The imprints of bars on the vertical stellar population gradients of galactic bulges

2.6 MB·

by  Alireza Molaeinezhad

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed16January2017 (MNLATEXstylefilev2.2)
The imprints of bars on the vertical stellar population gradients of
galactic bulges
A. Molaeinezhad1⋆, J. Falco´n-Barroso2,3, I. Mart´ınez-Valpuesta2,3, H.G. Khosroshahi1,
A. Vazdekis2,3, F. La Barbera6, R.F. Peletier5, M. Balcells2,4
1SchoolofAstronomy,InstituteforResearchinFundamentalSciences(IPM),POBox19395-5746Tehran,Iran
2InstitutodeAstrof´ısicadeCanarias,E-38200,LaLaguna,Spain
3Depto.Astrof´ısica,UniversidaddeLaLaguna(ULL),E-38206LaLaguna,Tenerife,Spain
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4IsaacNewtonGroupofTelescopes,Apartado321,38700SantaCruzdeLaPalma,CanaryIslands,Spain
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5KapteynAstronomicalInstitute,UniversityofGroningen,Postbus800,9700AVGroningen,theNetherlands
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6INAF OsservatorioAstronomicodiCapodimonte,I-80131Napoli,Italy
2

n
a
Accepted2017January09.Received2017January09;inoriginalform2016July26
J

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ABSTRACT

Thisisthesecondpaperofaseriesaimedtostudythestellarkinematicsandpopulationproper-
]
A tiesofbulgesinhighly-inclinedbarredgalaxies.Inthiswork,wecarryoutadetailedanalysis
ofthestellarage,metallicityand[Mg/Fe]of28highly-inclined(i>65o)discgalaxies,from
G
S0 to S(B)c, observedwith the SAURON integral-fieldspectrograph.The sample is divided
h. intotwocleansamplesofbarredandunbarredgalaxies,onthebasisofthecorrelationbetween
p thestellarvelocityandh3 profiles,aswellasthelevelofcylindricalrotationwithinthebulge
- region.Wefindthatwhilethemeanstellarage,metallicityand[Mg/Fe]inthebulgesofbarred
o
andunbarredgalaxiesarenotstatisticallydistinct,the[Mg/Fe]gradientsalongtheminoraxis
r
t (awayfromthedisc)ofbarredgalaxiesaresignificantlydifferentthanthosewithoutbars.For
s
barredgalaxies,stars thatare verticallyfurtheraway fromthe midplaneare in generalmore
a
[ [Mg/Fe]–enhancedand thus the verticalgradientsin [Mg/Fe] for barredgalaxiesare mostly
positive, while for unbarred bulges the [Mg/Fe] profiles are typically negative or flat. This
1
result, together with the old populations observed in the barred sample, indicates that bars
v
arelong-lastingstructures,andthereforearenoteasilydestroyed.Themarked[Mg/Fe]differ-
6
enceswiththebulgesofunbarredgalaxiesindicatethatdifferentformation/evolutionscenarios
6
arerequiredtoexplaintheirbuild-up,andemphasizestheroleofbarsinredistributingstellar
4
materialinthebulgedominatedregions.
3
0
Keywords: galaxies:bulges–galaxies:kinematicsanddynamics–galaxies:abundances–
.
1 galaxies:evolution–galaxies:formation–galaxies:stellarcontent.
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:
v
1 INTRODUCTION face-on disc galaxies and the stellar population gradients along
i
X the bars major axis (see Sa´nchez-Bla´zquez 2016, and references
IntheeraofIFUspectroscopy,thespatiallyresolvedspectroscopic
therein).Moorthy&Holtzman(2006)analysedlong-slitspectraof
r information, along with modern stellar population models, have
a 38 face-on spirals of type S0–Sbc and reported a trend towards
providednewinsightsintoourunderstandingoftheroleofbarson
youngeragesforthebulgesofbarredgalaxies.Theyconcludedthat
theformationandevolutionofdiscgalaxies. Barredgalaxiesrep-
apositiveagegradientisadirectindicatoroftheexistenceofabar.
resent aconsiderable fractionoftheentirediscgalaxypopulation
Pe´rez&Sa´nchez-Bla´zquez (2011) studied the age and metallicity
(e.g. Eskridgeetal. 2000; Knapenetal. 2000; Whyteetal. 2002;
gradients in the bulge region of a sample of 20 early-type barred
Grosbøletal.2004;Marinova&Jogee2007).Theyareconsidered
galaxies.Usingthelong-slitspectraalongthebarmajoraxis,they
askeydriversoftheinternalsecular evolutionindiscgalaxiesby
foundthat atagiven velocitydispersion (σ),thebulgesof barred
re-distributionoftheangularmomentum,triggeringofstarforma-
galaxies seem to be more metal rich than the bulges of unbarred
tion and the morphological transformation of galaxies in general,
galaxies.Theymeasuredmostlynegativemetallicitygradientsfor
as demonstrated observationally (e.g. Kormendy 2013) and theo-
the bulges in their sample, suggesting a strong link between the
retically(e.g.Athanassoula2013).
metallicity of the bulge and the presence of a bar. They also in-
Most stellar population studies of barred systems focus on
dicatedthatbulgepopulationsdistributionsandgradientmeasure-
mentscouldbebiasedagainsttheexistenceofthebulgesubstruc-
⋆ Email:[email protected] tures,suchasanuclear ringoraninnerdisc.Similarresultshave
(cid:13)c 0000RAS
2 Molaeinezhadetal.
beenobtained byCoelho&Gadotti(2011),that claimthecurrent (see Bureau&Athanassoula 2005; Athanassoula 2016, for more
star formation is enhanced in the centres of barred galaxies, and details).Thisisfollowedbyintroducingamethodtoquantifycylin-
consequently the measured mean stellar age of bulges is strongly dricalrotationthatisrobustagainstinnerbulgesubstructuressuch
affectedbythebar. asinner discs, which arelikelymore common inbarred galaxies.
A detailed and unbiased study of the extended stellar pop- Our results also confirmed high levels of cylindrical rotation in
ulation in the bulges of barred and unbarred galaxies requires barred galaxies withboxy/peanut bulges. In the current work, we
high inclinations to ensure minimal contamination by the stel- studythestellarpopulationpropertiesandinparticular,thevertical
lar disc. Nonetheless, the number of works, dedicated to study gradientsalongtheminoraxisinthebulgesof28highly-inclined
the stellar populations in highly-inclined barred galaxies is still (i > 65o)discgalaxies,observedwiththeSAURONintegral-field
verylowandislimitedtoafew nearbycases(e.g.Jablonkaetal. spectrograph.UsingthekinematiccriteriaintroducedinPaperI,we
2007;Williamsetal.2011;Pe´rez&Sa´nchez-Bla´zquez2011).Un- areabletosecureanunbiasedclassificationofbarredandunbarred
derstandingthevariationsofstellarpopulationparametersofbulges galaxiesinoursample,andcomparethestellarpopulationproper-
atdifferent heights fromthediscprovides stronger constraintson tiesandverticalgradientsineachclassofbulges.
theformationandevolutionscenariosofbulges.Morespecifically, The paper is organised as follows. In Section 2 we describe
comparing thevertical gradientsofthestellarpopulation parame- thesampleandgiveasummaryofopticalpropertiesandkinemat-
tersinbulgeswithandwithoutbarscanprovideveryimportantnew icsofthegalaxiesinoursample.Section3isdedicatedtotheline-
informationtoexpandourunderstandingofbarsnurtureanditsrole strengthmeasurementsandpresenttheresultsofourstellarpopula-
informationandevolutionofdiscgalaxies. tionanalysis.InSection4webrieflydiscussthepossiblescenarios
Using long-slit spectroscopy, Jablonkaetal. (2007) studied to interpret these results. Finally, we summarize our findings and
theverticalgradientsofstellarpopulationpropertiesinthebulges drawourconclusionsinSection5.
of 32 edge-on galaxies, spanning a wide range of Hubble types,
fromS0toSc.Theyfoundthatthecentralpartsofthebulgesare
mostly younger and more metal rich than their outer regions and
2 SAMPLE&IFUOBSERVATIONS
theouterpartsofthebulgeshavehigher[α/Fe]thantheinnerparts.
