Table Of ContentAstronomy&Astrophysicsmanuscriptno.HRSandSDSS˙arxiv (cid:13)c ESO2017
January3,2017
Bars as seen by Herschel & Sloan
GuidoConsolandi1,MassimoDotti1,Alessandro Boselli2,GiuseppeGavazzi1,andFabioGargiulo1
1 DipartimentodiFisicaG.Occhialini,Universita`diMilano-Bicocca,PiazzadellaScienza3,I-20126Milano,Italy
e-mail:[email protected]
2 AixMarseilleUniversite´,CNRS,LAM(Laboratoired’AstrophysiquedeMarseille),Marseille,France
7
ABSTRACT
1
0
We present an observational study of the effect of bars on the gas component and on the star formation properties of their host
2
galaxies ina statisticallysignificant sample of resolved objects, the Herschel Reference Sample. Theanalysis of optical and far–
n infraredimagesallowsustoidentifyaclearspatialcorrelationbetweenstellarbarsandthecold-gasdistributionmappedbythewarm
a dustemission.Wefindthattheinfraredcounterpartsofopticallyidentifiedbarsareeitherbar–likestructuresordeadcentralregions
J inwhichstarformationisstronglysuppressed.Similarmorphologiesarefoundinthedistributionofstarformationdirectlytracedby
2 Hαmaps.Thesizesofsuchopticalandinfraredstructurescorrelateremarkablywell,hintingatacausalconnection.Inthelightof
previousobservationsandoftheoreticalinvestigationsintheliterature,weinterpretourfindingsasfurtherevidenceofthescenario
] inwhichbarsdrivestronginflowstowardtheirhostnuclei:youngbarsarestillintheprocessofperturbingthegasandstarformation
A clearlydelineatestheshapeofthebars;oldbarsonthecontraryalreadyremovedanygaswithintheirextents,carvingadeadregion
G ofnegligiblestarformation.
. Keywords.Galaxies:structure–Galaxies:starformation–Galaxies:evolution
h
p
-
o 1. Introduction preventingthegasfromfragmentinganddecreasingthecentral
r
SFR(e.g.Reynaud&Downes,1998;Haywoodetal.,2016).
st Stellar bars are common features in disc galaxies on a In order to test the two above-mentionedscenarios, in this
a broad range of stellar masses and local environments (e.g.
study we aim at mapping the distribution of gas in barred and
[ Jogeeetal., 2004; Shethetal., 2008; Barazzaetal., 2008;
unbarredgalaxies.The mostdirectprobewouldrequirethe di-
Nair&Abraham, 2010; Mastersetal., 2012; Skibbaetal.,
1 rectimagingofmolecularandneutralatomicgas.Unfortunately,
v 2012; Me´ndez-Abreuetal., 2012; Gavazzietal., 2015). such informationis available onlyfor a very limited sample of
4 Because of their elongated shape, bars can exert a significant
galaxies,andisoftenaffectedbyeitheratoolowangularreso-
6 gravitational torque onto the host galaxy stellar and gaseous
lution or a very limited field of view. However these problems
3 components, making these features one of the main drivers
can be overcome because the molecular gas distribution corre-
0 of galactic evolution (see e.g. Kormendy&Kennicutt, 2004;
lates strongly with the distribution of the cold dust component
0 Kormendy,2013;Sellwood,2014,forrecentreviews).Inpartic-
. (Bosellietal.,2002).Wetakefulladvantageofsuchleverageby
1 ular,theinteractionbetweenthebarandtheISMwithinthebar
usingthefar–infrared(FIR)imagesfromtheHerschelReference
0 extent results in fast inflows of gas toward the galactic center
Survey (HRS, Bosellietal., 2010). We compare the Herschel
7 (Sanders&Huntley, 1976; Robertsetal., 1979; Athanassoula,
datawiththeopticalimagesfromtheSloanDigitalSkySurvey
1 1992; Sakamotoetal., 1999). Such inflows can trigger nuclear
(SDSS,Yorketal.,2000).Westudythecorrelationbetweenthe
: bursts of star formation (SF, as observationally confirmed by
v occurrenceofbarsinopticalimagesandofeitherbar-likestruc-
Xi Hoetal., 1997; Martinet&Friedli, 1997; Hunt&Malkan, tures or central zones of no emission in the HRS. We further
1999; Jogeeetal., 2005; Laurikainenetal., 2010), and, if the
measure the extent of such optical and infrared structures and
r gas infall proceeds unimpeded, accretion episodes onto the
a check whether they are correlated. For galaxies showing both
central massive black hole (if present, e.g. Shlosmanetal.,
anopticalbarandaninfraredbar-relatedstructurewelinktheir
1989;Berentzenetal.,1998).
