Stellar population constraints on the ages of galactic bars

1.8 MB·

by  Phil A. James

Mon.Not.R.Astron.Soc.000,1–9(2015) Printed20January2016 (MNLATEXstylefilev2.2)
Stellar population constraints on the ages of galactic bars
P. A. James⋆ & S. M. Percival
Astrophysics Research Institute, Liverpool John Moores University,IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
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Accepted .Received;inoriginalform
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ABSTRACT
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Wepresentastudy ofthestellarpopulationswithinthecentralregionsoffournearby
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barredgalaxies,anduseanoveltechniquetoconstrainthedurationofbaractivity.We

focus on the star formation ‘desert’, a region within each of these galaxies where star
]
A formation appears to have been suppressed by the bar. New Hβ spectroscopic data
arepresented,andusedtoproducespectroscopiclineindiceswhicharecomparedwith
G
theoreticalpredictionsfrompopulationsynthesismodelsforsimplestellarpopulations
.
h andtemporallytruncatedstarformationhistories.Thisanalysisshowsthatthedearth
p ofstarformationactivityintheseregionsappearsto havebeencontinuingforatleast
- 1 Gyr, with timescales of several Gyr indicated for two of the galaxies. This favours
o
modelsinwhichstrongbarscanbe long-livedfeaturesofgalaxies,butourresultsalso
r
t indicate a significant diversity in stellar population ages, and hence in the implied
s
histories of bar activity in these four galaxies.
a
[
Key words: galaxies: spiral - galaxies:stellar content - galaxies: structure.

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v
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7 1 INTRODUCTION Bournaud, Combes & Semelin 2005; Hozumi & Hernquist
4 2005; Hozumi 2012).
Bars are a common feature of spiral galaxies, with near-IR
0 Simulations by Berentzen et al. (2007) and
imagingrevealingthatthemajorityhavesomebarstructure
. Curir, Mazzei, & Murante (2008) show that bars can
1 in theircentral regions, while strong bars are seen in about
0 one-third of spirals (Knapen et al. 2000; Marinova & Jogee be stable for at least 5 and 10 Gyr respectively,
6 while Combes et al. (1990) find characteristic bar life-
2007).Muchtheoreticalandobservationalefforthasrecently
1 times of a Hubble time or more. Other studies (e.g.
been devoted to the possible role of bars in driving mor-
: Lang, Holley-Bockelmann, & Sinha2014) suggest that bars
v phological change in their host galaxies. In particular, tidal
may be induced by external phenomena such as galaxy
i torques from bars are thought to lead to central concen-
X – galaxy tidal interactions, which would predict a broad
tration of disk gas, with the resulting star formation caus-
r ing thebuild-upof ‘pseudobulges’ (Kormendy & Kennicutt spread of bar ages.
a
Observationally, P´erez, S´anchez-Bla´zquez, & Zurita
2004, and references therein). However, the strongest evi-
(2009) and P´erez & S´anchez-Bla´zquez (2011) have at-
dence for this process is from theoretical simulations, and
temptedtoaddressthisquestionusingopticalspectroscopy
the direct observational evidence is not yet compelling. A
to determine ages and metallicities along bars in 20
centralissueisthelifetimeanddutycycleofbars:isabarred
nearby early-type barred galaxies, finding a wide range
galaxyalwaysabarredgalaxy,andifnot,howlongdoesthe
in luminosity-weighted ages of the stellar populations.
bar phaseendure? Theoretical results are not clear.
However, the age of the stellar population is not simply
Oneaspectofthisquestionisthelong-termstabilityof
related to the age of the bar itself, and indeed Wozniak
barsinthepresenceofacentralmassconcentration suchas
(2007)presentsmodelspredictingsignificantagedifferences
a super-massive black hole. The most comprehensive study
within a given bar due to trapping of young stars at
into this question, by Shen & Sellwood (2004) found that
orbital resonances. Thus, we suggest here a complementary
bars are robust to the effects of plausible central masses
technique, based on the influence of the bar on the star
(up to several per cent of the disk mass), a result that
formation (SF) activity in its surrounding environment,
was confirmed by the models of Debattista et al. (2006).
ratherthan within thebar itself.