Theyfoundnomeaningfuldifferencesbetweenthestellarpopula- 2.1 SampleSelection&Properties
tionsofnearlyedge-onbarredandunbarredgalaxies,althoughthey
Oursampleconsistsof28highly-inclined(i > 65o)galaxieswith
were not in the position to precisely distinguish bulges with and
highS/NIFUobservations,thatcanbeusedtostudythestellarpop-
withoutbars.Ithasbeenwidelyacceptedthatboxy/peanut-shaped
ulationgradientsalongtheminoraxis.Thesampleisdrawnfrom
bulges (hereafter BP bulges), found in about half of edge-on
thecomplete,magnitudelimitedsampleof260nearbygalaxiesin
galaxies,arebarsviewedfromtheside(e.g.Kuijken&Merrifield
theATLAS3Dsurvey(Cappellarietal.2011),complementedwith
1995; Merrifield&Kuijken 1999; Bureau&Freeman 1999;
the sample of highly inclined galaxies introduced in Paper I. For
Athanassoula 2005). Williamsetal. (2011) studied the stellar
theATLAS3Dsurvey,wefocusedondiscgalaxies,andonlykept
kinematicsandpopulationsinfiveedge-ongalaxies,dominatedby
those galaxies whose inclination, as measured via the best fitting
BPbulges,usinglong-slitspectroscopy.Theymeasuredthevertical
mass-follow-light JAMmodels (Cappellarietal. 2013), areabove
gradientsinthebulgesandclaimedthatBPbulgesintheirsample
65◦. This criterion allows us to access the clean vertical extent
donotformahomogeneousclassofobjects.Later,Williamsetal.
of the bulge, lessdisrupted by contamination of dust, disc and/or
(2012),analysedthecentralandthemajoraxisprofilesinthestellar
central components. We visually inspected the sample to discard
populations of 28 highly inclined S0–Sb disc galaxies, presented
galaxieswithirregular morphology or low qualityIFUdata(with
inBureau&Freeman(1999)andChung&Bureau(2004),mostly
poorspatialsamplingintheregionsofinterest).Inadditionwedis-
with BP bulges. They found that at a given σ, gradients in BP
cardedgalaxieswheretheemissionlinecleaningofthespectrawas
bulges are shallower than those in elliptical galaxies, but that the
particularlyproblematicintheshortwavelengthrangeprovidedby
stellar populations at the very centres of disc galaxies with or
theSAURONspectrograph.Ourfinalsamplecomprises20galaxies
withoutBPbulgesdonotdifferfromthoseofearly-typegalaxies.
from the ATLAS3D sample (hereafter S2), along with 8 galax-
As we stated earlier, only a limited number of studies have
ies from our original sample of Paper I (hereafter S1). The basic
been dedicated to investigate the stellar population properties of
photometricpropertiesofthese28galaxiesarepresentedinTable1.
bulgesinhighly-inclinedbarredgalaxies,andontopofthat,there-
sultsaremostlyinconsistent(seeSa´nchez-Bla´zquez2016,andthe
referencestherein).Thismightbe,atleastinpart,duetothelack
2.2 Observations
ofareliableandrobustmethodtodistinguishbarredandunbarred
galaxies in highly-inclined systems. The presence of a bar is not TheSAURONobservationsofbothsamplesS1andS2aredescribed
generallyapparentifthebarisorientedclosetoorexactlyparallel indetailinPaperIandCappellarietal.(2011),respectively.Briefly,
totheline-of-sight,eveninedge-ongalaxies,(seee.g.Bureauetal. the spectroscopic observations of the S1 sample were carried out
2004; Athanassoula 2016; Molaeinezhadetal. 2016). Therefore, between October 1999 and 2011 with the SAURON integral-field
previousresultsmightbehighlybiased(seePeletieretal.2007). spectrograph (Baconetal. 2001) attached to the 4.2-m William
Inthefirstpaperofthisseries(Molaeinezhadetal.2016,here- Herschel Telescope (WHT) of the Observatorio del Roque de los
after Paper I) we studied the connection between bulge morphol- MuchachosatLaPalma,Spain.Weusedthelowspatialresolution
ogy and kinematics in 12 mid to highly-inclined disc galaxies, mode ofSAURON which givesa33′′ ×41′′ field-of-view (FoV),
observed with the SAURON integral-field spectrograph. We sug- with a spatial sampling of 0′.′94 × 0′.′94. This setup produces
gested that the strong positive correlation between the stellar ve- 1431 spectra per pointing over the SAURON FoV. Additionally,
locity (V) and the h3 Gauss–Hermite moment appears to be the a dedicated set of 146 lenses provides simultaneous sky spectra
most reliable indicator for the presence of bars among all other 1.′9 away from the main field. The spectral resolution delivered
bar diagnostics, even in cases with end-on orientation of the bar by the instrument is ∼4.2 A˚ (FWHM) and covers the narrow
(cid:13)c 0000RAS,MNRAS000,000–000
Barsandthestellarpopulationsofbulges 3
Table1.Propertiesofoursampleofgalaxies
Galaxy Sample PA Vhel MK T–type incl. σ0 Dust mcyl Bar zdisc xB zB
(deg) (kms−1) (mag) (deg) (kms−1) (arcsec) (arcsec) (arcsec)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
NGC3098 S2 88.5 1397 −22.72 −1.5 90 126.2 N 0.54±0.20 N? 1.5 10 9
NGC4026 S2 177.5 985 −23.03 −1.8 84 158.1 N 0.54±0.13 Y 3.0 12 10
NGC4036 S2 261.2 1385 −24.40 −2.6 75 181.9 F 0.26±0.24 N 2.0 12 9
NGC4179 S2 142.8 13 −23.18 −1.9 86 167.5 N 0.59±0.12 Y 2.0 13 10
NGC4251 S2 99.0 1066 −23.68 −1.9 80 128.8 N 0.73±0.08 Y 2.0 12 9
