII] EW) (SDSS DR7)
4 900 2.0 300
3 1.5
Port) Port)
ux) ( 2 450 W) ( 1.0 150
Ha Fl Ha E
log( log(
1 0.5
0 0 0.0 0
0 1 2 3 4 0.0 0.5 1.0 1.5 2.0
log(Ha Flux) (SDSS DR7) log(Ha EW) (SDSS DR7)
4 800 2.0 400
3 1.5
Port) Port)
NII] Flux) ( 2 400 NII] EW) ( 1.0 200
log([ 1 log([ 0.5
0 0 0.0 0
0 1 2 3 4 0.0 0.5 1.0 1.5 2.0
log([NII] Flux) (SDSS DR7) log([NII] EW) (SDSS DR7)
Figure2.EmissionlinefluxesandequivalentwidthsforHβ,[OIII],Hα,and[NII]measuredinthisworkforasubsetofSDSSgalaxiesincomparisonwith
themeasurementspublishedintheDataRelease7.Observed,nondust-corrected,valuesareused.Emissionlinefluxesandequivalentwidthsareslightly
higherinthisworkby∼0.1dex.
(cid:13)c 2012RAS,MNRAS000,1–14
6 Thomasetal.
Ang) 6
m^2/ 5 spSpec-4486-55588-152
Density (10^-17 erg/s/c 1234 z: 0.354
Flux 04
2
0
-2
4000 5000 6000 7000 8000 9000
Lambda (λ)
Ang)
m^2/ 15 spSpec-4629-55630-664
Density (10^-17 erg/s/c 1050 z: 0.267
Flux 4
2
0
-2
4000 5000 6000 7000 8000 9000
Lambda (λ)
Figure3.ExamplespectrawithandwithoutemissionlinesforBOSSgalaxiesaroundz ∼ 0.3inobserved-framewavelengthwiththebestfitspectrum
composedofthestellarpopulationandtheemissionlinetemplatesoverplotted(redline).Thebottomspectrumisthevariance,theresidualfromthefitis
plottedinthebottomsub-panel.
componentthatonlyaffectsthespectralshapeadoptingaCalzetti surementsonthelines.ThelineEWestimatesarealsoconsistent
(2001)obscurationcurve.Theemissionlinefluxesprovidedhave tobetterthan5percentinthered,andto10−15percentinthe
beencorrectedfordustextinctionobtainedinthisway.Notethat blue,againwithdispersionsconsistentwiththemeasurederrors.
wedonotcorrectforMilkyWayforegroundextinctionbeforethe
emissionlinesanalysis.Thisdoesnotaffecttheemissionlinemea-
surements,butdoesimplythattheresultingE(B-V)valuesneedto 3.4 ExamplespectrainBOSS
becorrectedforMilkyWayextinctionaposteriori.
Fig.3showstwoexamplespectraforBOSSgalaxiesaroundz ∼
WecompareemissionlinemeasurementsonasubsetofSDSS 0.3 with the best fit spectrum composed of the stellar population
DR7galaxieswiththeMPA-JHUvaluespublishedinDR8(Brinch- andtheemissionlinetemplatesoverplotted.Itcanbeseenthatthe
mann et al. 2004; Tremonti et al. 2004). Observed, not dust- code is successful at identifying true emission line features over
corrected,valuesareused.Fig.2showsthiscomparisonforemis- therelativelyhighnoiselevel.Thestellarpopulationfitsaregood
sion line fluxes and EWs of H , [OIII], H , and [NII]. It can be enough to make measurement of stellar kinematics and emission
β α
seenthatthemeasurementsgenerallyagreewellshowingtightcor- line fluxes. Results and reliability will be assessed in the follow-
relations with small scatter and only small offsets. Our measure- ingsections.Asmentionedearlier,amorecomprehensiveapproach
mentsofemissionlinefluxestendtobeslightlylargerthaninDR7 withawidersetoftemplateswillberequiredtoderivereliablestar
by∼ 0.1 dexwithadispersionof∼ 0.2 dex.OurvaluesofEWs formationhistoriesfromthespectra.Note,however,thataccurate
showasystematicoffsettoslightlylargervaluesby∼0.05dexfor derivationofstellarpopulationparametersfromindividualBOSS
[OIII],H ,[NII]withadispersionof∼0.1dexandby∼0.15dex spectra is challenging because of the modest S/N ratios, and the
α
forH withadispersionof∼ 0.2 dex.TheoffsetinH EWin- stacking of spectra to obtain better S/N ratios is advisable (Chen
β β
creasesforsmallvalues.Still,thecomparisonissatisfyingoverall, etal.2012;Thomasetal.2012).
andresidualdiscrepancieswillmostlikelybecausedbydifferences
inthetreatmentofreddening,absorptionlinecorrection,andcon-
tinuumfitting.
