Outflows and complex stellar kinematics in SDSS star forming galaxies

4.5 MB·

by  Claudia Cicone

Astronomy&Astrophysicsmanuscriptno.sdss_sf_v8_anda (cid:13)cESO2016
January20,2016
Outflows and complex stellar kinematics in
SDSS star forming galaxies
C.Cicone,1,2,3,R.Maiolino2,3,andA.Marconi4,5
1 InstituteforAstronomy,DepartmentofPhysics,ETHZurich,Wolfgang-Pauli-Strasse27,CH-8093Zurich,Switzerland
e-mail:[email protected]
2 CavendishLaboratory,UniversityofCambridge19J.J.ThomsonAvenue,CambridgeCB30HE,UK
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3 KavliInstituteforCosmology,UniversityofCambridge,MadingleyRoad,CambridgeCB30HA,UK
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4 DipartimentodiFisicaeAstronomia,UniversitàdeglistudidiFirenze,ViaG.Sansone1,50019,SestoFiorentino(Firenze),Italy
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n 5 INAF-OsservatorioAstrofisicodiArcetri,LargoE.Fermi5,50125,Firenze,Italy
a
J
Receivedxxxxxx/Accepted:xxxxxx
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ABSTRACT
]
A
Weinvestigatethepropertiesofstarformation-drivenoutflowsbyusingalargespectroscopicsampleof∼160,000local“normal”star
G forminggalaxies,drawnfromtheSloandigitalskysurvey(SDSS),spanningawiderangeofstarformationrates(SFRs)andstellar
masses(M ).ThegalaxysampleisdividedintoafinegridofbinsintheM −SFRparameterspace,foreachofwhichweproducea
. ∗ ∗
h compositespectrumbystackingtogethertheSDSSspectraofthegalaxiescontainedinthatbin.Weexploitthehighsignal-to-noise
p ofthestackedspectratostudytheemergenceoffaintfeaturesofopticalemissionlinesthatmaytracegalacticoutflowsandwould
- otherwisebetoofainttodetectinindividualgalaxyspectra.Weadoptanovelapproachthatreliesonthecomparisonbetweenthe
o
line-of-sight velocity distribution (LoSVD) of the ionised gas (as traced by the [OIII]λ5007 and Hα+[NII]λλ6548,6583 emission
r
t lines)andtheLoSVDofthestars,whichareusedasareferencetracingvirialmotions.Significantdeviationsofthegaskinematics
s fromthestellarkinematicsinthehighvelocitytailoftheLoSVDsareinterpretedasasignatureofoutflows.Ourresultssuggestthat
a
theincidenceofionisedoutflowsincreaseswithSFRandspecificSFR.Theoutflowvelocity(v )isfoundtocorrelatetightlywiththe
[ out
SFRforSFR>1M⊙yr−1,whereasatlowerSFRsthedependenceofvoutonSFRisnearlyflat.Theoutflowvelocity,althoughwitha
1 muchlargerscatter,appearstoincreasealsowiththestellarvelocitydispersion(σ∗),andweinfervelocitiesashighasvout ∼(6−8)σ∗.
v Strikingly,wedetectthesignatureofionisedoutflowsonlyingalaxieslocatedabovethemainsequence(MS)ofstarforminggalaxies
5 intheM∗–SFRdiagram,andtheincidenceofsuchoutflowsincreasessharplywiththeoffsetfromtheMS.Thisresultsuggeststhat
1 starformation-drivenoutflowsmayberesponsibleforshapingtheupperenvelopeoftheMSbyprovidingaself-regulatingmechanism
7 forstarformation.Finally,ourcomplementaryanalysisofthestellarkinematicsrevealsthepresenceofblueasymmetriesofafew10
4 kms−1 inthestellarLoSVDs.Theoriginofsuchasymmetriesisnotclear,butapossibilityisthatthesetracethepresence,inlocal
0 galaxiesofalargenumberofhighvelocityrunawaystarsandhypervelocitystarsinradialtrajectories.
.
1 Key words. galaxies: general – galaxies: ISM –galaxies:evolution –galaxies: stellar content – ISM:kinematicsand dynamics –
0 ISM:evolution
6
1
:1. Introduction cosmologicalinflowsofcoldgasintogalaxies(e.g.Dekeletal.
v
i 2009;Bouchéetal.2010),orbursty,triggeredbydynamicalpro-
XBaryons in galaxies are engaged in a complex intertwining of
cessessuchasgalaxyinteractions(Scoville2013).Recentstud-
mechanisms whose combined effects drive galaxy evolution.
r iessuggestedthatthelattermodeofstarformationdoesnotcon-
aKey players of this game are gas, stars and active galactic nu-
tributesignificantlytothecosmicstarformationrate(SFR)den-
clei (AGN). Gas is the primary ingredient of both star forma-
sity, accountingfor only 10% of it even at z ∼ 2, i.e. the peak
tion and black hole accretion, and its properties are therefore
of the cosmic SFR density (Rodighieroetal. 2011). Although
crucial for shaping galaxies. Star formation and AGN activity
westilllackconclusiveobservationalevidenceforthepresence
can in turn heavily influence the physical, chemical and kine-
ofcoldaccretionflowsfeedinggalaxyhalos(butseepromising
matical conditions of gas within galaxies, thereby exerting a
observationsby Cantalupoetal. 2014 and Hennawietal. 2015
“feedback” on galaxy evolution. In a simplistic way, our cur-
atz ∼ 2), the hypothesisthatthe buildingup of baryonicmass
rent knowledge of the processes occurring in galaxies and in-
is a secular process has been suggested by the observation of
volving gasand stars can be summarised as follows. The pres-
a tight correlation between stellar mass (M ) and star forma-
∗
ence of gas within galaxies is a key prerequisite for star for-
tionrateinstarforminggalaxies(Bouchéetal.2010;Lillyetal.