NGC4270 S2 109.8 2331 −23.69 −2.0 80 139.6 N 0.58±0.12 Y 1.5 13 10
NGC4346 S2 98.8 832 −22.55 −2.0 77 127.0 N 0.63±0.11 Y 3.5 12 12
NGC4425 S2 25.8 1908 −22.09 −0.6 90 82.8 N 0.53±0.15 Y 3.0 14 8
NGC4435 S2 10.0 791 −23.83 −2.1 68 152.8 D 0.51±0.14 Y? 3.0 11 9
NGC4461 S2 8.1 1924 −23.08 −0.8 71 133.0 N 0.58±0.15 Y 3.5 13 9
NGC4474 S2 79.4 1611 −22.28 −2.0 89 87.9 N 0.55±0.17 Y? 2.0 10 9
NGC4521 S2 166.3 2511 −23.92 −0.1 90 185.8 N 0.71±0.08 Y 2.0 10 8
NGC4710 S2 27.4 1102 −23.53 −0.9 88 104.7 D 0.54±0.20 Y 3.0 19 13
NGC4762 S2 29.6 986 −24.48 −1.8 90 133.7 N 0.66±0.13 Y 1.0 10 7
NGC5103 S2 140.6 1273 −22.36 – 90 111.2 N 0.37±0.21 N 2.0 9 8
NGC5326 S1 130.0 2520 −23.77 – 65 144.9 N 0.35±0.20 N 2.5 10 8
NGC5353 S2 140.4 2198 −25.11 −2.1 80 281.2 D 0.57±0.11 Y 3.0 15 9
NGC5422 S1 152.3 1838 −23.69 −1.5 90 161.8 N 0.69±0.08 Y 2.5 16 10
NGC5475 S1 166.2 1671 −22.88 – 79 115.0 N 0.25±0.25 N 2.5 12 8
NGC5574 S2 62.7 1589 −22.30 −2.8 89 81.9 N 0.51±0.23 Y 2.0 11 9
NGC5611 S2 64.6 1968 −22.20 −1.9 74 137.4 N 0.40±0.24 N 1.5 10 9
NGC5689 S1 84.0 2160 −24.00 – 81 157.4 D 0.64±0.09 Y 3.0 17 13
NGC5707 S1 35.0 2212 −23.22 – 80 131.8 N 0.25±0.20 N 1.5 11 8
NGC5746 S1 170.0 1727 −24.99 – 81 202.8 D 0.61±0.09 Y 3.5 22 15
NGC5838 S1 40.1 1341 −24.13 −2.6 72 246.0 N 0.47±0.17 Y 2.0 11 9
NGC5854 S2 54.8 1663 −23.30 −1.1 74 104.7 N 0.38±0.26 Y 3.5 13 10
NGC5864 S2 65.6 1874 −23.62 −1.7 74 110.7 N 0.59±0.12 Y 1.5 13 10
NGC6010 S1 102.9 2022 −23.53 – 90 159.2 D 0.45±0.19 Y 3.5 11 10
NOTES:(1)Galaxyname.(2)Sample–S1:Molaeinezhadetal.(2016)sample.S2:ExtractedfromtheATLAS3Dsurvey.(3)Positionangle(N–E)ofthe
dustfreeminoraxis.(4)Heliocentricvelocity,takenfromCappellarietal.(2011).ForgalaxiesnotintheATLAS3Dsample,valuesaretakenfromNED.(5)
TotalgalaxyabsolutemagnitudeinK-band,takenfromPeletier&Balcells(1997)andCappellarietal.(2011),forsamplesS1andS2,respectively.(6)
MorphologicalT–typefromHyperLeda(Patureletal.2003).(7)Inclinationsderivedfromthebest-fittingmass-follow-lightJAMmodelling(Cappellarietal.
2013),orifnotavailable,derivedfromdiscellipticityinR-bandfromPeletier&Balcells(1997),correctedforfinitediscthickness.(8)Centralvelocity
dispersion,definedasthemaximumσwithinthebulgeanalysiswindow,fromourdata.(9)Dustfeatures–D:dustydisc,F:dustyfilament,N:none(Taken
fromCappellarietal.2011).(10)Levelofcylindricalrotation(mcyl)withinthebulgeanalysiswindowanditsuncertainty.Valuesofmcylaregenerally
between1(purecylindricalrotation)and0(nosignofcylindricalrotation).(11)Detectionofabar,basedonthekinematicanalysisasinPaperI.The
Questionmark(?)indicatescaseswherebardetectionisuncertain.(12)Verticalextentoftheregion,closetothediscplanewhichismostlikelydisruptedby
contaminationofdustand/orcentralcomponents.(13)and(14)Analysiswindowofthebulgealongthemajorandminoraxisrespectively.See§2formore
details.
spectral range 4800-5380 A˚. The IFU observing strategy and the easily distinguishable if it is not inclined enough or has not ver-
datareductionoftheS2asdescribedinCappellarietal.(2011)is tically thickened enough to be clearly recognized above the disc
similartotheS1sample,whichguaranteesthehomogeneityofdata plane, and/or beyond a possible large classical bulge. It is worth
foroursample.Thestellarkinematicsdata,publishedhereforS1 notingthatthroughout thisstudy,thebulgeofagalaxyisreferred
isthesameaswaspresentedinPaperI.KinematicsmapsoftheS2 totheexcessoflightthatisabovethemainexponentialdisc.
samplewerepresentedindetailsinKrajnovic´etal.(2011).Inorder A major concern in such studies is the disc contamination, that
to ensure meaningful and accurate stellar population parameters could severely bias the bulge morphology analysis. For this pur-
all the data were Voronoi binned, following Cappellari&Copin pose,aswedidinPaperI,wehaveusedGalfit(Pengetal.2002)to
(2003),toatargetSNR=60.Wedescribeourprocedurestoextract findthebestexponentialdiscmodelandsubtractitfromtheorig-
thepopulationparametersin§3. inalimage.Thesesubtractedimageshavebeenusedtodelimitthe
regionofinterestinourgalaxies:the’photometricbulge’.Theex-
tentofthebulgeintheradial(x)andvertical(z)directionshasbeen
2.3 Samplemorphologicalandkinematicclassification evaluated by integrating the residual images, along the minor (or
major)axis,anddeterminingtheregioninwhichmorethan90per
2.3.1 Morphologicalanalysis centoflightcomesfromthebulge.Wehavemadeuseofi−band
imagesfromtheSloanDigitalSkysurveyDR10(Ahnetal.2014)
Inourstudyofthestellarpopulationpropertiesofbulges,thechoice
for this purpose. In addition, we have produced unsharp masked
of highly-inclined galaxiesensures minimal contamination bythe
(median filtering)imagestohighlight thehigh frequency features
stellardisc.However,attheseinclinationsthebarcomponentisnot
(cid:13)c 0000RAS,MNRAS000,000–000
4 Molaeinezhadetal.
oftheimagesandrevealpossiblestructureswithnoradialsymme-
tries.ThesearepresentedinAppendixA,together withthebulge
regionobtainedfromourGalfitanalysis.Theradialandverticalex-
tentofourbulges(inarcsec)arelistedinTable1.