4 VELOCITYDISPERSION
Wenotethatthissameversionofthecodeisbeingusedwithin
Inthissectionwepresentthemeasurementsoftheline-of-sightve-
the Galaxy and Mass Assembly (GAMA) project (Driver et al.
locitydispersions.Weshowtheirerrordistributions,comparewith
2011)toderivesimplestellarkinematicsandemissionlinefluxes.
independentmeasurementsanddiscusstheirredshiftevolutionin
TheGAMAspectroscopyandacomparisonofGANDALF-based
theBOSSsample.
emission line measurements with other independent methods are
presented in Hopkins et al. (2012). In this comparison, particular
carehasbeentakeninthehomogenisationofthetreatmentofdust
4.1 Errordistribution
reddening.Theresultreinforcestheaboveconclusion.Itisshown
thatthemeasurementsofemissionlinefluxesareconsistenttobet- Thetypicalsignal-to-noiseratioineachBOSSgalaxyspectrumis
terthan5percent,withadispersionconsistentwiththeerrormea- about5A˚−1.Thisisconsiderablylowerthantheaverageprovided
(cid:13)c 2012RAS,MNRAS000,1–14
SpectroscopicpropertiesofSDSS-III/BOSSgalaxies 7
1200 1100
2.5•104
400 400
452786 35639
2.0•104 92.7% 7.3%
mber of galaxies11..05••110044 σ (Port) (km/s)230000 600 σ (Port) (km/s)230000 550
nu
5.0•103 100 100
0 0 0 0 0
0.0 0.1 0.2dσ / σ0.3 0.4 0.5 0 100σ (SDSS2 0p0ipeline) (3k0m0/s) 400 0 100 σ (C2h0e0n) (km/s3)00 400
Figure 4. Left-hand panel: Distributions of the relative error in the velocity dispersion measurements. Centre and right-hand panels: Comparison with
independentmeasurementsofvelocitydispersionsbasedontheoriginalSDSSDR7pipelinecodebyBoltonetal.(2012)andthecodebyChenetal.(2012).
25
20 Stacked Spectrum z = 0.511
15 SNR = 21.24 400
10
Flux Density (10^-17 erg/s/cm^2/AA)---05101234510123451012345345 sSsSsSsSppppNNNNSSSSRRRRpppp eeee====cccc ----65543333 ....66661569111496305557----55555555124170489851----777777775535 vidual median - stack) (km/s) -2200000
-101245 spSpec-3 647-55241-775 σ (indi
23 SNR = 5.43 -400
1
0 0 100 200 300 400
4-5100 5000 5500 6000 6500 7000 σ (median of individuals in stack) (km/s)
LLLLLaaaaammmmmbbbbbdddddaaaaa (((((AAAAAAAAAA)))))
Figure5.IndividualspectraandtheirstackofasingleBOSSgalaxywith Figure6.MedianvelocitydispersionmeasuredonindividualBOSSspec-
repeated1-hourobservations.TheS/Nratioperresolutionelementisgiven traversusthedifferencebetweenmeasurementsonindividualandstacked
bythelabels.TheS/Nratioofthestackedspectrumishigherbyabouta spectra.Eachstackedspectrumisthesumofindividualspectraofthesame
factorfourcomparedtotheindividualspectra. objectfromrepeatedplateobservations.SeeFig.5foranexample.Theblue
symbolsarethemedianinbinsofvelocitydispersion.Theagreementisvery
good,hencevelocitydispersionmeasurementsarereliableevenattypical
BOSSσ.Theoffsetatthehigh-σendisatthetailofthedistributionand
bySDSS-IspectroscopyowingtothefaintnessoftheBOSStargets.
onlyaffectsasmallfractionofBOSSgalaxies(seeFig.reffig:sigmared).