mation. In local galaxies, all known star formation takes place
2013),dubbedstarformation“mainsequence”(MS).TheMSis
in dense molecular clouds, mostly in giant molecular clouds
awell-establishedpropertyofthestarforminggalaxypopulation
(GMCs, see review by Scoville 2013). The star formation pro-
atz∼0(Noeskeetal.2007;Pengetal.2010)andisbelievedto
cess can be either smooth and quiescent, probably fostered by
Articlenumber,page1of27
A&Aproofs:manuscriptno.sdss_sf_v8_anda
persistatleastuptoz∼4(Schreiberetal.2015),withanintrin- iescurrentlyavailable,theSloandigitalskysurvey(SDSS).The
sicscatterofonly∼0.3dex. SDSS,inconjunctionwiththecataloguesprovidedbytheMPA-
Once star formation is activated, supernovae enrich the in- JHUanalysis,constitutesahugedatabase,whosenoteworthypo-
terstellar medium (ISM) with metals and, along with young tentialintermsofstudyingthegeneralpropertiesofgalaxiescan
and massive stars, inject energy and momentum into the ISM, befurtherexploitedbystackingmultiplespectratogethertoob-
thereby altering the physical conditions of the gas surround- tainveryhighsignal-to-noisecompositespectra(e.g.Chenetal.
ing the sites of star formation and at the same time driving 2010;Andrews&Martini2013).Thestackingtechniquehasnu-
multi-phase galactic-scale outflows that propel part of the gas merousadvantages,anditisparticularlyusefulwhensearching
out of these regions, and in some cases can even expelit from for faint spectral features, such as those impressed by galactic
thegalaxy.Theinvestigationofstellarfeedbackhastriggereda outflows,whichcanbemissedinthenoiseofsinglegalaxyspec-
wealthofboththeoreticalandobservationalworks.Fortheoret- tra.
ical work we refer to the early review by Chevalier (1977) for WedividetheM∗−SFRparameterspaceintoafinegridof
energy-drivenoutflows,toMurrayetal.(2005)formomentum- bins, each of which includes from several tens to thousandsof
driven outflows, and to Hopkinsetal. (2014) for cosmological sources. The spectra within each bin are then stacked together
zoom-in simulations exploring the combined effect of multiple to produce a single, very high signal-to-noise composite spec-
stellar feedbackmechanismson galaxyevolution.For observa- trumthatisrepresentativeofthespectrumofa“typical”galaxy
tionalworkwerefertothereviewsbyVeilleuxetal.(2005)and inthatbin,inwhichwecanstudytheemergenceoffaintbroad
Erb(2015),thelatteronefocussingonhighredshift. wingsofopticalemissionlinesthatmaytraceionisedoutflows.
Gas heating and removal resulting from intense episodes Moreover,since stellar mass and star formationrate are intrin-
of star formation is believed to play a key role in galaxy sically correlated in most star-forming galaxies (because most
evolution. For example, stellar feedback may be the main re- of them follow the MS), by stacking galaxies in bins of SFR
sponsible for: (i) the low efficiency of star formation observed andM∗ wecaninvestigatethetrendsbetweentheoutflowprop-
in both low and high mass galaxies (Behroozietal. 2010; erties and these two parameters independently, thus breaking
Papastergisetal. 2012), althoughformore massive galaxiesan theirdegeneracyresultingfromtheMS.TheSDSSdatabaseand
additionalfeedbackfromAGNsisofteninvoked(e.g.reviewsby the use of the stacking technique allow us to explore the inci-
Cattaneoetal. 2009 and Kormendy&Ho 2013); (ii) the lower dence and properties of ionised outflows over a wide dynami-
gas-phase metal content of low-mass galaxies with respect to calrangeof galaxyproperties,i.e. M∗ ∈[2×107,6×1011]M⊙
more massive galaxies (Davéetal. 2011a), and (iii) the conse- andSFR∈[2×10−3,2×102]M⊙ yr−1,whichisunprecedented
quent enrichment of the circumgalactic medium (CGM) with inoutflowstudies.
metals (MacLow&Ferrara 1999). Moreover, it has been sug- A rather controversial issue in studies of galactic outflows
gestedthatgalacticoutflowsdrivenbystarformationmayhave throughemissionlineprofilesisthedefinitionof“broadcompo-
aidedthereionizationoftheearlyUniverse,bycreatinggas-free nent”,whichissupposedtotracegasinoutflow,andof“narrow
pathsaroundyoungstarclustersthusfacilitatingtheleakageof component”,which instead traces virial motions in the galaxy.
ionising photons from the first galaxies (Heckmanetal. 2011; Suchanissuehasnotbeenaddressedinasystematicwaybypre-
Erb2015). viousstudies:mostof themjustrely onthe χ2 improvementto
Although blueshifted absorption lines at optical and UV establishthenecessityofabroadcomponenttofittheemission
wavelengths are probably the most direct and accessible lineprofile(e.g.Harrisonetal.2012;Westmoquetteetal.2012;
tool to identify outflows of ionised (and atomic) gas, es- Bellocchietal. 2013; Arribasetal. 2014). Moreover, even if a
pecially in distant galaxies, nebular emission lines such as broad componentis present, its association with galactic flows
[OIII]λ5007, Hα and [NII]λλ6548,6584 are also widely em- isnotstraightforward:abroadcomponentinemissionmaytrace
ployed to study galactic outflows, both in local star form- thehighvelocitytailofvirialisedmotionswithinthegalaxy,or
ing galaxies (e.g. Heckmanetal. 1990; Veilleuxetal. 1995; galaxy interactions. In this study we attempt to overcome this
Lehnert&Heckman1996;Sotoetal.2012;Westmoquetteetal. problembyadoptinganovelstrategy,consistingindirectlycom-
2012; Rupke&Veilleux 2013; RodríguezZaurínetal. 2013; paringtheline-of-sightvelocitydistribution(hereafter,LoSVD)
Bellocchietal. 2013; Cazzolietal. 2014; Arribasetal. 2014) of the ionised gas with the LoSVD of the stars, which is taken
and at higher redshifts (e.g. Shapiroetal. 2009; Newmanetal. asareferencetracingvirialmotions.Accordingly,gaseousout-
2012; Harrisonetal. 2012;Cano-Diazetal. 2012; Genzeletal. flowsareidentifiedbydeviationsofthegaskinematicsfromthe
2014;FörsterSchreiberetal.2014;Carnianietal.2015).How- stellarkinematics,andinparticularbythepresenceofanexcess
ever,despitetheincreasingnumberofobservationalstudiestar- ofgasvelocitywithrespecttothestarsinthehighvelocitytail
getinggalacticoutflowsandtheremarkableadvancesinthefield, oftheLoSVD.