Some other studies contradict this conclusion, with mod-
ThispaperbuildsontheresultsofarecentHαspectro-
els that indicate a possible role for central mass concentra-
scopicstudyof15nearbybarredgalaxies(James & Percival
tionsin dissolving barsrapidly(Bournaud & Combes 2002;
2015, henceforth JP15). These galaxies had previously
beenidentified(James, Bretherton, & Knapen2009)ashav-
⋆ E-mail:[email protected] ing a specific pattern of emission in continuum-subtracted
(cid:13)c 2015RAS
2 P. A. James & S. M. Percival
narrow-band Hα imaging, with a pronounced dip in this
emission in the radial range swept out by the bar. This re-
gion was termed the ‘star formation desert’ (SFD), a ter-
minology we follow in the present paper. This behaviour is
clearlyapparentintheHαimagespresentedforthe4galax-
ies in the present study, in Figs. 1–4, all showing regions
devoid of Hα emission over a radial range that corresponds
closely with the bar extent. This characteristic SF pattern
is confirmed by other indicators, e.g. three of the galaxies
studiedhere(allexceptforNGC2712) haveGALEXultra-
violet light profiles presented by Gil dePaz et al. (2007),
all of which show distinctive peaks or plateaus of red far-
UV – near-UV colours in the radial range corresponding to
theSFD.Furtherconfirmation comes from Hakobyan et al.
(2015)whodemonstrateadeficitofcore-collapsesupernovae
in barred galaxies over the radial ranges corresponding to
SFDs, again confirming the suppression of SF in these re-
gions.
Note that, in adopting a functional definition of the
regionsstudiedintermsofstar-formationproperties,wede-
liberately side-stepthethornyterminological andclassifica-
tion issues of whetherwe arestudyingbulges,outer bulges,
pseudo-bulges, disks, lenses or ‘barlenses’.
The deep long-slit optical spectroscopy presented in
JP15 shows that most of theseSFD regions do in fact have
diffuse Hα line emission, at a very low level that is not
pickedupbynarrow-bandimaging. However,suchemission
was found always to have [NII]/Hα line ratios very differ-
ent from those found from star-formation regions; this and
the extended diffuse morphology led us to conclude that
the emission is not powered by SF, but more probably by
post-AsymptoticGiantBranchstarsorshocksfromthebar.
Such emission was also found, using rather different meth- Figure 1. NGC 2543, showing R-band (upper frame) and
ods,ingalaxiesstudiedbySingh et al.(2013).Thissuppres- continuum-subtractedHα(lowerframe)images.Northisupand
sion of SF over intermediate radial ranges was anticipated east to the left, and the image covers an area of 200 arcsec by
by Reynaud& Downes (1998), who proposed bar-induced 172arcsec.
shocks as an efficient method for producing such suppres-
sion effects.
ingofbaractivity.Themainconclusionsaresummarisedin
In this paper, we attempt to constrain the longevity of Section 6.
bar activity by an analysis of the SFD regions of 4 of the
galaxiesstudiedinJP15.WepresentadditionalHβlong-slit
spectroscopic observations for these galaxies, and compare
2 GALAXY SAMPLE AND OBSERVATIONS
measured absorption line strengths with predictions from
population synthesismodels toconstrain thetimesince the
Thegalaxiesstudiedhereare4low-redshift,strongly-barred
cessation of SFactivity. Oneimportant aspect of this anal-
galaxies,allofwhichareviewedatmoderateanglesofincli-
ysis is a robust and self-consistent treatment of the effects
nation. Threeare of typeSBb,and oneis classified asSBa.
oflineemissiononobservedabsorption-linestrengths,given
The main properties of the galaxies are listed in Table 1,
ourfindingofnear-ubiquitousbutlow-levellineemission in
including distances (column 4) which are derived from a
these regions.
Virgo-infallcorrectedrecessionvelocityforthemostdistant
Thestructureofthispaperisasfollows. Section2con- galaxy(NGC2543),andfromvelocity-independentdistance
tains a description of the observed galaxies and their main indicators such as Cepheid variables or type Ia supernovae
properties,andliststhenewspectroscopicobservationsused fortheotherthree.Columns5–7inTable1listthegalaxyin-
in this analysis. In Section 3 we describe the data reduc- clinationtotheline-of-sight,thepositionangleofthegalaxy
tionthatwasperformedtoextractabsorptionlinestrengths majoraxis,andthepositionangleofthebar,measuredfrom
from long-slit spectroscopy. The data analysis presented in ourown R-band imaging, in that order.
