Astronomy&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. 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