asystematicandunbiasedinvestigationofstarformation-driven Summarising, the main goal of this work is to investigate
outflows in a large sample of galaxies is still missing. Indeed, theincidenceandpropertiesofgalacticoutflowsof ionisedgas
since the outflow signature can be very faint and difficult to asafunctionofawiderangeofgalaxyproperties,breakingthe
detect in individual normal galaxies, previous studies focused degeneracybetween SFR and M∗. Moreover,for the first time,
on rather small samples, mostly characterised by “extreme” we push outflow studies to stellar masses and SFRs as low as
properties(e.g.powerfulandmassivestarbursts,ultra-luminous 2×107M⊙and2×10−3M⊙yr−1,respectively,aregimethathas
infrared galaxies (ULIRGs)), hence not representative of the notbeenprobedbeforeinthisfield.
general galaxy population, although significant improvement AH0=70kms−1Mpc−1,ΩM=0.27,ΩΛ=0.73cosmologyis
has been recently made in this regard (e.g. Chenetal. 2010; adoptedthroughoutthispaper.
Martinetal.2012;Rubinetal.2014;Chisholmetal.2014).
In this paper we study star formation-driven outflows of
2. Methods
ionised gas as traced by faint broad wings of optical emission
lines ([OIII]λ5007, Hα and [NII]λλ6548,6584) by using the InthissectionwepresenttheSDSSsampleofstarforming,non
largestandmostunbiasedspectroscopicsample of localgalax- activegalaxiesselected forthis study(§ 2.1). We also describe
Articlenumber,page2of27
C.Cicone, etal.:OutflowsandcomplexstellarkinematicsinSDSSstarforminggalaxies
Fig.1:TheSDSSsampleofstar-forming,nonactivegalaxiesselectedforthisstudy,plottedintheM -SFRparameterspace.The
∗
gridof50binsusedforthestackingisshown,withthebinscolour-codedaccordingtothenumberofspectraenclosed.Theexact
numberofsourceswithineachbinisindicatedbytheyellowtext.ThewhitedashedanddottedlinesrepresenttheMSrelationof
localstar-forminggalaxieswithits±1σscatter(=0.3dex).TheMSrelationhasbeentakenfromPengetal.(2010)andadaptedto
thisdatasetasexplainedin§2.2.
indetailthestackingprocedure(§2.2)andthefittingtechniques gins. In the vast majority of local isolated (inactive) galax-
applied to the stellar continuum and to the [OIII]λ5007 and ies, the most plausible source of extended LINER-type excita-
Hα+[NII]λλ6548,6583nebular lines to obtain the line-of-sight tion are evolved (post-AGB) stars (CidFernandesetal. 2011;
velocity distributions of stars and ionised gas from the galaxy Yan&Blanton 2012; Singhetal. 2013; Belfioreetal. 2015).
compositespectra(§2.3-2.6). Thisisingeneralnotthecaseforstronglystarforminggalaxies
suchas(U)LIRGs,wherethereisevidenceforextendedLINER
excitationassociatedwithwidespreadshocks.Indeed,boththe-
2.1.Sampleselection
oretical models (Allenetal. 2008) and spatially resolved inte-
Ourgalaxysampleisentirelydrawnfromthespectroscopicsam- gralfield spectroscopy(IFS) observations(Monreal-Iberoetal.
ple of the Sloan digitalsky survey,data release 7 (SDSS DR7, 2006, 2010; Soto&Martin 2012; Richetal. 2014, 2015) have
Abazajianetal. 2009). We operate a first selection by includ- shownthatgalaxyregionsdominatedbyshockexcitationexhibit
ing only those galaxiesenclosed in the MPA-JHU catalogue 1, opticallineratiostypicalofLINERs.
whichprovidesintrinsicandderivedspectralparametersforeach Since extended shocks can be associated with strong
source, such as redshift, emission line fluxes, stellar mass, and outflows produced by vigorous star formation (e.g.
starformationrate(Tremontietal.2004;Kauffmannetal.2003; Sharp&Bland-Hawthorn 2010), our sample selection, by
Brinchmannetal.2004;Salimetal.2007).TheMPA-JHUanal- leaving out LINERs and Composite galaxies, may be biased
ysisencompassesonlySDSSDR7sourcesatz<0.7. against (U)LIRGs experiencing strong feedback from star
Among the MPA-JHU galaxies, we selected only sources formation.Although this is certainly a caveat to keep in mind,
classifiedas“star-forming”(or“HII”)accordingtotheirlocation the nature of the widespread shocks that may dominate the
onthelog([OIII]/Hβ)vslog([NII]/Hα)diagram(i.e.the“BPT” ionised gas excitation over extended regions in (U)LIRGs is
diagram,Baldwinetal.1981),byadoptingthesamedivisioncri- most likely linked to the merger process. This argument is
teriaasKauffmannetal.(2003).Suchselectionisneededinor- based on the lack of observational evidence for shock-like
dertoavoidcontaminationfromAGNs,whichisofprimaryim- excitation significantly affecting the optical line fluxes in
portancetoconstrainthestarformation-drivenfeedbackmecha- isolated(U)LIRGs(Monreal-Iberoetal.2010;Richetal.2015).
nismsinactioninnormallocalgalaxies. Indeed,the pointmadebyresolvedIFSstudies(Soto&Martin
We note that the BPT selection may introduce a potential 2012;Richetal.2014,2015)abouta possiblemisclassification
biasinoursampleofstarforminggalaxies.Thisisduetothefact of star forming galaxies hosting widespread shocks, when
thatpurelystarforminggalaxiesmaystillexhibitextendedareas aperture-integrated spectra are used, concerns only (U)LIRGs
wherethe opticalline ratiosare typicalof low-ionisationemis- in a merger stage. In contrast, for isolated (U)LIRGs, the
sionlineregions(LINERs).Asaconsequence,an“HII”galaxy contribution from shocks to extended optical line emission
maybeerroneouslyclassifiedas“Composite”or“LINER”when is negligible (Monreal-Iberoetal. 2010; Richetal. 2015).