2.3.2 Kinematicclassification
Bureau&Athanassoula (2005) suggested, using N-body simula-
tions, a number of major-axis stellar kinematic features that can
beusedasbardiagnosticsinhighlyinclinedsystems:(1)adouble-
hump rotation curve; (2) a broad central velocity dispersion peak
with a plateau (and possibly a secondary maximum) at moderate
radii and (3) an h profile correlated with velocity over the pro-
3
jectedbarlength.Allthesediagnosticshavebeensuccessfullyex-
pandedbeyondthediscplanebyIannuzzi&Athanassoula(2015),
whostudiedtheimprintsofbarsonthe2Dline-of-sightkinemat-
icsofsimulateddiscgalaxies.InPaperI,wetestedthisapproach
on12mid-tohighlyinclinedgalaxies,allofthemincludedinthe
S1 sample, and confirmed that the correlation between the stellar
lainveeroyfrseiglihatblveelboacritdyia(gVn)osatnicdttohoel.h3ThGeausussc–cHesesrmofittehipsarmamethetoedr iiss Figure 1. Distribution of the level of cylindrical rotation (mcyl) for the
galaxies with positive V–h3 correlation (red histogram) and those with
theabilitytounveilthehiddenbar,whenthebarisnotclearlyvis-
mostlynegativeornullcorrelation(bluehistogram).
ibleinphotometricdata,duetoitsorientationorstrength.Wehave
takenadvantage ofthisapproachtodetect thepresenceofbarsin
oursampleofgalaxies.Thediagnosticplotsshowingthedegreeof requiresthedeterminationofoftenuncertainoffsetstoaccountfor
correlationcanbefoundinAppendixA. differencesinthefluxcalibrationbetweenmodelsandobservations.
Inaddition,atypicalkinematicfeatureofmostbarredgalaxies The wavelength range provided by SAURON (4800–5380 A˚)
withBPbulgesiscylindricalrotation(e.g.Kormendy&Illingworth constrains the set of spectral indices that can be used to estimate
1982; Bureau&Freeman 1999; Falco´n-Barrosoetal. 2006; stellar population parameters, i.e. age, metallicity, and [Mg/Fe].
Molaeinezhadetal. 2016). In such systems, the mean stellar Wehaveusedthethreeavailableindicesthataremeasuredacross
rotationspeedshowslittledifferenceatdifferentheightabovethe the full field for all galaxies, namely the Hβo age indicator (i.e.
disc plane. In Paper I, we introduced a method to quantify the theoptimizedHβ indexofCervantes&Vazdekis(2009)),aswell
levelofcylindrical rotationinbulges. Thisquantity(m )shows asthemetallicity-and[Mg/Fe]–sensitiveindicesFe5015andMgb
cyl
theimportance of thisproperty inbulgesand isgenerally avalue (Trageretal.1998).Noticethatthe[Mg/Fe]abundanceratioisof
between+1whichindicatespurecylindricalrotationand0where particularinterestforthepresentstudy,asitmeasures–forafixed
there is no sign of cylindrical rotation within the bulge analysis IMF – the different timescale of the ejection of magnesium and
window. Figure 1 shows the distribution of m for the galaxies ironintotheinterstellarmedium.Thus,the[Mg/Fe]isbelievedto
cyl
in our sample, separated into two categories of galaxies with bea”chemicalclock”toestimatestar-formationtimescalesforthe
positive V–h3 correlations and those with negative or null V–h3 bulkofastellarpopulation(e.g.Thomasetal.2011;Vazdekisetal.
correlations. Asexpected, thegalaxieswithrelativelyhigh values 2015;Mart´ın-Navarro2016).
of m are mostly those showing positive correlation between Our analysis relies on α–MILES stellar population mod-
cyl
V and h3, while those with lower mcyl values do not show a els, from Vazdekisetal. (2015, hereafter V15). The models, con-
significant correlation. The m values and related uncertainties structedfromMILESstellarspectra,havebeencomputed,inaself-
cyl
arepresentedinTable1. consistentmanner,atbothsolarscaleandfor[α/Fe]=+0.4,overa
Asourgoalistohavetwocleanclassesofbarredandunbarred rangeofmetallicities,withtheaidoftheoreticalstellarspectrafrom
systems, weusethecombination of bothbar diagnostictools,the Coelhoetal. (2005, 2007), and using BaSTi (either scaled-solar
correlationV–h3andthelevelofcylindricalrotation,todefinethe or α–enhanced) isochrones from Pietrinfernietal. (2004, 2006).
galaxies belonging to each class. In this study, ”barred” galaxies Notice that throughout the present work, we adopt the notation
refertothosesystemsthatshowremarkablypositivecorrelationbe- [Mg/Fe], rather than [α/Fe], as our estimate of abundance ratios
tweenV andh3 intheareasabovethemid-planewithinthebulge reliesentirelyonMg-andFe-sensitivefeatures,i.e.Mgb5177and
analysiswindow,whilethosegalaxieswithnon-positiveV–h3and Fe5015.
m <0.55aremarkedas”unbarred”system.Notsurprisingly,all The α–MILES models cover the (optical) MILES spectral
cyl
galaxieswithBPbulgesinoursamplebelongtothe”barred”class. range (λλ ∼ 3500–7400 A˚), in the metallicity and age range of
−2.27 6 [Z/H] 6 +0.4 and 0.03 6 t 6 14 Gyr, respectively.
For bothscaled-solar and α–enhanced SSPmodels, wehavepro-
duced an interpolated grid of model line-strengths with a step of
3 STELLARPOPULATIONQUANTITIES
∼0.06Gyrinage,and∼0.017dexinmetallicity.Thescaled-solar
Besidesstellarkinematics,wehavealsomeasuredline-strengthin- andα–enhancedgridsaretheninterpolated/linearly-extrapolatedto
dicesoftheS1andS2galaxiesintherecentlydefinedLineIndex cover the [Mg/Fe] range from −0.1 to +0.8 dex, with a step of
System (LIS) LIS-8.4A˚ (Vazdekisetal. 2010, hereafter VAZ10). 0.009dexin[Mg/Fe].Noticethattheextrapolationdoesnotaffect
Thismethodhastheadvantageofcircumventingtheuseoftheso- atallourresults,astheintegrated,averageprofilesof[Mg/Fe]for
calledLick/IDSfittingfunctionsforthemodelpredictions, which barredandunbarredgalaxiesexceedsonlyslightly(by∼0.05dex;
(cid:13)c 0000RAS,MNRAS000,000–000
Barsandthestellarpopulationsofbulges 5
NGC5689
(a)
77 0.0 3.0 6.0 9.012.0
66 z_b (arcsec)
1.00
55
1.50
44 2.00
3.00
33 -0.66 7.1000.00
-0.35-0.25 14.00
0.0 06.1 05.26
22
11 22 33 44 55
(b)
5
14.00
0. 470.00
4
0.26
0.15
0.06
3
-0.25
-0.35
2
-0.66
1 -0.96
-1.26
1 2 3 4 5 6 7
Figure2.Leftcolumn,panel(a):Theindex-indexdiagramofHβoversus[MgFe50]’forallVoronoibins(greyfilledcircles)withinthebulgeanalysiswindow
ofNGC5689,arepresentativebarredgalaxywithBPbulgeinoursample.Leftcolumn,panel(b):TheMgbversusFe5015diagramforthisgalaxy.Overlaidare
theMILESSSPmodelsfordifferentagesandmetallicities.Thegreenandredgridscorrespondtothescaled-solarandα–enhancedSSPmodels,respectively.