Despitethislimitation,measurementsofstellarvelocitydispersion
canbemade.ThisisdemonstratedbyFig.4,whereweshowthe
distribution in formal measurement errors in the left-hand panel.
4.3 ComparisonwithhigherS/Nspectra
Errorsarerelativelylarge,asexpectedfromthelowsignal-to-noise
ratios,butstillacceptable.Thereisastrongpeakatatypicalrelative Additionallytothecomparisonwithindependentmeasurementswe
errorof14percent,and93percentoftheobjectshavearelative investigatedthedependenceonS/Nratio.Tothisendwemadeuse
errorbelow30percent. ofrepeatedplateobservationsinthecourseofthelasttwoyears.
Weconstructedasampleof574BOSSgalaxiesmainlyfromplates
3615and3647withatleastsix1-hourobservationseach.Wethen
stacked the spectra from the individual exposures to a high S/N
4.2 Comparisonwithindependentmeasurements
spectrumforeachobject.Fig.5showstheindividualspectraand
Totestthereliabilityofthesemeasurementswecompareourve- thefinalstack.Itcanbeseenthattheresultingspectrumhasasig-
locitydispersionswithalternativemeasurements.Inthecentreand nificantly higher S/N ratio. The latter increases by about a factor
right-handpanelsofFig.4wecompareourresultswithtwoinde- fourfrom∼ 5to∼ 20perresolutionelement.Itisimportantto
pendent measurements, one from Shu et al. (2011); Bolton et al. notethatthisstackedspectrumrepresentsthestackforoneindivid-
(2012) based on the original SDSS pipeline and the other from ualobject.
Chenetal.(2012).Thevelocitydispersionsareslightlyhigherby Finally, we measured stellar velocity dispersion on both the
∼ 10 km s−1 than the ones of Bolton et al. (2012) at a disper- individualandthestackedspectra.Theresultofthisexerciseispre-
sionof20 kms−1 andslightlysmallerby∼ 10 kms−1 thanthe sentedinFig.6,whereweplotthemediandifferencebetweenthe
onesofChenetal.(2012)atadispersionof20kms−1.Boththese measurementsonindividualandstackedspectraasafunctionofthe
offsetsareacceptablewithinthemeasurementerror.Thisresultis measurementsonindividualspectra.Theagreementisverygood.
encouragingandunderlinesthereliabilityofstellarvelocitydisper- Thereisnosystematicoffsetbetweenmeasurementsonindividual
sionmeasurementsontheBOSSspectradespitetherelativelylarge (lowS/N)andstacked(highS/N)spectra.Thereissomeincrease
errorandlowsignal-to-noiseratio. atthehigh-σend,suggestingthatwetendtooverestimatevelocity
(cid:13)c 2012RAS,MNRAS000,1–14
8 Thomasetal.
1.0 1.0 1.0
0.8 0.00 < z < 0.15 0.8 0.15 < z < 0.30 0.8 0.30 < z < 0.45
7236 objects 35146 objects 86569 objects
0.6 0.6 0.6
fraction0.4 fraction0.4 fraction0.4
0.2 0.2 0.2
0.0 0.0 0.0
0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500
σ (km/s) σ (km/s) σ (km/s)
1.0 1.0 1.0
0.8 0.45 < z < 0.60 0.8 0.60 < z < 0.75 0.8 0.75 < z < 2.00
242943 objects 77062 objects 3829 objects
0.6 0.6 0.6
fraction0.4 fraction0.4 fraction0.4
0.2 0.2 0.2
0.0 0.0 0.0
0 100 200 300 400 500 0 100 200 300 400 500 0 100 200 300 400 500
σ (km/s) σ (km/s) σ (km/s)
Figure7.Distributioninvelocitydispersionforspectrawithanerroroflessthan30percentintheredshiftintervals0 < z (cid:54) 0.15,0.15 < z (cid:54) 0.3,
0.3<z(cid:54)0.45,0.45<z(cid:54)0.6,0.6<z(cid:54)0.75,andz>0.75.Theredlineindicatesthepositionofthepeakforthefourcentralredshiftintervals.The
distributionisapproximatelyGaussianandthereisnoevolutionwithin0.15<z<0.75.Atredshiftsbelowz∼0.15andabovez∼0.75,thedistributions
areskewedandslightlyoffsettowardlowerandhighervelocitydispersions,respectively.