Section 4 includes a description of the iterative procedure Two sets of spectroscopic observations are used in the
adopted to correct line indices for the effects of line emis- analysis of thesegalaxies. Oneset was takenwith theIsaac
sion,andpresentsthetheoreticalpopulationsynthesismod- Newton Telescope (INT) in thered end of theoptical spec-
elswithwhichthecorrectedindicesarecompared.Section5 trum,specificallytargetingtheregionincludingtheHαline
containsthediscussionofthemainresultsintermsofstellar and the nearby [NII] lines. The reduction and analysis of
populationagesandtheirconsequencesforourunderstand- these spectra were described fully in JP15 and so will not
(cid:13)c 2015RAS,MNRAS000,1–9
Age constraints for galactic bars 3
Table 1.Propertiesoftheobservedgalaxies.
Name Classn. Velocity Distance Inclination GalaxyPA BarPA
kms−1 Mpc ◦ ◦ ◦
NGC2543 SB(s)b 2471 38.6 55 45 90
NGC2712 SB(r)b 1815 31.2 57 178 28
NGC3185 (R)SB(r)a 1217 23.4 47 130 113
NGC3351 SB(r)b 778 10.2 47 13 110
Figure 2. NGC 2712, showing R-band (upper frame) and Figure 3. NGC 3185, showing R-band (upper frame) and
continuum-subtractedHα(lowerframe)images.Northisupand continuum-subtractedHα(lowerframe)images.Northisupand
east to the left, and the image covers an area of 200 arcsec by east to the left, and the image covers an area of 200 arcsec by
172arcsec. 172arcsec.
be discussed further here. Spectra were extracted for two othersource, whichmust betakenintoaccountwhen inter-
SFD regions in each of thefour galaxies discussed here. All pretingabsorption-line strengths in the present analysis.
eight of these regions showed evidence for low-level diffuse Additional spectroscopy, presented here for the first
emissionintheHα6563˚Aand[NII]6584˚Alines,andinev- time, was taken using the William Herschel Telescope
erycasetheratiosoftheemissionstrengthinthesetwolines (WHT)withtheIntermediateDispersionSpectrographand
wereinconsistentwithSFastheexcitationmechanism.The Imaging System (ISIS), in long-slit spectroscopy mode, on
[NII]/Hαratiosalllayintherange0.51–1.77,afterapplica- the night of 27 December 2010. The total exposure times
tion of a conservative correction for underlying Hα absorp- were 1.0 - 1.7 hours per galaxy, as listed in column 10 of
tion,whereassimilarly correctedspectrafromSFregionsin Table 2. ISIS was used with the R600B grating and a 1.5′′
thefullgalaxy sampleshowed amean ratioof 0.330±0.013. wideslit,givingaspectralresolutionof∼2.5˚Aatthewave-
Thus the conclusion from JP15 is that these SFD regions lengthoftheHβ 4861 ˚Alinewhichwastheprimary goalof
have strongly, and probably completely, suppressed SF ac- theWHTspectroscopy.Thisresolutionissufficienttodeter-
tivity,butthattheydohavediffuselineemissionfromsome minethespectralindiceswerequire,fromdirectly-measured
(cid:13)c 2015RAS,MNRAS000,1–9
4 P. A. James & S. M. Percival
Table 2.Detailsofthelong-slitspectroscopicobservations takenwiththeINTandWHT.
Name BarPA Date Inttime SlitPA AM RMAX Date Inttime SlitPA AM
◦ INT s ◦ kpc WHT s ◦
NGC2543 90 20140204 1×1200 0 1.01 4.5 20101227 5×1200 0 1.02-1.13
NGC2712 28 20140207 3×1200 170 1.04-1.05 3.3 20101227 5×1200 170 1.04-1.05
NGC3185 113 20140204 3×1200 140 1.01 3.3 20101227 3×1200 150 1.01
NGC3351 110 20140205 3×1200 20 1.05-1.06 2.5 20101227 4×1200 20 1.05-1.12
in Section 4 does rely on Hα to Hβ emission line ratios
and will be somewhat affected. However, it is important to
note that all observations were taken at low airmass, with
a maximum value of 1.13 and most observations being be-
lowairmass1.06,sotheseatmosphericdispersioneffectswill
be very small. For all observations with both the INT and
WHT, the slit was positioned to pass directly through the
nuclear peak of continuum emission as seen on the acquisi-
tion TV of thetelescope.
3 DATA REDUCTION
Data reduction followed standard long-slit CCD spec-
troscopy procedures of bias subtraction, flat fielding, wave-
length and flux calibration, and will not be described in
detail here. Flat fielding made use of tungsten arc spec-
tra, while wavelength calibration was provided by copper-
neon+copper-argon emission-line lamp spectra which were
alwaystakenimmediatelybeforeorafterthespectrumwhich
they calibrated, at the same telescope pointing angle, to
avoid differential flexure effects. Spectrophotometric stan-
dard star observations from the same night were used to
providea fluxcalibration.