morphology to the star formation distribution as traced by Hα
Only recently Verleyetal. (2007), Cheungetal. (2013),
images (Kennicutt, 1998). Finally a qualitative comparison to
Gavazzietal.(2015)andFanalietal.(2015)suggestedthatthe
thefewavailableHI-mapstracingtheatomicgasdistributionis
promptgasremoval,ifconvertedintostarsontheshortdynam-
accomplishedowingtothehighresolutionmapsfromtheVIVA
ical time-scale of the galaxy nucleus, quenches any SF in the
survey(Chungetal.,2009).
central few kpc region of the galaxy. This scenario has strong
implicationsfortheevolutionofthe SFrate (SFR) observedin
field disc galaxies as a function of their stellar mass, with the
2. TheHerschelReferenceSample
declineofthespecificSFR(sSFR)observedinmassivegalaxies
possibly linkedto the formationof a stellar bar (Gavazzietal., The galaxies analyzed in this work have been extracted from
2015; Consolandietal., 2016). On the other hand,even if bars the Herschel ReferenceSurvey,a volume-limited(15≤ D ≤ 25
donotremoveallthegaswithintheirextent,theyareexpected Mpc), K-band-selected sample of nearby galaxies spanning a
toperturbthegaskinematicsbypumpingturbulenceintheISM, widerangeofmorphologicaltypes,fromellipticalstodwarfir-
1
G.Consolandi:BarsinHRS
Fig.1.Examplesforthecategoriesclassifiedinthiswork.Fromlefttoright:NGC4548(HRS-208),NGC4579(HRS-220),NGC
4689 (HRS-256).The top row shows the SDSS RGB image of the galaxy while the second row shows the correspondingPACS
images. Green circles illustrate qualitatively the circular region used to measure the extensions of structures. In the third row is
reportedtheHαimageandinthefourththeHImapfromtheVIVAsurvey.Ineachframea1arcminutescaleisgiven.
regulars,andstellarmasses(108.M .1011M )thathasbeen intermsofsensitivity,angularresolutionanddusttemperature.
∗ ⊙
observedinguaranteedtimewithHerschel(Bosellietal.,2010). AtthisfrequencytheFIRemissiongivesthedistributionofthe
cold dust component, which is a direct tracer of the molecu-
Sincethepresentworkaimsatperformingavisualcompari-
lar gas phase of the ISM (e.g. Boselli et al. 2002) from galac-
sonoftheISMandofthestellarmorphologyintheHRSgalax-
tic to sub-kpc scales (Corbellietal., 2012; Smithetal., 2012;
ies, we need a sufficient spatial resolution in both the IR and
Bolattoetal.,2013;Sandstrometal.,2013).Ontheopticalside,
the optical images as well as a good sensitivity and little dust
the i-band is only little affected by dust and is the best SDSS
obscurationin the opticalband. For this purposewe character-
tracer of the stellar mass of a galaxy,and is preferredto the z-
ize the morphological properties of the stellar component and
bandforitshighersensitivity,whiletheHαdataaretakenfrom
oftheISMusingtheSDSSimagesinthei-band(Corteseetal.
Bosellietal.(2015).TheSDSS,PACSandHαimagesareavail-
2012)andthe160µmmapsobtainedwiththePACSinstrument
ableontheHeDaMdatabase(http://hedam.lam.fr/).
(Cortese et al. 2014), respectively.At 160 µm the resolution is
FWHM=11.4arcsec,whilethepixelsizeofthereducedmaps Further on, we limited the analysis to the 261 late-type
is 2.85 arcsec pixel−1 (Cortese et al. 2014). This photometric galaxies of the sample in order to avoid contamination from
bandhasbeenchosenamongthoseavailableforthewholesam- slow rotators (namely ellipticals, which do not develop bars,
plegalaxies(22µmfromWISE,Cieslaetal.2014;100-500µm, Sellwood,2014)andfromearly-typedisks(includingS0s)that
Ciesla et al. 2012;Cortese etal. 2014)as the best compromise havetoolittlecoldgastotestthebar-relatedquenchingprocess
2
G.Consolandi:BarsinHRS
(Bosellietal., 2014). Finally we exclude galaxies with an axis Consolandi,2016).Followingthisprocedureweextractaradius
ratiolowerthan0.4toavoidamajorinclinationbiasinourmor- ofthebarinthei-bandforeachgalaxyand,wededucethebars
phologyclassification and measures, leaving a final subsample strength following (Laurikainenetal., 2007) from the peak of
of165late-typeface-ongalaxies. the ǫ profilein the i-band.We findthat≈ 95%of galaxiesthat
we classified as barred harbor strong bars (ǫ > 0.4). Although
thisquantitativemethodhasnotbeenappliedtofar-IRdatapre-
3. Results viously, ellipse fits can nevertheless be derived for the 160µm
imagesandtheǫ andP.A.profilesexaminedforabarsignature.