its optical emission is averaged over a large aperture (such as Therefore,thetypesofshocksthatcanaffectintegratedoptical
the SDSS spectroscopic fibre). Extended LINER-like gas ex- line fluxes to the point that an “HII” galaxy is classified as
citation in purely star forming galaxies can have different ori- “Composite”or “LINER”aregalaxy-wideshocksproducedby
a complex interplay of merger-driven processes, and not just
1 Availableathttp://www.mpa-garching.mpg.de/SDSS/ by star-formation-driven outflows. For their sample of local
Articlenumber,page3of27
A&Aproofs:manuscriptno.sdss_sf_v8_anda
ULIRGs, Monreal-Iberoetal. (2006) explicitly concluded that !"#$%&’#()%*$+&
the LINER-typeopticalline ratiosobservedinthe extranuclear
regions, several kiloparsecs away from the circumnuclear
starburst, are due to shocks associated with tidally-induced ,$%#$%&-.*/001(.2&’#()%*$+&
large-scale flows produced during the merger and not to star
formation-driven“superwinds”(whichinsteadtendtodominate Fig.2:Sketchofthe“Drizzle”methodappliedtotheSDSSspec-
the gas kinematics in the inner regions of these ULIRGs that tra.
are characterised mostly by HII-type excitation). A similar
conclusion was reached by Monreal-Iberoetal. (2010) who
investigatedthe excitationsource of ionised gasin a sample of properlyonlybyusingspatially-resolvedobservations.Inorder
32localLIRGsusingIFSdata,andfoundthatthepresenceand tobeabletostudyoutflowsin“normal”galaxies(i.e.ingalax-
relevance of shock-type excitation is more strongly correlated iesthatare neitherextremestarburstsnor AGN hosts)we need
withtheinteractionclassofthesystemthanwiththestarforma- towaitforthecompletionofongoinglargeIFSsurveyssuchas
tionactivity(withisolatedgalaxiesbeinglargelydominatedby the CALIFA, SAMI and MaNGA surveys, although these will
HII-typeexcitation). still have a statistics that is orders of magnitudes smaller than
In summary, based on the observational evidence avail- that providedby the SDSS. Anotherpotential source of bias is
givenbygalaxyinclination,whichcanhaveanimportanteffect
able from IFS studies, our BPT sample selection could lead to
onthedetectabilityofoutflows(e.g.Chenetal.2010).However,
two possible biases: (i) a bias against ageing isolated galaxies
dominated by old stellar populations and/or (ii) a bias against in terms of outflow properties,we expectthe stacking of thou-
merger-driven shocks. The first bias would affect massive “re- sandsofspectratoaverageouttheeffectsofinclination,andso
wedonotexpectourresultstobegreatlyaffected.
tired” galaxies with stellar populations older than 1 Gyr and
little ongoing star formation (e.g. CidFernandesetal. 2011; Ourfinalselectionconsistsof157,639galaxyspectra,whose
Singhetal. 2013), hence with very little or no star forma- distribution in the M∗−SFR diagram is shown in Fig. 1. The
tion feedback at work. The second bias would affect mergers, stellar mass of each galaxy was derived from a fit to the ob-
which are only a minor fraction of the local galaxy popula- servedspectralenergydistribution(SED), obtainedwith SDSS
tion (Patton&Atfield 2008) and so unimportant in our sam- broad-band photometry (Brinchmannetal. 2004; Salimetal.
ple2. Moreover, even if hypothesising that our analysis under- 2007), which therefore takes into account the total emission
representsmerging(U)LIRGsexperiencingstrongfeedback,our from the galaxy. The SFR estimate is instead based on the
conclusions (e.g. § 4.2) would not be affected but would ac- Hα intrinsic luminosity following the procedure described by
tually be even strengthened: in this case we would be under- Brinchmannetal.(2004),withoutapplyinganyaperturecorrec-
representinggalaxieswithhighSFRs/sSFRs(i.e.galaxiesabove tiontotheSDSSfibre-limitedspectrum:theSFRsemployedin
the Main Sequence in the M -SFR diagram, e.g. Combesetal. this work are therefore the “fibre SFRs” providedin the MPA-
∗
2013; KilerciEseretal. 2014) with strong outflows, hence our JHU catalogue. Such choice is justified by the fact that, in this
investigationand the relevantresults couldbe regardedas con- study,weinferthepropertiesofthegaseousandstellarkinemat-
servative. ics by using exclusivelythe informationprovided by the fibre-
limited SDSS spectroscopy,hence we do not have information
In addition to the BPT selection of “HII” galaxies, we re-
on the gas (and so on possible outflows) on scales larger than
quireasignal-to-noisegreaterthanfiveinallfouremissionlines
thosesampledbythefibre.Therefore,webelievethat,giventhe
usedintheBPTdiagram(i.e.[OIII],Hβ,[NII],Hα).Asacon-
obviouslimitations of SDSS spectroscopy(which can be over-
sequence,themostpassivegalaxies,whichdonotshowpromi-
comeonlywithIFSstudies),itismoresensibletolookforpos-
nentnebularlines,areunder-representedinoursample.Wenote
siblecorrelationsbetweenthepropertiesofionisedoutflows(on
that, by imposinga thresholdSNR onthe [OIII]and[NII] line
thescalessampledbytheSDSSfibre)andthefibreSFRs,rather
fluxes,wemayintroduceabiaswithmetallicity.However,since
than the aperture-corrected SFRs. For the stellar mass instead
localgalaxiesdefineatightsurfaceintheM −Z−SFRplane
∗
wehavechosentousethe“total”M becausegalacticoutflows
(i.e.the“fundamentalmetallicityrelation”,e.g.Mannuccietal. ∗
are expectedto feelthe effect of the entire galaxygravitational
2010;Lara-Lópezetal.2010),wedonotexpectgalaxieswithin
potentialwell,whichislinkedtothetotalstellarmass.