ColouredsymbolsindicateaverageVoronoibinvaluesatvariousheightsfromthemajoraxis.Middlecolumn:Theabsorptionline-strengthmapsofHβo,Mgb
andFe5015,respectively(fromtoptobottom).RightColumn:computedSSP-equivalentmapsofage,metallicityand[Mg/Fe],respectively.
seeFig.7)thevalueof[Mg/Fe]=+0.4forwhichV15α–enhanced green and red grids correspond to the scaled-solar and [Mg/Fe]–
modelshavebeencomputed. enhancedSSPmodels,respectively.Themiddlecolumnofthefig-
Foreachspectrum,wederivetheSSP-equivalentage,metal- ureshowstheHβo,Fe5015,andMgbindexmaps,whiletheright
licity,and[Mg/Fe],byminimizingthefollowingequation: column presents the age, metallicity and [Mg/Fe] maps obtained
withtheprocedureoutlinedabove.
(EW −EW )2
χ2 = X obs,i mod,i , (1)
σ2
i obs,i
3.1 Comparisonofthemeanstellarpopulationparametersof
where the index i runs over the available indices (Hβo, Fe5015, bulgesofbarredandunbarredgalaxies
and Mgb), EW and EW are observed and model line-
obs,i mod,i
strengths, while σ denote uncertainties on observed line- Themajorityofstellarpopulationstudiesinbulgesfocusonthein-
obs,i
strengths. The minimization is performed over the grid of SSP tegratedpropertiesinsideacertainaperture,nearthecentre.These
modelpredictions,withvaryingage,metallicity,and[Mg/Fe](see studiescouldthereforestronglydependonthepresence(ornot)of
above).Uncertaintyonbest-fitparametersareestimatedfromN = differentsubcomponents (e.g.ayoungstellarcentraldisc,nuclear
1000bootstrapiterations,wherethefittingisrepeatedaftershifting cluster; see Peletieretal. 2007). Thiseffect is particularly impor-
observedlinestrengthsaccordingtotheiruncertainties. tantinhighly-inclinedgalaxies,wherethemaindisccontamination
Figure 2 shows the line-strength indices and resulting stel- isaproblematicissue.Toavoidthesedifficulties,welimitouranal-
lar population parameters for NGC5689, a representative barred ysisofthebulgestellarpopulations toawindow wellbeyond the
galaxywithBPbulgeinoursample. Intheleftcolumn, panel (a) discplane.Morespecifically, theregionsof interestinour bulges
showstheHβo versus[MgFe50]’ forallVoronoi bins(greyfilled arelimitedtotheclean(e.g.dustfree)sideofthegalaxies,between
circles)withinthebulgeanalysiswindow ofNGC5689. Coloured −xBand+xBalongthemajoraxisandzdisctozBalongtheminor
symbols indicate representative values of Voronoi bins at various axis of galaxies. The z indicates the vertical extent of the re-
disc
heightsfromthemajoraxis(see§3.2formoredetails).Overlaidare gion,closetodiscplanethatmightbecontaminatedbydustand/or
theMILESSSPmodelsfordifferent agesandmetallicities.Panel centralcomponents(evaluatedbyvisualinspectionofthevelocity
(b) presents the Mgb versus Fe5015 diagram for this galaxy. The maps).
(cid:13)c 0000RAS,MNRAS000,000–000
6 Molaeinezhadetal.
Figure3.DistributionofmeanSSP-equivalentage,metallicityand[Mg/Fe] Figure4.DistributionofmeanSSPequivalentage,metallicityand[Mg/Fe]
withinthebulgeanalysis windowofbarred(redhistogram)andunbarred versusthedynamicalmass(MJAM)estimates,withinoneeffectiveradius
(bluehistogram) galaxies inoursample.PK−S indicates theprobability (Reff)ofbothclassesofbarredandunbarredgalaxiesinoursample.Over-
thatthetwodistributionsaredrawnfromthesamepopulation. plotedasgreyfilledcirclesshowstellarpopulationsofgalaxies,measured
inthecentralregionsofthegalaxies,withinacircularaperturewitharadius
ofzdisc.Horizontaldottedlinerepresentsthemeanstellarpopulations of
thecentralapertures.
In order to investigate stellar populations of bars and the in-
fluence of bars on bulges of nearby galaxies, we compare the
key stellar population parameters within the bulge analysis win- dependentofthepresenceofthebar,aremoremetalrich,younger
dowsofoursample.Figure3showsthedistributionsofthemean and less [Mg/Fe]–enhanced in the central regions. Jablonkaetal.
SSP-equivalentage,metallicityand[Mg/Fe]forourtwoclassesof (2007)arguedthattheouterregionsofbulges,whicharegenerally
bulges.AKolmogorov–Smirnovtest(hereafterK–S)indicatesthat olderandmoremetalpoor thancentralregionsconsistofthefirst
the difference between the two populations of bulges is not sig- stars to form, supporting those scenarios in which star formation
nificant.ThisisinagreementwithresultsofJablonkaetal.(2007) proceeds fromthe outer partsinwards. Here, weprovide adiffer-
whofoundnosignificantdifferencebetweenthestellarpopulations entexplanationandinterprettheobservedstellarpopulationprop-
ofearlytype(S0–S(B)b)edge-onbarredandunbarredgalaxies. ertiesinthecentralregionsofbulgesasduetohigher(andpossibly
Akeyaspecttoconstrainbulgeformationscenariosisthecor- unsought) contamination level of maindisc components instellar
relationofthestellarpopulationpropertiesofbulges,withtheto- populationpatternsofbulges.
tal mass of their parent galaxies. Figure 4 shows the distribution
of the mean SSP-equivalent age, metallicity and [Mg/Fe] of the
3.2 Verticalgradients
bulges (within the bulge analysis window) versus the dynamical
mass (M ) estimates from (Cappellarietal. 2013), within one Exploringthepossibleinfluenceofbarontheformationandevo-
JAM
effective radius (R ) of galaxies. The central stellar populations lution of bulges is not possible without understanding the varia-
eff
(within a circular aperture with a radius of z ) are also shown tions of stellar population parameters with radius. Here, we have
disc
(grey filled circles). As this figure shows, no significant correla- takenadvantageofthespatiallyresolvedstellarpopulationmapsto
tionsofthestellarpopulationparameterswiththedynamicalmass study thevertical gradients of the SSP-equivalent age, metallicity
arefound,exceptformetallicity,wheremoremassivegalaxieshar- and[Mg/Fe]inoursample.
bouronaveragemoremetalrichbulges.Ingoodagreementwiththe Forthispurposewehavedefined,ontheclean(i.e.dust-free)
resultsbyJablonkaetal.(2007),mostofbulgesinoursample,in- sideofourgalaxies,asetofpseudo-slits(from−xBto+xB),par-
(cid:13)c 0000RAS,MNRAS000,000–000
Barsandthestellarpopulationsofbulges 7
alleltothemajoraxisofthebulge,atvariousheightsfromthema- galaxieswithlowestinclinationsinoursample.Wehaveconfirmed
joraxis.Theslitscoverthewholeverticalextentofthebulgefrom that theaverageand slope valuesobtained for thefull sampleare
z = 0toz = zB.Theouterslits(withrespecttothemajoraxis) consistentwiththosefortheleastinclinedgalaxiesalone.