Foreachgalaxywevisuallyinspectthei-bandSDSSimagesand
Since we are trying to measure a regionof decreasedemission
lookforthepresenceofanevidentstellarbar.Separatelywealso
possibly surrounded by a ring-like emitting structure, we also
visuallyinspectthePACSimageslookingforacentralcarvedre-
extractaradialsurfacebrightnessprofilefromconcentricellip-
gionwithlittletonoemissionthat,ifpresent,isdistributedalong
ticalaperturescenteredonthegalaxyandellipticityfixedtothe
abar-likecomponent(seeFig.1,HRS208)orinasmallnuclear
outer infraredisophotes. The derivedsurface brightnessprofile
regionsurroundedbyaring-likestructure(seeFig.1,HRS220).
thereforehasarelativemaximumincorrespondenceofthering-
InFig.1,fromlefttoright,weshowthreeillustrativecasesrep-
likestructure.Inthe caseswherenoinfraredbarisdiscernible,
resentingtheinfraredmorphologiespossiblyassociatedwithop-
theradiusatwhichthisoccursisagoodproxyoftheextension
ticalbars(columnoneandtwo,HRS208and220)andanormal
ofthenonemittingregion.
spiralgalaxy(lastcolumn,HRS254).Foreachgalaxywe give
Inthe160µmimages,thismethodsucceedsatextractingthera-
fromtoptobottomtheSDSSRGB,the160µm,thecontinuum
diusofthenonemittingregionorofthebarin75%ofthebarred
subtracted Hα images and the HI map from the VIVA survey
galaxies. Because of the irregular and clumpy distribution of
(Chungetal.,2009)thatunfortunatelyoverlapsoursampleonly
lightat160µmthefitoftheisophotesdoesnotconvergein25%
withfewgalaxies.
of the galaxies. Therefore, in order to preserve the statistics of
Wefind51barredgalaxies(≈30%ofthesample)inthei-band,
ouralreadylimitedsample,weplotinFig.2theradiusobtained
out of which 75% show in the corresponding 160µm images
averagingthemeasuresoftheopticalbarsmadebytheauthors
anelliptical/circularareawheretheonlyemissionisdistributed
versusthosefromthe160µmdata(blackemptydots)andthose
on a bar- or ring-like structure. On the other hand, we find 63
fromthecontinuumsubtractedHαimages(redemptydots).All
galaxies(≈38%ofthesample)hostingthedescribedmorpholo-
radiiarenormalizedtothei-band25thmagisophoteradiusofthe
giesinthe160µmimagesoutofwhich38(≈65%)galaxiesare
galaxy,takenfromCorteseetal.(2012),anderrorsareevaluated
foundbarredinthecorrespondingopticalimage.Thefrequency
fromthestandarddeviationofourmeasurements.Theblackand
of galaxies hosting an infrared feature that also show a corre-
red dashed lines indicate the bisector fit (Isobeetal., 1990) to
spondingopticalbar,andtheoccurrenceofopticalbarsshowing
the data respectively for the 160µm (slope ∼ 0.89±0.11) and
an infrared feature are ≈ 65% and ≈ 75%, respectively.These
the Hα data (slope ∼ 1.35±0.08).The slope of the fit relative
percentagesrise to ≈ 85% and ≈ 96% if we include 16 galax-
to the optical versus Hα data is strongly influenced by the ex-
ies classified as barred by other literature classifications found
tremelydeviantpoint(associatedtoHRS322)visibleinFig.2.