agivenbinintheM −SFRparameterspacetoshowsignificant
∗
variationsinmetallicity.Therefore,themaineffectofimposing
SNRconstraintsonthe[OIII]and[NII]emissionlinesisthatour 2.2.Stackingprocedure
sampleselectiondoesnotallowustopopulatecertainregionsof
the M∗ −SFR parameterspace, butwe are notintroducingdif- The M∗−SFR parameter space sampled by our SDSS galaxy
ferential biases within the galaxy bins that are included in our selectionisdividedintoafinegridof50bins,asshowninFig.1,
sample. and the spectra within each bin are stacked together to obtain
veryhighsignal-to-noisecompositespectra.Eachbinisrequired
Ageneralcaveatforthisstudyisthatatthemedianredshift
ofoursample,z =0.073,the3arcsec-diameteroftheSDSS toincludeaminimumof50sources,butmostbinscontainover
med
spectroscopicfibresamplesonlythegalaxyemissionwithinr. 100galaxies,andthecentralonesupto∼ 20,000.Throughout
thispaper,wewillcodeeachstackbyitsaveragelog(SFR)and
2.1kpc.Moreover,thespectroscopicfibresamplesawiderange
of differentprojected sizes within our galaxy sample. Aperture log(M∗)(ID=log(SFR)_log(M∗)).
effects are an obvious limitation of our study that will need to WealsoshowinFig.1thepositionofthemainsequenceof
be taken into account in future works and that can be tackled localstar-forminggalaxiesandits±0.3dexscatter(Salimetal.
2007;Pengetal.2010).Sinceweusefibre-limitedSFRs,theref-
2 Oursampleislargelyrepresentativeofthelocalstarforminggalaxy erence MS for this study differs slightly from the MS relation-
population,incontrasttopreviousoutflowstudiesfocussingmostlyon ship derived by Pengetal. (2010), who instead used aperture-
thelocal (U)LIRGpopulation (for whichthemerger fractionismuch correctedSFRs.ThemaineffectofusingSFRsthatarenotcor-
higher). rected for the SDSS aperture is a downward shift of the MS
Articlenumber,page4of27
C.Cicone, etal.:OutflowsandcomplexstellarkinematicsinSDSSstarforminggalaxies
relation by ∆log(SFR) = −0.4 dex. This shift was estimated
by selecting, in our sample, galaxies with 0.02 < z < 0.085
(to match the redshift range of Pengetal. 2010) and 9.0 <
log(M [M ])<11.0(i.e.thestellarmassrangewherethemean
∗ ⊙
relationshipbetweenlog(SFR)andlog(M )islinear)andbyfit-
∗
ting the mean trend between log(SFR) and log(M ) with a lin-
∗
earrelationship,whoseslopewasconstrainedtobethesameas
Pengetal.(2010).However,westressthattheaimofthiswork
isnottoprovidea“general”fittothelocalMSrelationofstar-
forminggalaxies,and thatthe MSrelationshipshownin Fig. 1
should only be used as a reference for this galaxy sample. We
referto the worksbyPengetal. (2010);Schreiberetal. (2015)
andGavazzietal.(2015)formoredetaileddiscussionaboutthe
MSofstarforminggalaxies.
Prior to stacking, the galaxy spectra in each bin are de-
reddened and normalised to their extinction-corrected Hβ line Fig. 4: Instrumental profile of the SDSS spectrographs, esti-
flux,whichisrelatedtothestarformationrateinthegalaxy.The mated from bright, non-blendedemission lines in the arc lamp
reddening E(B− V) is estimated from the observed Hα-to-Hβ images, selected within the wavelength range of interest for
fluxratio,byusingtheGalacticextinctioncurveasdescribedin this study (i.e. HgI λ5460.753Å for the blue camerasand NeI
Cardellietal.(1989)andbyassumingR = A /E(B−V)=3.1.
V V λ6598.953Å for the red cameras). For each camera (blue/red)
The spectra are then shifted to the rest-frameby using the red-
ofthetwospectrographs(1/2),weoverplottedtenrepresentative
shiftsprovidedbythePrinceton-1Dspectroscopicpipeline3and
profilesfromasmanydifferentframes(correspondingtodiffer-
contextuallyre-binnedbyusingatechniquebasedon“Drizzle”,
ent plates) obtained by averaging the arc lamp spectra over all
the method for linear reconstruction of under-sampled images
320 fibres. The black solid line corresponds to the mean pro-
(Fruchter&Hook2002).Thedrizzlingconsistsinmappingthe
file.Theinsethighlightsthepresenceoflow-levelinstrumental
original redshifted SDSS spectrum into a rest-frame pixel grid wings(correspondingto.3/1000ofthepeak),whicharemore
(theoutputspectrum)bytakingintoaccounttheirdifferentpixel
prominentfortheredcameras.
sizesandrelativeshift,assketchedinFig.2.Inpractice,theflux
containedintheinputpixelisaveragedintotheoutputpixelwith
a weightthatis proportionalto theareaof overlapbetweenthe
twopixels.Theresultingspectrahaveawavelengthseparationof emission lines, which are the targets of this study. The nebular
0.8Åpixel−1, i.e.slightlyfinerthantheoriginalSDSSspectra. emissionlinesandISMabsorptionfeaturesaremaskedfromthe
Therest-framegalaxyspectraineachbinarefinallyaveragedto- stellarfittopreventitfrombeingaffectedbynon-stellarfeatures.
gethertoproduceasinglecompositespectrum(thefinalstack), When the fit is providedwith a large enoughnumberof stellar
whichisthennormalisedtoitsmean. features in the non-masked portion of the spectrum (which is
alwaystrueinthespectralregionweareinterestedin),thispro-
cedure can accurately determine the stellar contamination un-
2.3.Stellarsubtraction
derthemaskednebularlines.Figure3showsthestellarcontin-
uum fit and subtraction performedfor two differentstacks. We
The subtraction of the stellar continuum emission from the
notethatthepPXFprocedure,togetherwiththehighresolution
stackedspectraisofcrucialimportanceforthisstudy,whichis
aimedatdetectingfaintwingsofnebularemissionlines.Forthe PÉGASE-HR templates, producesan excellentfit to the stellar
purpose of stellar subtraction, we select as stellar templates a continuumunderlyingthenebularfeaturesinthegalaxystacks.
largesetofsyntheticspectraofsinglestellarpopulations(SSPs)
computed with the PÉGASE-HR v3.0 code4 (LeBorgneetal.