dispersionsabove∼ 300 kms−1.Thisonlyaffectsasmallfrac- conclusionisdrawninMarastonetal.(2012),whoshowthatthe
tionofBOSSgalaxiesinthetailofthedistribution,however(see distributionofphotometricstellarmassesisapproximatelyuniform
Fig.7).WefindnoevidenceforasystematictrendwithS/Nratio. uptoz∼0.6.InBeifiorietal.(2012)weusethesevelocitydisper-
Hence,overallthisexercisedemonstratesthatvelocitydispersion sionmeasurementsforthederivationofdynamicalgalaxymasses
measurementsonBOSSspectraarereliable(withintheirerrors)in anddiscusstheevolutionofthedynamical-to-stellarmassratiosof
spiteoftherelativelylowS/Nratios. BOSSgalaxieswithredshift.
4.4 Velocitydispersionsandtheirevolutionwithredshift
In Fig. 7 we show the distributions of velocity dispersions with
anerrorlessthan30percent,correspondingto93percentofthe
sampleintheredshiftintervals0−0.15,0.15−0.3,0.3−0.45,
5 EMISSIONLINEPROPERTIES
0.45−0.6,0.6−0.75,andforz>0.7.Theredlineindicatesthe
positionofthepeakforthefourcentralredshiftintervals.Ingen- InthissectionwepresenttheemissionlinepropertiesofourBOSS
eral,thedistributioninvelocitydispersionisapproximatelyGaus- galaxy sample. The aim is to discuss the prevalence of various
sianwithawellpronouncedpeakatσ=240km/s.Thisrelatively galaxytypesandemissionlineclassesintheBOSStargetselection
high value is consistent with the goal of the BOSS galaxy target algorithm.Therefore,inthepresentanalysiswefocusongalaxies
selectionalgorithmdesignedtotargetmassivegalaxies(Padman- withredshiftsz<0.45,sothatthefullsetofkeydiagnosticemis-
abhanetal.2010,2012;Marastonetal.2012). sionlinesHβ,[OIII],Hα,and[NII]canbemeasuredintheBOSS
Fig. 7 further shows that the distribution in velocity disper- spectrum.Thissamplecontains111,476galaxyspectraoutofthe
sion is largely independent of redshift within 0.15 < z < 0.75. 492,450analysedinthefullredshiftrange.Itshouldbenotedthat
Thepositionofthepeakofthedistributiondoesnotchange,while in this way we deselect a large fraction of CMASS galaxies, the
thewidthbecomessomewhatlargerathigherredshiftscausedby redshift distribution of which peaks at z ∼ 0.6. The analysis of
slightly larger measurement errors (see also Shu et al. 2011). At AGNfractionsatthesehigherredshiftsrequiresalternativemeth-
redshifts below z ∼ 0.15 and above z ∼ 0.75, the distributions odsfortheidentificationofAGN(Trouilleetal.2011),whichgoes
are skewed and slightly offset toward lower and higher velocity beyondthescopeofthepresentpaper.
dispersions,respectively.Thisuniformityofthedistributioninve- Wefurtherrestrictouranalysistothosespectrawherethefull
locity dispersion as a function of redshift between 0.15 and 0.75 setofkeydiagnosticemissionlinesH ,[OIII],H ,and[NII]can
β α
is an important feature of the BOSS target selection, as it estab- bedetectedwithanAoNabove2toallowforaproperanalysisof
lishesadirectlinkbetweenthegalaxysamplesatvariousredshift, theemissionlinecharacteristicsthroughemissionlineratiodiag-
whichallowsustoaccuratelyprobetheredshiftevolutionofmas- nosticdiagrams(Baldwinetal.1981).Thisresultsinasampleof
sivegalaxies.TargetselectioninBOSShasbeendesignedtothis 4,237spectra(3.8percentofthetotal).Thefractionsofgalaxies
purpose and, using stellar velocity as proxy for galaxy mass, the withsignificantdetectionsoftheemissionlinesH and[NII]are
α
present results shows that this goal has been achieved. A similar considerablyhigher(around30percent).