in the NASA Extragalactic Database (NED). These are mostly
Thisoutlierischaracterizedbyaverysmallerror,asalltheau-
weakbarsthataredifficulttorecognizevisuallyandwhoseex-
thorsconsistentlymeasuredthesameradiuswithverylittlescat-
tentisdifficulttoquantify.Forthisreasonweexcludetheseob-
ter. We stress, however,that the semi-major axisof thisgalaxy
jects fromour furtheranalysis. In order to quantitativelyrelate
intheHαdistributionisperpendiculartotheopticalbar(seethe
theregionofstarformationavoidancetothepresenceofanop-
opticalandHαimageintheonlinematerial)thustheimportant
ticalbar,wemeasurethesizeofthesestructuresvisuallyinthe
discrepancy is mostly due to projection effects. If we exclude
opticalandinthe160µmandthendothesameusingellipsefits
thispointfromthefit,theslopebecomes0.79±0.11.Finally,in
toisophotes.Thetwoapproachesareusefulbecausetheeyecan
green,weplotthebestlinearfitofthecomparisonofopticalver-
effectivelyrecognizefeaturesandtheirextentevenifsomewhat
sus160µmradiimeasuredwithIRAF(slope∼0.87±0.10).All
subjective, while ellipse fits are objective measures that never-
fits show a strong consistency between them even when evalu-
thelesscanbestronglyaffectedbyotherstructuresinthegalax-
atedwithindependentmethods.Tofurthercheckapossiblebias
ies.Fouroftheauthors(GC,MD,FG,GG)manuallyevaluated
due to inclination,we derivedthe same fits for a subsample of
theextentofopticalbarsbymeasuringtheradiusofthecircular
galaxieswithaxisratiogreaterthan0.7(≈ 40%ofthesample),
regioncircumscribingthe bar, avoidingpossible HII regionsat
findingfullyconsistentresults.
theendofit.Ontheotherhand,inthe160µmimages,whenan
infrared bar is present we measure the radius of the circle cir-
cumscribingthebarwhile,whennoclearbarisdiscernible,we 4. Discussionandconclusions
measuretheinnersemi-majoraxisofthering-likestructuresur-
roundingthedepletedregion(asdepictedinFig.1).Fortheop- The study and comparison of frequenciesof occurrence of bar
ticalbarsshowingtheregionofavoidanceinthe160µmimages related features in the optical and FIR, as traced by the stellar
we also visually inspect the continuum subtracted Hα images continuumandbywarmdustemission,respectively,resultsina
findingsimilarmorphologiesandrepeatthesamemeasure. fractionofgalaxieshostinganopticalbarof∼30%whileazone
Using IRAF1 task ellipse, ellipticity (ǫ) and position angle of avoidance with or without an infrared bar is found in 38%
(P.A.) radial profilesof the isophotesof each sample galaxy in of the 160µm images. The percentagesof common occurrence
each considered band. In optical broad-bands, it is well tested suggeststhatFIRimagesareaneffectivewayofidentifyingthe
that the radiusat which there is a peak in the ellipticity profile presenceofabarinagalaxy.
anda related plateauin theP.A. profileis a goodproxyforthe For the galaxies hosting both an optical bar and a central
extensionofthebar(Jogeeetal.,2004;Laurikainenetal.,2010; zoneofavoidanceinthe160µmimages,wemeasuredtheangu-
lar size of both structureswith independentmethods, finding a
1 IRAF(ImageReductionandAnalysisFacility)isasoftwareforthe good correspondence.First, we measured the extent of bars in
reductionandanalysisofastronomicaldata. optical images, while in the FIR images we measured the ex-
3
G.Consolandi:BarsinHRS
(Sakamotoetal., 1999) andaffectsthestar formationofbarred
galaxies(Verleyetal., 2007;Gavazzietal.,2015):assoonasa
barstartsgrowing,thegasisinitiallyperturbedandcompressed
alongthebar,whereitformsstarswhilegraduallylosingitsan-
gular momentum;as the time goes by, the gas is swept by the
barintosub-kpcscales,leavingagas-depletedandSFquenched
regionofthesizeofthebaritself,withorwithoutacentralknot
ofSFdependingontheconsumptiontimescaleoftheoriginally
infallinggas.
Acknowledgements. WethanktheanonymousRefereeandtheEditor,Francoise
Combes,fortheirconstructivecriticism.Thisresearchhasbeenpartlyfinanced
bytheFrenchnationalprogramPNCG,andithasmadeuseoftheGOLDmine
database(Gavazzietal.2003,2014b)andSDSSWebsitehttp://www.sdss.org/.
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4