2004),withbothlowmetallicities(Z = 0.004)andsolarmetal- 2.4.SDSSspectralinstrumentalprofile
licities (Z = 0.02).ThePÉGASE-HRspectraltemplatescover
Our investigation of the kinematics of the ionised gas in lo-
thewavelengthrangeλ∈[4000,6800]Å,withaspectralresolv-
cal star forming galaxies, as traced by the [OIII]λ5007 and
ingpowerofR = 10,000atλ = 5500Å,andarecomputedfor
Hα+[NII]λλ6548,6583nebularemissionlines,isaimedatfind-
a wide range of ages (from 10 Myr to 2 Gyr), by assuming a
ing broad wings of the LoSVD of the ionised gas that may be
Salpeterinitialmassfunction(IMF,Salpeter1955).
signatureofoutflows.Thisisdonebycomparing,ineachgalaxy
We fit the PÉGASE-HR templates to the nebular line-free
stack,theLoSVDofthegaswiththeLoSVDofthestars.Asit
regionsofthecompositegalaxyspectrabyusingtheIDLimple- willbeexplainedin§2.5,theLoSVDofthestarsisderivedby
mentation of the penalised pixel-fitting (pPXF) method devel-
fittingthe stellar continuumaroundλ = 5500Å, i.e.in a spec-
oped by Cappellari&Emsellem (2004). The stellar continuum
tralregionwhere the stellar fit producesthe best results thanks
fitting and subtraction is performed in two wavelength ranges,
to the large number of stellar absorption lines and the scarcity
i.e.λ ∈ [4800,5100]Å andλ ∈ [6200,6790]Å,selected toin-
ofnebular(ISM)features.However,inorderto extractthe real
clude the [OIII]λ5007and the Hα +[NII]λλ6548,6583nebular
kinematicsof stars and gas from the observedline profiles(ei-
ther from the nebular emission lines, in the case of the gas, or
3 Theseareheliocentricredshiftsderivedfromacombinationofstel-
from the stellar features seen in absorption, in the case of the
lar absorption lines and nebular emission lines. For additional in-
formation we refer the reader to D. Schlegel’s Princeton/MIT SDSS stars), the LoSVDs of gas and stars must be deconvolvedfrom
Spectroscopy webpage (http://spectro.princeton.edu) and to theinstrumentalspectralprofile.Therefore,itisofprimaryim-
Boltonetal.(2012). portance for our study to determine the instrumental profile of
4 http://www2.iap.fr/pegase/pegasehr/ theSDSSspectrographs.
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Fig.3:Stellarcontinuumfitandsubtractionovertwowavelengthrangesincludingthe[OIII]λ5007emissionline(toppanels)and
theHα+[NII]emissionlines(bottompanels),performedfortwogalaxystacks:ID=-2.0_8.5(left)andID=0.5_11.5(right).Ineach
plot,theupperpanelshowsthestackedspectrum(black)andthebestfittothestellarcontinuum(blue),whilethelowerpanelshows
the residualspectrum.Theredshadedregionsmarkthe spectralwindowscontainingnebularfeaturesandthereforemaskedfrom
thestellarcontinuumfit.
Thepresenceofscatteredlightinthespectrometermayaffect stellarkinematics.ThepPXFprocedureadoptsaGauss-Hermite
thespectralinstrumentalprofile,resultinginextendedwings.For parametrisationforthestellarline-of-sightvelocitydistributions,
thepurposeofourinvestigation,weusethearclampcalibration which accounts for possible asymmetries in the stellar absorp-
spectra to retrieve and analyse the spectral instrumentalprofile tionlineprofiles.
oftheoriginalSDSSspectrographs(adoptedforSDSS-II).The
average SDSS instrumental profile profile, shown in Fig. 4, is
ToinferthestellarLoSVDs,wefirstconvolvethestellartem-
impressively clean from wings down to . 3/1000of the peak,
plates (i.e. the PÉGASE-HR synthetic spectra) with the SDSS
where some low-level instrumental wings start to appear. This
instrumentalspectralprofileobtainedin§2.4.Whenperforming
effect is however negligible for our study, since it clearly can-
thestellarcontinuumfitbyusingthesestellartemplates(i.e.the
notaccountforthe muchstrongerbroad(andasymmetric)line
onesconvolvedwiththeinstrumentalprofile),thepPXFproce-
wingsthatwedetectinourgalaxystacks,asitwillbeshownin
dureyieldsthe“real”stellarLoSVD,i.e.thestellarLoSVDde-
§3.
convolvedfrom the instrumentalprofile. As already mentioned
Smeeetal. (2013) analyse, in their Figure 35, the spectral in§2.4,inordertoextractthestellarkinematics,itispreferable
resolving power (R(λ) = λ/(2.35∗ σλ)) of the original SDSS to perform the continuum fitting around λ ∼ 5500 Å, because
spectrographsasafunctionofwavelength.Theresolvingpower thisspectralregionisabundantinstellarabsorptionfeaturesand
ofSDSSisquitedifficulttoconstrainbetweenλ = 5000Åand relativelyfreeofnebularemissionlines.Forthisreason,toinfer
λ = 6000Å, becausein thisspectralregionR doesnotdepend the stellar LoSVDs, we fit the stellar continuumin wavelength
monotonicallyonwavelength,duetothetransitionfromtheblue rangesselectedbetweenλ=5100Åandλ=5850Åorbetween
to the red camera of the spectrographs (and some overlap be- λ = 6050Åandλ = 6250Å.Therangeλ ∈ [5850,6050]Åis
tween the two). In this study we adoptR = 1850 (correspond-
avoidedbecauseofthepresenceoftheNaIDλλ5890,5896Åres-
ing to σ =69kms−1), whichis the averagespectralresolving
v onanceabsorptiondoublet,whichiscontaminatedbythegalaxy
powerinferredfromthearclampimages(Figure4)andisonthe
ISM.
lowersideof(butstillwellwithin)thenominalrangequotedin
theliteraturefortheSDSSspectrographs(i.e.R∈[1800,2000]).
An example of continuum fitting aimed at extracting the
stellar kinematics is reported in Fig. 5 for two different galaxy
2.5.TheLoSVDofthestars stacks,i.e.ID=−0.5_11.0andID=−1.5_9.5.Wenotethatthe
ID = −0.5_11.0stackshowsasignificantasymmetryinitsstel-
ThepPXFmethoddevelopedbyCappellari&Emsellem(2004) larLoSVD.ThepresenceofasymmetriesinthestellarLoSVDs
allowsusnotonlytoaccuratelyfitthestellarcontinuumandsub- hasbeenevidencedalsoinotherstacksandwillbediscussedin
tractitfromthestackedspectraofgalaxies,butalsotoextractthe §3.3.