(cid:13)c 2012RAS,MNRAS000,1–14
SpectroscopicpropertiesofSDSS-III/BOSSgalaxies 9
2 2 2
CMASS Seyfert CMASS Seyfert CMASS Seyfert
0.00 < z ≤ 0.15 1.21% 0.15 < z ≤ 0.30 3.53% 0.30 < z ≤ 0.45 59.37%
1 1 1
βH) βH) βH)
OIII]/ 0 OIII]/ 0 OIII]/ 0
log([ log([ log([
Star Forming LINER Star Forming LINER Star Forming LINER
-1 95.77% 0.40% -1 77.34% 1.63% -1 21.89% 6.84%
Composite Composite Composite
2.62% 17.50% 11.89%
% AoN>2: 6.03 % AoN>2: 1.97 % AoN>2: 1.06
-2 -2 -2
-2 -1 0 1 -2 -1 0 1 -2 -1 0 1
log([NII]/Hα) log([NII]/Hα) log([NII]/Hα)
2 2 2
LOWZ Seyfert LOWZ Seyfert LOWZ Seyfert
0.00 < z ≤ 0.15 9.49% 0.15 < z ≤ 0.30 16.04% 0.30 < z ≤ 0.45 39.50%
1 1 1
βH) βH) βH)
OIII]/ 0 OIII]/ 0 OIII]/ 0
log([ log([ log([
Star Forming LINER Star Forming LINER Star Forming LINER
-1 19.22% 29.26% -1 4.74% 46.39% -1 3.76% 31.66%
Composite Composite Composite
42.02% 32.83% 25.08%
% AoN>2: 15.60 % AoN>2: 2.49 % AoN>2: 0.36
-2 -2 -2
-2 -1 0 1 -2 -1 0 1 -2 -1 0 1
log([NII]/Hα) log([NII]/Hα) log([NII]/Hα)
Figure8.Emissionlineclassification(Baldwinetal.1981)forBOSSgalaxiesatvariousredshiftbins(redshiftincreasingfromlefttoright)forCMASS
galaxies(toppanels)andLOWZgalaxies(bottompanels).Objectsatz<0.45areselectedtowarrantdetectabilityofHαand[NII].Thefractionofobjects
forwhichallfouremissionlinesaredetected(requiringtheamplitude-over-noiseratioofallfourlinestobelargerthantwo)isgiveninthebottom-leftcorner
ofeachpanel.TheempiricalseparationbetweenstarforminggalaxiesandAGN(dashedline)isfromKauffmannetal.(2003b),andthetheoreticalextreme
starburstlinefromKewleyetal.(2001)toidentifypureAGNemission(solidcurvedline).AdividinglinedefinedbySchawinskietal.(2007)isusedto
distinguishbetweenLINERandSeyfertemission(solidstraightline).Thefractionofeachgalaxiesineachoftheemissionlineclassesaregivenbythelabels
inthepanels.Overall,thefractionofobjectswithdetectedemissionlinesissmallinBOSS.Thereisamarkeddifferenceintheemissionlineproperties
betweentheBOSSgalaxysamplesathighandlowredshift,mainlycausedbyselectioneffects.Furthermore,thereisastrikingdifferencebetweentheLOWZ
andCMASSsamples(seetextfordetails).
5.1 BPTclassification Furthermore, there is a striking difference between the LOWZ
and CMASS samples. Besides the fact that the number of galax-
We use the emission lines H , [OIII], H , and [NII] to perform
β α iesselecteddecreaseswithincreasingredshiftforLOWZandin-
the standard classification based on the diagnostic diagram first
creases for CMASS owing to the design of the target selection,
introduced by Baldwin et al. (1981) and Veilleux & Osterbrock
the overall fraction of galaxies with detected emission lines (see
(1987),andwidelyusedinstudiesofSDSSgalaxies(e.g.Kauff-
label in bottom-left corner in each panel) is different for LOWZ
mannetal.2003b;Milleretal.2003;Hopkinsetal.2003;Brinch-
andCMASSandchangeswithredshift.Ingeneral,thefractionof
mannetal.2004;Kewleyetal.2006;Schawinskietal.2007;Wild
starforminggalaxiesamongBOSSgalaxiescanbeexpectedtobe
etal.2007;Stasin´skaetal.2008).Inthismethodtheemissionlines
low,independentlyofredshift,becauseofthebiastowardsmassive
ratios[OIII]/H and[NII]/H areusedasindicatorsfortheionisa-
β α galaxiesatallredshifts.
tionsourceoftheinterstellargasingalaxies.Thediagramhasbeen
proventobeverypowerfulatseparatingstarforminggalaxiesfrom
objectswithAGN.