arewidertocompensate forthesmallnumber ofcontributeddata Theseresultsleaveuswithaveryintriguingpicturewithvery
points(Voronoibins),sothatthenumberofbinswithinallslitsis similar average stellar population values for barred and unbarred
almostthesame.Theverticalprofilesareconstructedbyfindingthe galaxies, consistent mean age profiles, slightlysteeper metallicity
meanvalueofstellarpopulationparameterinquestionineachslit gradientsinbarredgalaxies,butmostnotablyadrasticdifferentin
atdifferentheightsfromthediscplane,normalisedtozB.There- thevertical[Mg/Fe]gradientbetweenthem.Inthenextsectionwe
latederrorsarecalculatedasthestandarderrorofthemeansineach willdescribetheimplicationsoftheseresultsandoutlinepossible
pseudo–slit.Inordertoevaluatetheverticalgradientsinage,metal- scenariosthatmightexplainthem.Wewillalsocomparethemwith
licityand[Mg/Fe]inourbulges,wefitanerror-weightedstraight thosefromtheclosestbarredgalaxyknown:theMilkyWay.
line to the individual profiles excluding the regions close to disc
plane (z ), which are most likely affected by contamination of
disc
dustand/orcentralcomponents.Theslopeofthelines(verticalgra-
4 DISCUSSION
dients)andtherelatederrorsforallgalaxiesinoursamplearelisted
inTable2. Theresultspresentedhereposeachallengetomanytheoriesabout
Figure5demonstratesthemeansandtheverticalgradientsof the formation and evolution of barred galaxies. A common ap-
the SSP-age, metallicity and [Mg/Fe] of bulges as a function of proachtoinvestigatetheirinfluenceontheformationandevolution
level of cylindrical rotation(mcyl) for all galaxies inour sample. ofbulgesistocomparethestellarpopulationpropertiesofgalaxies
Theseresultsrevealatrendtowardssteeperpositiveverticalgradi- withandwithoutbars.Ifbarsfollowadifferentevolutionarypath,
entsin[Mg/Fe]forbulgeswithhigherlevelofcylindricalrotation, wewouldexpect different abundance patternsinbulgesof barred
whicharemostlybarredsystems(lowerrightpanel).However,as andunbarredgalaxies.
noted earlier, cylindrical rotation cannot be considered a reliable Most bulges studies in the literature show similar and rela-
kinematicpropertytoidentifythefullpopulationofbarredgalax- tivelybroaddistributionsinmeanSSP-equivalentagesandmostly
ies,asthisfeaturestronglydependsonbarorientationandgalaxy no vertical gradients in age, independent of the bulge clas-
inclination. sification (e.g. Falco´n-Barrosoetal. 2002; Jablonkaetal. 2007;
IntheleftpanelsofFig.6,wepresenttheverticalprofilesof Peletieretal.2007;Morellietal.2008).Thisisinagreementwith
theSSPequivalentmetallicityandageforthe2classesofbulgesin earlier findings for early-type galaxies (e.g. Mehlertetal. 2003;
oursample,togetherwiththeerror-weightedaveragedprofiles.To Sa´nchez-Bla´zquezetal.2006).Furthermore,theflatnessofthever-
derive theaveraged profiles,theindividual data pointsarebinned ticalageprofilesinbarredgalaxiesinour samplecouldinprinci-
sothateachnon-overlappingbincontainsthesamenumberofdata plesupportthoseformationscenariosofbulgesinwhichthebulge
points.Theerrorweightedaverageandthecorrespondingerrorin formswithverylittleinfluencefromthedisc(e.g.mergersormono-
eachbinaredeterminedusingthemaximum-likelihoodmethod(see lithic collapse), as expected in bulges of unbarred galaxies. It is
Taylor1982),sothatdatapointswithsmalleruncertaintiesineach worthnotingthatthefactthatoldstellarpopulationsareobservedin
bincontributedmoretothecomputedmeanthanthosewithlarger bulgesofgalaxiesdoesnotnecessarilymeanthatbarsareoldstruc-
errorbars.Therightpanelsshowthedistributionofaveragevalues tures,asbarmighthaveformedrecentlyoutfromoldstars,coming
in each class. The individual SSP-equivalent population profiles, fromthedisc(Sa´nchez-Bla´zquezetal.2011).Thefactthatbarred
alongtheminoraxisforallbulgesinour samplearepresented in galaxiescontainoldstellarpopulationsis,however,inmarkedcon-
Appendix B. The figure shows that the mean age profiles for our trasttotheinternalsecularevolutionscenarioexpectedinthesesys-
sample are rather consistent, in mean values and slope, for both tems,inwhich thebarred bulgesareproduced through radial and
barredand unbarred galaxies.Themetallicitygradient isnegative verticaltransportofdiscmaterial(whichistypicallyyoung),dueto
in both cases too, although it appears to be somewhat flatter for instabilities and resonances (see Martinez-Valpuesta&Shlosman
unbarred systems. Mean values, though, are very similarfor both 2004; Moorthy&Holtzman 2006; Kormendy&Kennicutt 2004).
typesofbulges. Originally,itwasthoughtthatifbulgesformedsecularly,starsthat
More interestingly, as shown in Fig. 7, the distribution havebeenscatteredfurthestfromthediscaretheoldest starsand
and gradients of [Mg/Fe] are remarkably different for barred thereforeapositiveagegradientisexpected(Freeman2008).Con-
and unbarred galaxies. [Mg/Fe] gradients are mostly positive in sideringthestellar agepatternof bulgesinour sample, andmore
barred galaxies, while for unbarred ones they are mostly flat. A specificallybarredgalaxies,onecouldna¨ıvelyconcludethatapure
Kolmogorov-Smirnov (K-S) test gives a probability of 10−4 that secular evolution scenario via the bar buckling process, which is
vertical [Mg/Fe] gradients of the barred and unbarred bulges are thewidelyacceptablemechanismtoformbulgesinbarredgalaxies,
drawnfromthesamedistribution. cannotexplaintheobservedpropertiesofbulges,becausethenthe
starsatlargescaleheightsshouldbeyounger.Theonlysolutionwe
A major concern, in this kind of studies is that our gradient see that bars were formed long timeago, and were not destroyed
measurements could beseverely biased against the physical scale later on. The disc itself was later rejuvenated with newly formed
height of the bulges. To test this, we have computed the correla- starsfrominfallinggasclosetothemidplane,asweareseeingin
tion between the [Mg/Fe] gradients scaled to the physical size of our own Milky Way. This leads to these bulges having old ages
thebulgealongtheminoraxis,inkpcunitsandnormalisedtozB, overall,exceptinthemidplane,whereingeneralyoungerstarsare
inarcsec.Ourresults,notshownhere,areveryconsistent inboth beingseen.
caseswithoutmajordifferences,exceptforfewcaseswithrelatively Both sample of bulges show negative metallicity gradients
smallbulges.Wehavealsoinvestigatedwhetherthegradientsmea- along the minor axis, a behaviour that have been formerly inter-
suredinourgalaxiesdependonthepotentiallevelofcontamination preted as a direct evidence against the secular evolution scenario
introducedbytheprojectionofmaindiscbehindthebulgesinthose ofbulges.However, morerecent numericalmodelsshow that,the
(cid:13)c 0000RAS,MNRAS000,000–000
8 Molaeinezhadetal.
Figure5.Meanandvertical gradients ofthestellar age,metallicity and[Mg/Fe]ofbulgesasafunctionofthelevelofcylindrical rotation(mcyl)forall
galaxiesinoursample.Ineachpanel,theverticaldashedlinesindicatethemeanlevelofcylindricalrotationforbarredandunbarredbulges,respectively,while
thehorizontaldashedlinesshowtheaveragedstellarpopulationproperties(upperpanels)andverticalgradients(lowerpanels),respectively.