Articlenumber,page6of27
C.Cicone, etal.:OutflowsandcomplexstellarkinematicsinSDSSstarforminggalaxies
!"#$
!%#$
Fig. 5: Stellar continuum fit over the range λ∈[5250,5550]Å (left) and resulting stellar LoSVD (right) derived for two galaxy
stacks,i.e.ID = −0.5_11.0(panela)andID = −1.5_9.5(panelb).Intheleftpanels,thestackedspectrumisplottedinblack,and
thebestfittothestellarcontinuumisinblue.Thereddashedregionsmarkthespectralwindowsmaskedfromthestellarcontinuum
fit because of the presence of nebular features. In the right panels, the red dashed line indicates the LoSVD profile obtained by
using,inthepPXFprocedure,a simpleGaussianfunctiontomodelthestellarfeatures,whilethebluesolidlineshowsthestellar
LoSVDprofileobtainedbyallowingtheuseofHermitepolynomialstoaccountforpossibleasymmetriesofthestellarabsorption
features(Cappellari&Emsellem2004).ThestellarLoSVDobtainedforthestackID = −0.5_11.0showsaclearblueasymmetry.
Thefitisexcellent(χ2 =0.8)forbothstacks.
red
2.6.TheLoSVDoftheionisedgas Anemissionlinecentredatavelocityv,correspondingtoa
i
centralwavelengthλ, will be fitted with a finalfunctionofthe
To infer the LoSVD of the ionised gas we fit, for each stack, i
form:
the[OIII]λ5007,Hαand[NII]λλ6548,6583emissionlineswith
afunctionthatresultsfromtheconvolutionoftheSDSSinstru- λ−λ
mentalprofile,R(v),withanotherfunctionL(v),whichbydefini- Fi(λ)= F(v)∗[ϕiδ(v−vi)]= F(v)∗"ϕiδ c λ i #, (4)
tioncorrespondstothe“real”LoSVDofionisedgas.Toaccount (cid:16) i (cid:17)
for potential broad wings of the LoSVD, possibly tracing out- thatis,theobservedLoSVD,F(v),convolvedwithaDiracdelta
flows of ionised gas, L(v) is parametrised by the sum of three function centred at the vacuum wavelength of the line λ and
i
Gaussians, lk(v)(k =1,2,3),eachcentredata velocityv0,k, with whoseintegralϕiisproportionaltothelineflux.FollowingEq.4,
areaak andstandarddeviationσk,i.e. thefinalfunctionF[OIII](λ)thatweemploytofitthe[OIII]λ5007
emissionlineprofileinthecompositespectrais:
3 3
L(v)= lk(v)= aknke−(v−v0,k)2/2σ2k, (1)
Xk=1 Xk=1 F[OIII](λ)=
“wohbesreervnekd=”1L/oqSV2Dπσ,2kFi(sv)t,hreensuolrtminaglifsraotimonthfaecctoorn.vTohluetrieofnoroef,tthhee ϕ[OIII]Xk=31akNke−(cid:2)c(λλ−[λO[IOIIII]I])−v0,k(cid:3)2(cid:14)2(cid:0)σ2k+σ2R(cid:1). (5)
“real” LoSVD (L(v)) and the SDSS spectral resolution profile
Similarly,wefittheHα+[NII]emissionlineswithafunctionof
(R(v)),isgivenby:
theform:
3 3
F(v)=L(v)∗R(v)= lk(v)∗R(v)= fk(v), (2) Ftot(λ)=FHα(λ)+F[NII]6548(λ)+F[NII]6583(λ)=
Xk=1 Xk=1 3
wherewehaveusedthedistributivepropertyoftheconvolution ϕHα akNke− c(λλ−HλαHα)−v0,k 2 2 σ2k+σ2R +
(indicatedbythesymbol∗)andwehavedefined f (v) ≡ l (v)∗ Xk=1 (cid:2) (cid:3) (cid:14) (cid:0) (cid:1)
k k
Rha(sv)t.hAesfoarmre:sult, since bothlk(v) and R(v) are Gaussians, fk(v) ϕ[NII] 3 akNke− c(λλ−HλαHα)−c(λ[NλIIH]α−λHα)−v0,k 2 2 σ2k+σ2R +
Xk=1 (cid:2) (cid:3)(cid:14) (cid:0) (cid:1)
fk(v)=akNke−(v−v0,k)2/2(σ2k+σ2R), (3) 3ϕ[NII] 3 akNke− c(λλ−HλαHα)−c(̺λ[NλIHI]α−λHα)−v0,k 2 2 σ2k+σ2R .
withNk =1/ 2π(σ2k +σ2R). Xk=1 (cid:2) (cid:3) (cid:14) (cid:0) (cid:1)
q
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Fig. 6: Examples of spectral fits to the [OIII]λ5007 (left column) and Hα+[NII] (right column) emission line profiles for four
representativegalaxybinswithdifferentSFRandM .Fromtoptobottom:ID=-1.58.0(lowSFRandlowM ),ID=-1.010.5(low
∗ ∗
SFRandhighM ),ID=0.58.5(highSFRandlowM ),andID=2.011.0(highSFRandhighM ).Ineachplot,theleftpanelshows
∗ ∗ ∗
thefittothedata(solidcurves,thetotalfitisinblue,themultipleGaussiancomponentsinalightershadeofblue).Forcomparison,
wehaveoverplottedtothedatatheaverageSDSSinstrumentalresolutionprofile(magentadashedcurve,fromFig.4).Intheright
panelofeachplotweshowthefinalLoSVDoftheionisedgasresultingfromthefit,hencedeconvolvedfromtheSDSSinstrumental
profile(detailsin§2.6).
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C.Cicone, etal.:OutflowsandcomplexstellarkinematicsinSDSSstarforminggalaxies
Fig.7:SameasFig.6butforatypicalbinontheMS,withintermediateSFRandM (ID=0.010.5).