5.1.1 LOWZ
ThevariouspanelsofFig.8showtheBPTclassificationsfor
the 4,213 BOSS galaxies in three redshift bins, with redshift in- Not surprisingly, the largest fraction of BOSS galaxies with de-
creasing from left to right (see labels in the panels). We adopt tectableemissionlines(15percent)isfoundintheLOWZsample
the empirical separation between star forming galaxies and AGN atredshiftsbelow0.15.Thisshouldbeexpectedasthecontamina-
(dashedline)asdefinedbyKauffmannetal.(2003b),andthethe- tionwithlower-massgalaxiesislargest(seeFig.7).Themajority
oreticalextremestarburstlinebyKewleyetal.(2001)toidentify ofemission-linegalaxieshassomeAGNcomponent,thefraction
pureAGNemission(solidcurvedline).Asiscommonlydone,we ofpurelystarforminggalaxiesonlybeing20percent.Thefraction
assume that the area between these two separating lines is popu- of galaxies with detected emission lines drops dramatically with
latedbygalaxieswithacompositeofstarburstandAGNspectra. increasingredshifttoonlyafewpercent.TheprevalenceofAGN
WefurtherusethedividinglinedefinedbySchawinskietal.(2007) andLINER-likeemissionincreasesfurtherwithincreasingredshift
todistinguishbetweenLINERandSeyfertemission(solidstraight above0.15.
line) based on SDSS galaxy classifications obtained through the WehavealsoexaminedthepresenceofBOSSemission-line
[SII]/Hαratio. galaxiesthatarelikelytobeionisedthrougholdstellarpopulations
It can be seen that, in contrast to the distribution in galaxy rather than AGN. Following Cid Fernandes et al. (2011), we as-
massesandstellarvelocitydispersion,thereisamarkeddifference sumethatsuchgalaxiescanbecharacterisedbyhavingH EWs
α
in the emission line properties between the BOSS galaxy sam- below3 A˚.Thisthresholdhasbeenderivedfromthedistribution
ples at high and low redshift, mainly caused by selection effects. ofH EWsinSDSSgalaxies,whichisstronglybimodal(seealso
α
(cid:13)c 2012RAS,MNRAS000,1–14
10 Thomasetal.
Bamfordetal.2008),withtwopeaksat1and16A˚ andaninterme-
diateminimumintheneighbourhoodof3 A˚ (CidFernandesetal. 1.2
2011).Wefindthattheseso-called’retired’galaxiestypicallyhave
LINERemission,butonlyaminority(aboutonequarter)ofgalax- 1.0 0.00 < z ≤ 0.15
nt 0.15 < z ≤ 0.30
ieswithLINERemissionlinecharacteristicsisfoundtobebetter u
describedbythisobjectclass. y co 0.8 0.30 < z ≤ 0.45
x
a
al
g 0.6
d
e
s
5.1.2 CMASS ali 0.4
m
Interestingly,therearesomeCMASSgalaxiesatlowredshiftsbe- nor
low 0.15, even though the selection cuts are designed to target 0.2
galaxiesabovez∼0.4.Fig.8showsthatmostoftheseobjectsare
0.0
starforminggalaxiesthatfellintotheCMASScolourselectioncut
7.5 8.0 8.5 9.0 9.5
despitebeingatlowredshift,mostprobablyduetodustreddening. metallicity
These galaxies are also relatively low-mass with velocity disper-
sions at the low-end of the distribution. With increasing redshift,
Figure 9. Gas metallicities for star forming BOSS galaxies in various
thispopulationprogressivelydisappearsfromtheCMASSsample. redshiftbins.Theverticaldashedlineindicatesthesolarvalue.Thelow
Atredshiftsabove0.3,closertotheactualredshiftrangetar- andhigh-zsamplesshowsimilarmetallicitydistributions.Thelow-redshift
getedbytheCMASSselectioncuts,the(veryfew)galaxieswith sample extends to a low-metallicity wing mainly composed of low-z
detectedemissionlinesaremostlyAGN.Interestingly,themajor- CMASSgalaxies.