Barred Galaxies Unbarred Galaxies
11.0
10.5
yr]) 10.0
ge[ 9.5
a
g( 9.0
o
l
8.5
8.0
0.2
-0.0
H]
M/ -0.2
[
-0.4
-0.6
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
z/z_B z/z_B
Figure6.Leftpanels:TheverticalprofilesoftheSSPequivalentmetallicityandageforthe2classesofbulgesinoursample.Thegreylinescorrespondtothe
individualprofilesforeachgalaxy.Foreachclassofbulges,theindividualdatapointsarebinnedsothateachnon-overlappingbincontainsthesamenumber
ofdatapoints.Hatchedregionsindicatetheerror-weightedaveragedprofilesandthecorrespondinguncertainty,followingthemaximum-likelihoodmethod,
ineachbin.Linesrepresentthebestlinearfitstothedatasets,whiledatapointsbelongtotheregions,closetothediscplanearenotincludedinthefitting
process.Rightpanels:Distributionoftheverticalgradientsinageandmetallicityforbothclassesofbulgesinoursample.Thegradientshavebeencalculated
index/zB,wherethezBistheverticalextentofthebulge/barinarcsec.
negative metallicity gradient in secularly formed bulges can be resultappearstorevealthatbothkindsofbulgeshavedifferentori-
consistentlyreproduced,dependingontheoriginaldiscradialgra- gins.Bulgesofunbarredgalaxiesareoftendenotedas’classical’or
dients(Martinez-Valpuesta&Gerhard2013),verticalgradientsof ’elliptical-like’ bulges based on the remarkable resemblance they
thecompositionoftwo-componentdisc(Bekki&Tsujimoto2011) haveinmanyproperties(frommorphology tostellarpopulations)
or both (DiMatteoetal. 2014). Other bulge formation scenarios, to elliptical galaxies. If, as suggested in the literature, they have
mergersandmonolithiccollapse,alsopredictsuchmetallicitygra- similarformationandevolutionchannels,thenbulgesofunbarred
dients (Eggenetal. 1962; Larson 1974; Arimoto&Yoshii 1987). galaxies have been formed through merger of monolithically col-
Therefore, thenegative metallicitygradients along theminor axis lapsedcloudsathighredshifts(Baughetal.1996),similartolow-
ofbulgesofbarredandunbarredgalaxiesdonotallowustosingle luminosityellipticals(e.g.Moorthy&Holtzman2006).Inthatsit-
outthebulgeformationandevolutionaryscenarios. uation the material that forms those bulges comes typically from
The most striking feature found in our data is that bulges of low-masssystemswherethe[Mg/Fe]istypicallylow.Whilethere
barred galaxies are in general more [Mg/Fe]–enhanced at higher issomelevelofenrichmentinthisprocess(thatiswhythetypical
latitudesthan similar regions of bulges in unbarred systems. This [Mg/Fe]valuesarehigherthanindiscmaterial),thefinalmixtureof
(cid:13)c 0000RAS,MNRAS000,000–000
Barsandthestellarpopulationsofbulges 9
lutionof thebar fractionindiscgalaxiesasafunctionofredshift
(seeShethetal.2008;Melvinetal.2014).
TheMilkyWay(MW)isaparticularcaseofabarredsystem
wherewecandirectlyresolvethedistributionofthedifferentkine-
maticandchemicalstellarpopulations.Thebulgeisknowntocon-
sistof predominantly oldstars(∼10Gyr)withaverysmall frac-
tion (up to 3.5%) of the stars younger than 5Gyr (Clarksonetal.
2008,2011).Theoriginoftheseintermediate-agestarsisstillun-
derdebateanddifferentmechanismshavebeensuggestedtointer-
prettheobservational results(seeGonzalez&Gadotti2016,fora
detailedreviewofallarguments).Themetallicitydistributionfunc-
tionofstartsintheGalacticbulgeisbroad,asymmetric(Rich1990;
Ibata&Gilmore1995;Zoccalietal.2003,2008)withaclearver-
tical gradient of ∼0.6dex/kpc decreasing towards high latitudes
(e.g. Zoccalietal. 2008; Nessetal. 2013). The vertical metallic-
itygradientoftheMWbulgeisaround-0.6dex/kpc(Zoccalietal.
2008), -0.06 dex/deg=-0.43 dex/kpc (Gonzalezetal. 2013) and -
Figure7.Theintegrated,errorweightedaverageprofilesofthevariationsof
0.45 dex/kpc (Nessetal. 2013). In any case, the three measure-
the[Mg/Fe],withincreasingheight(z)fromthediscplaneforbothbarred
mentsarehigherthantheaveragevaluewemeasuredinbulgesof
andunbarredgalaxiesinoursample.Theinsethistogramshowsthedistri-
barredgalaxies(∇[M/H]=-0.3dex/kpc).Inprinciple,itcouldseem
butionoftheverticalgradientsin[Mg/Fe]forbothclassesofbulgesinour
thattheMWissomehowdifferenttotherestofthegalaxies.This
sample.Gradientshavebeenmeasuredintherangezdisc <z <zB,nor-
malizedbytheverticalextentofthebulge(zB)inarcsec.PKS givesthe differencecouldbepartiallyduetodifferentmethodweusetomea-
probabilitythatthetwodistributionsaredrawnfromthesamepopulations, suretheverticalgradients.IncaseofMW,wearealwaysusingthe
asderivedfromaK-Stest. minoraxis,butinexternalgalaxiesweareaveragingovertheentire
extent of the bulge (including those bulge regions away from the
minoraxis).Inordertounderstandthedifferencewehaveusedthe
simulations by Martinez-Valpuesta&Gerhard (2013) to compute
allthatmaterialislowerthantheonetheycouldhaveiftheywere thegradients along theminor axisand alsoaveraging withinlon-
made of primordial stars (those that formed quickly at very early gitude−5◦ < l < +5◦.Theverticalgradientsdecreaseby28%,
epochs). from -0.46 dex/kpc to -0.33 dex/kpc respectively. Moreover, it is
Forbarredgalaxies, thehigh [Mg/Fe]abundance ratiofound worthnotingthat,whenlookingatbulgesofexternalgalaxieswe
at high latitudes implies that stars at higher z in barred galaxies are integrating foreground and background light, and in the MW
formed much faster compared to those in the inner regions. The wealwayshaveamagnitudelimitavoidingmanybackgroundstars.
reasonwhythisisnotclearlyseenintheagegradients,whereone ThesetwoexplanationscanexplainthedifferenceswiththeMW.
mayexpectamorepositivegradientinbarred,relativetounbarred Finally,theα-elementabundancesoftheMWbulgeshowthat
systems, is that beyond ∼6 Gyr it is more and more difficult to stars with [Fe/H]< −0.3 are more α-enhanced than metal-rich
distinguishformationeventsontime-scalesoftheorderoflessthan stars([Fe/H]> −0.3)(Cunha&Smith2006;Lecureuretal.2007;
1Gyr.Ouragediagnostic,theHβoindex,willnotgiveusthatlevel Richetal.2007).WhilethemeasurementsintheMWcomefrom
of precision with the typical observational uncertainties we have individual stars,theyseemtobeconsistent withthosefromunre-
in the Hβo equivalent width (see Cervantes&Vazdekis 2009, for solvedstellarpopulationsinourgalaxysample.Inthisrespect,as
details). shownforthestellarkinematicsinPaperI,theMWisnodifferent
Sinceyoungpopulationsarenotobservedathighzinbarred fromthetypicalbarredgalaxyinthenearbyUniverse.