∗
Inthisequation,[NII]indicatesthebluestlineofthe[NII]dou- 3.1.Thenoveltyofourapproach
blet,i.e.the[NII]λ6548line,and̺denotesthetheoreticalratio
Before presenting our results, we first shortly summarise what
between the rest-framevacuumwavelengthsof the [NII]λ6548
isnewinourapproachtothestudyofgalacticoutflows,byem-
and[NII]λ6583linesasreportedintheNISTdatabase.
phasizing the advantagesand strengths of this method. For the
Thekinematics(velocityandwidths)ofallcomponentsfit-
firsttime,westudygalacticoutflowsbydirectlycomparingthe
tingtheHα+[NII]complexissettobeequalforthethreetran-
LoSVDoftheionisedgas(astracedbythe[OIII],Hαand[NII]
sitions. Such a constrained approach is necessary to separate
nebular emission lines) with the LoSVD of the stars. This has
the partially blended Hα and [NII] emission lines and it im-
neverbeenattemptedbefore,primarilybecausepreviousstudies
proves the quality and reliability of the fit, as pointed out by
couldnotreachthe signal-to-noiseneededto extractthe stellar
Westmoquetteetal.(2012).Moreoverthismethodisjustifiedby
LoSVDsfromthedata,and(ifany)theyjustreliedonthecom-
the fact that in star forming galaxies and in absence of strong
parisonbetweenthevelocitydispersionofgasandstars.
shocks,theHαand[NII]emissionlinesarisefromadjacentre-
Furthermore,this techniqueholdsthe potentialofdetecting
gions.
the signature of outflows in sources spanning a wide dynamic
Thefit,whoseinputparametersareonlytheinstrumentalres-
range of physical parameters as shown in Fig. 1, and in par-
olution(σ )andthe(vacuum)centralwavelengthsofthediffer-
R ticular in faint, low mass (i.e. M as low as ∼2×107M ) and
ent transitions (λ), provides us with the relative line fluxes ϕ ∗ ⊙
i i moderatelystar-forming(i.e.SFRaslowas∼2×10−3M yr−1)
andwithothernineparameters,i.e.v ,σ anda (withthein- ⊙
0,k k k
galaxies.Themainlimitationswhenprobingoutflowsingalax-
dexk =1,2,3denotingthethreeGaussiancomponentsusedto
ieshavingsuchlow stellar massesandlowstar formationrates
model the observed LoSVD) from which, using Eq. 1, we re-
with SDSS spectroscopy are both the low signal-to-noise and
trieve L(v), i.e. the real LoSVD of the ionised gas. Figures 6
low spectral resolution of the associated spectra. In this study,
and7showexamplefitstothe[OIII]λ5007andHα+[NII]emis-
however,weovercomeboththeselimitations.Firstly,weusethe
sion lines as well as the resultingLoSVDs, for five representa-
stacking technique to significantly improve the signal-to-noise
tive galaxy stacks, having different SFR and M properties (in
∗
in the spectra (§ 2.2). Secondly, we minimise problems asso-
particular,lowandhighSFR, low andhighM ,andintermedi-
∗
ciated with the limited spectral resolution by deconvolvingthe
ateSFRandM ).InFigs.6and7wealsoshowforcomparison,
∗
“real”line-of-sightvelocitydistributionofgasandstarsfromthe
over-plottedto the observedspectralemission line profiles, the
instrumental spectral profile in the observed resolution-limited
average SDSS instrumental profile (from Fig. 4), to show that
composite spectra (as described in § 2.4-§ 2.6). This is made
the broadasymmetricwingswe detectedin a few ofthe stacks
possible thanksto the high signal-to-noiseof the stacked spec-
(e.g.ID=2.011.0)arenotinstrumental.
tra, especially for those galaxybins in which the observedline
profilesarenarrow(i.e.linewidthsareclosetotheinstrumental
resolution). In particular, since, in emission lines, outflows are
3. Results
tracedbyabroadcomponentsuperimposedonanarrowcompo-
In this section we present the results of our analysis of the dy- nent,ourdeconvolutiontechniqueinprincipleallowsustodetect
namical properties of ionised gas and stars based on stacked suchabroadcomponent,provideditisbroadenough,evenifthe
SDSSspectraoflocalstar-forminggalaxies,aimedatidentifying bulkofthe(narrow)emissionlineisnotresolved.
the presenceofgalactic outflows.Throughoutthe remainderof
thepaperandinthefigures,forthesakeofsimplicity,wewillre-
3.2.Propertiesofthekinematicsofgasandstars
place“[OIII]λ5007”and“Hα+[NII]λλ6548,6583”with“[OIII]”
and“Hα”,respectively.Thereadershouldhoweverkeepinmind
LoSVDprofiles
that“Hα”actuallyreferstotheHαand[NII]linecomplexasa
whole, since the LoSVDs were derived by fitting the Hα and In the following we analyse variations in the LoSVD profiles
[NII] lines simultaneously, as explained in § 2.6. The adopted of (ionised) gas and stars as a functionof stellar mass and star
strategyisbrieflysummarisedin§ 3.1. In§3.2we analysethe formationrate.Westressthat,sincethegalaxyspectrahavebeen
generalpropertiesofthe LoSVDs ofgasand stars; in § 3.3we stacked in bins of M and SFR, the trends with M and SFR
∗ ∗
studythe presenceofasymmetricwingsin the LoSVDprofiles canbeinvestigatedindependently,thusbreakingthedegeneracy
andfinallyin§3.4wediscusstheoutflowproperties. betweenSFRandM thatholdsforstarforminggalaxiesonthe
∗
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Fig.8:Line-of-sightvelocitydistributionoftheionisedgasastracedbythe[OIII]λ5007emissionline.Ineachpanelwehaveover-
plottedtheLoSVDsofgalaxybinsidentifiedbythesame(average)SFRandbydifferent(average)stellarmasses,colour-codedby
theirM .
∗
Fig.9:Line-of-sightvelocitydistributionoftheionisedgasastracedbytheHα+[NII]emissionlines.Ineachpanelwehaveover-
plottedtheLoSVDsofgalaxybinsidentifiedbythesame(average)SFRandbydifferent(average)stellarmasses,colour-codedby
theirM .
∗
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