ity of these AGN are Seyfert. It should be emphasised, however,
thatAGNfractionsderivedthroughopticalemissionlineratiosare
criticallydependentonS/Nratios.InparticularLINERs,typically
havingloweremissionlinefluxes,tendtodropoutofthesample
astheS/Ndecreases.Hence,therelativelylowfractionofBOSS 6.1 The(g−r)vs(r−i)diagram
galaxieswithLINERemissionathighredshiftsismostlikelyase-
lection effect, and the effect of S/N ratio needs to be considered In Fig. 10 we present the colour-colour plot g − r vs r − i of
carefullyinscientificanalyses. all BOSS galaxies with z < 0.45. Galactic extinction-corrected
modelmagsareused.Thesolidlinelabelledd indicatesthemajor
⊥
colourselectioncut
5.2 Gasmetallicities d⊥ ≡(r−i)−(g−r)/8=0.55 (1)
Following Saintonge et al. (2011), we use the prescription from
that has been used to separate the high-redshift CMASS and
Pettini&Pagel(2004)toderivegasmetallicitiesofthestarform-
the low-redshift LOWZ samples (Padmanabhan et al. 2012). The
inggalaxiesfromthe[OIII]/H and[NII]/H emissionlineratios.
β α colour r−i is an excellent redshift indicator aroundredshifts of
We caution here, however, that metallicity estimates made using z = 0.4, because the 4000A˚ break passes from the g-band into
differentlineratiosshowsomesystematicdifferences(Kewley&
ther-band.Asaconsequence,galaxiesbecomesignificantlyred-
Ellison 2008). The resulting distributions of gas metallicities for
der in r −i and slightly bluer in g −r with increasing redshift
various redshift bins are shown in Fig. 9. As is to be expected,
beyondaredshiftofz = 0.4(Eisensteinetal.2001).Thispattern
metallicitiesarehighandpeakaroundsolarvalues(dashedline).
isdisplayedbytheBOSSgalaxydatainFig.10.Galaxiesabove
Thereisnonotabledifferencebetweentheredshiftbins0.15−0.3 thesolidline(d (cid:62)0.55)definetheCMASSsampleandtypically
⊥
and0.13−0.45.Thelow-redshiftsamplebelowredshift0.15,in- haveredshiftsz (cid:38) 0.4,whileeverythingbelowthatlineisinthe
stead,extendstoalow-metallicitywingmainlycomposedoflow-z LOWZ sample with typical redshifts of z (cid:46) 0.4. The solid lines
CMASSgalaxies.Thisextensiontolowmetallicitiesisalsovisible
labelled’LRG’and’2SLAQ’aretheselectioncutsusedbyEisen-
inFig.8throughthepresenceofstarforminggalaxieswithhigh
steinetal.(2001)todefinetheSDSSLRGsampleandbyCannon
[OIII]/H andlow[NII]/H ratiosinthelowestredshiftbin(see
β α etal.(2006)todefinethe2SLAQLRGsample,respectively.The
Fig.8).
dashed line is the dividing line between early-type and late-type
fromMastersetal.(2011).
The correlation seen in the LOWZ sample is a sequence of
varyingredshiftinwhichbothg−randr−ibecomeredderwith
increasingredshiftuptoz =0.4.Thesequenceisrelativelytight,
6 TARGETSELECTIONCOLOUR-COLOURSPACE
becauseredshifteffectsdominatetheobservedcolours.Thedistri-
WehaveshownthatasmallbutsignificantfractionofBOSSgalax- butionofCMASSgalaxiesintheg−rvsr−icolour-colourplane
ies contains emission lines, encompassing all ionisation classes isquitedifferent.Astheobservedg−r colourdependsstrongly
fromstarformingtoAGN.Itisinterestingtoinvestigate,whether onthestarformationhistoryofthegalaxy,awiderangeing−r
these emission line properties follow some distinct pattern in the coloursiscovered.Theobservedr−icolour,instead,isgenerally
colour-colour diagram used for target selection in BOSS. In the redwitharelativelysmalldynamicalrangeandmuchlessdepen-
following we focus on redshifts below z ∼ 0.45 so that we can dentonthestarformationhistory.Passivelyevolvinggalaxiespop-
benefitfromthefullBPTclassificationandseparateemission-line ulatethetopright-handsectionofthediagram,asindicatedbythe
freegalaxiesfromstarformingandAGN. LRGselectionline.
(cid:13)c 2012RAS,MNRAS000,1–14