galaxies, it is clear that the bar was made of stars that formed On the theoretical side, there has been mounting evidence
longtimeago.Thealternativeisthatthebarwillselectivelypush supporting the long-lasting nature of bars (e.g. Athanassoula
old material vertically from a disc made of young and old stars 2003; Martinez-Valpuestaetal. 2006; Debattistaetal. 2006;
(like the ones we observed in the nearby Universe), which is not Athanassoulaetal. 2013). More recently Debattista et al. (2016,
easytoexplain.Thehigh[Mg/Fe]observedinthoseouterregions private communication) studied, in the context of the MW anal-
seem to imply that whatever process that formed these stars with ysis, the influence of an evolving long lasting bar on the stellar
such high [Mg/Fe] must have formed very quickly. We are thus populations in the bulge dominated region, offering a somewhat
saying that the bars we observed today (at least in our sample) alternative scenario. By means of a pure N–body simulation,
were bars that formed long time ago. Numerical simulations of they show that the stellar populations of disc material that have
barred galaxies suggest they are made from disc instabilities and differentinitialin-planekinematicsseparatewhenabarforms.The
gothroughaparticularlyimportant bucklingphasewherealotof populationthathashotterradialkinematicsalsohashottervertical
material gets pumped-up to higher latitudes above the disc plane kinematics and becomes the vertically thick and box-shaped part
(see Martinez-Valpuesta&Shlosman 2004; Athanassoula 2005; of the bar, whilethe radiallycooler populations isverticallythin.
Debattistaetal. 2006; Martinez-Valpuestaetal. 2006; Sahaetal. In this scenario, the declining metallicity profile towards higher
2012).Iftrue,ourobservationsaresettinganimportantconstraint latitudesandverticallyrisingabundanceratiointheGalacticbulge
onthetimethatprocessoccurred.Themeanagevaluesandespe- are interpreted as a result of this separation of initially nearly
ciallythepositive[Mg/Fe]gradientssuggestthisprocesstookplace co-spatialpopulations.Thisisaninterestingevolutionaryscenario,
veryearlyoninthelifeofgalaxies.Theseresultsgoalongthelines but how the original superposition of differentiated discs could
ofrecentworksintheliteraturepointingattheearlyformationof be formedisstill unknown. Whilethisscenario isalso consistent
bars(seee.g.Seideletal.2015,2016)andthoseanalysingtheevo- with our observations of more distant galaxies, we are not yet in
(cid:13)c 0000RAS,MNRAS000,000–000
10 Molaeinezhadet al.
position to robustly disentangle the different chemo-kinematic workingonthispaper.A.M.alsoacknowledges theIsaacNewton
componentsofourgalaxies,asdoneintheMW.Infact,whilewe GroupofTelescopes(ING),theInstitutodeAstrof´ısicadeCanarias
agree on the final set of observed properties in the galaxies we (IAC) and the Kapteyn Astronomical Institute for hospitality and
see today, we provide a different explanation to the way we get support while this paper was in progress. The authors acknowl-
to those properties. Current attempts to do so are limited to very edgesupportfromtheSpanishMinistryofEconomyandCompet-
dramatic cases of large-scale counter-rotating disc components itiveness (MINECO) through grants AYA2009-11137, AYA2016-
(e.g. Coccatoetal. 2013; Johnstonetal. 2013). Meanwhile, more 77237-C3-1-P and AYA2014-58308-P. Funding for SDSS-III has
chemo-dynamical simulations are also necessary to confirm this been provided by the Alfred P. Sloan Foundation, the Participat-
issueandexplainpossiblesegregationprocesses,inducedbybars, ingInstitutions,theNationalScienceFoundation,andtheU.S.De-
toproduce the verticalgradients instellarpopulation of secularly partment of Energy Office of Science. The SDSS-III web site is
formedbulges. http://www.sdss3.org/.
5 CONCLUSIONS
Thispaper investigates theimprintsof barson the stellarpopula-
tionproperties of bulgesand morespecifically, thevertical gradi-
entsofstellarpopulations.Forthispurpose,wehavecarriedouta
detailed analysis of thestellar age, metallicityand [Mg/Fe] of 28
highly-inclined (i > 65o) disc galaxies (i > 65o) , from S0 to
S(B)c,observedwiththeSAURONintegralfieldspectrograph.The
choiceofhigh-inclinationgalaxiesensuresminimalcontamination
bythestellardisc.Followingtheapproach,appliedinPaperI,the
sampleisdividedintotwocleansamplesofbarred(n=21)andun-
barredgalaxies(n=7).Comparingthestellarpopulationproperties
and vertical gradients of bulges inthese twoclasses, wefind that
while:
• The distribution of the mean stellar age, metallicity and
[Mg/Fe]inthebulgesofbarredandunbarredgalaxiesarenotsta-
tisticallydistinct.
• Galaxiesinoursamplespanawiderangeofverticalgradients
inSSP-equivalentstellaragesandthereisnosignificantdifference
betweenbarredandunbarredgalaxies.
• Bothclassesofbulgespresentnegativemetallicityandthegra-
dientsarenotstatisticallydistinctforbarredandunbarredgalaxies.
AsimilarpatternhasbeenreportedfortheBPbulgesofMW.
Weseeastrongdifferenceintheverticalgradientin[Mg/Fe]in
barredandunbarredgalaxies.Theverticalgradientsin[Mg/Fe]for
barredgalaxiesaremostlypositive,whileforunbarredgalaxiesthe
profileisalmostflat.Inotherwords,bulgesofbarredgalaxiesare
ingeneralmore[Mg/Fe]–enhancedathigherlatitudesthanregions,
close to disc plane and therefore demonstrate a positive [Mg/Fe]
gradientsalongtheminoraxis.
Suchhighlevelof[Mg/Fe]ratioinbarredbulgestogetherwith
the relatively old population of bar material, as old as unbarred
bulgesinoursample,suggeststhatbarsarelong-lastingstructures
madeofoldmaterial.Theoriginofsuch[Mg/Fe]–enhancedstarsis
notyetclear,butasimpleinterpretationwouldbethatthebuckling
phaseleadingtotheformationofthebartookplacelongtimeago
(>10Gyr).Moredetailedchemo-dynamicalstudiesarestillneeded
toshedmorelightonthisissue.Whatitseemsclearisthattheposi-
tiveverticalgradientin[Mg/Fe]isacharacteristicfeatureofbarred
bulgesandthereforedifferentevolutionmechanismsarerequiredto
interpretthestellarpopulationdifferencesobservedbetweenbulges
ofbarredandunbarredgalaxies.
ACKNOWLEDGMENTS
A.M. wishes to thank the School of Astronomy, IPM and the
Iranian National Observatory (INO) for providing support while
(cid:13)c 0000RAS,MNRAS000,000–000