For each of the four galaxies, two spectra were ex-
tracted, one from each side of the nucleus, by co-adding
spectral information from appropriate partsof theslit. The
selection of these regions was described in JP15; the Hβ
spectra from the WHT were extracted in exactly the same
Figure 4. NGC 3351, showing R-band (upper frame) and way,usingidenticalslitregions(although,giventheslightly
continuum-subtractedHα(lowerframe)images.Northisupand differentslitPAforNGC3185,theregionsofthegalaxyob-
east to the left, and the image covers an area of 266 arcsec by servedfortheHαandHβ spectraareslightlyoffset).These
214arcsec. regions were defined to lie completely within the SFD of
each galaxy, i.e. the radial range swept out by the bar and
theslitregionsareindicatedbytheblackbarsshownonthe
equivalentwidth(EW)valueswithinthebandpassesdefined continuum-subtracted Hα images in Figs. 1–4. In all cases,
by Trager et al. (1998), as described in Section 4. no emission is apparent along the chosen slit positions in
Columns 5 and 10 of Table 2 give the position angle theseHαimages.Thespectrafromoppositesidesofthenu-
(PA) on the sky of the slit for the INT and WHT obser- cleus in each galaxy could have been co-added to improve
vations respectively. For three of the four galaxies, the PA signal-to-noise. This was not done, partly to enable the in-
usedwasidenticalforthetwosetsofobservations,whilefor vestigation of spectral variation within galaxies, and partly
the other, NGC 3185, the slit PA differed by 10 degrees. because of data reduction complications resulting from ve-
It should also be noted that the spectra were not observed locity differences between these regions caused by galaxy
at the parallactic angle, and so atmospheric dispersion will rotation.Thus,theendresultofthebasicdataanalysiswas
leadtosomedifferentialfluxlossasafunctionofwavelength. 8flux-calibrated,sky-subtractedspectrafortheSFDregions
ThemajorityoftheanalysispresentedhereisbasedonEW ofthe4barredgalaxies,coveringthespectralregionaround
measurementswhichareentirelyunaffectedbyatmospheric the Hβ stellar absorption feature, and matching the 8 Hα-
dispersioneffects,buttheemissionlinecorrectiondescribed region spectra already described in Section 2.
(cid:13)c 2015RAS,MNRAS000,1–9
Age constraints for galactic bars 5
50 Myr
4.0
100 Myr 3.0
250 Myr
2.0
500 Myr
1 Gyr
2 Gyr
4 Gyr
1.0
4700 4800 4900 5000 0.0 2.0 4.0 6.0
Figure 5. Simulated spectra showing the variation in the Figure6. AgridofindicesfromBaSTIstellarpopulationmod-
strength of the Hβ absorption feature for solar metallicity sim- els,showingthestrongcorrelationofHβEWwithageofasimple
plestellarpopulations,showingtheextremedepthofthefeature stellarpopulation.Thegridpointsandoverlaiddatapointscorre-
inyoungandintermediate-agestellarpopulations andhencethe spondtoequivalent widthsofthe appropriatefeatures usingthe
sensitivity of Hβ spectroscopy to small fractional admixtures of wavelengthrangesdefinedinTrageretal.(1998).Thedatapoints
suchpopulations. aredetermined fromspectra taken inthe ‘desert’ regions onop-
positesidesofthenucleus ofthe4barredgalaxies(solidsquare,
NGC2543;opensquare,NGC2712;opencircle,NGC3185;solid
4 DATA ANALYSIS circle,NGC3351)andtheHβindexstrengthshavebeencorrected
for the presence of diffuse line emission. SSP ages are indicated
4.1 Calculation of absorption line indices totherightofthefigure;themetallicitiesusedincalculatingthe
grid points are [Fe/H] = –1.27, –0.96, –0.66, –0.31, +0.06 and
In order to derive useful properties, principally ages, from
+0.4,fromlefttoright.
our spectra, we performed comparisons with theoretical
spectra taken from the BaSTI stellar population synthe-
sis data base (Pietrinferni et al. 2004; Percival et al. 2009). scaled-solar element abundance ratios with total heavy ele-
In all cases, the procedures adopted were identical for the mentabundancesranging from well below tojust aboveso-
observed and theoretical spectra, and involved direct inte- lar,andcoveredagesfrom50Myrto14Gyr.Thepredicted
grationofspectralflux.EWvaluesweremeasuredusingthe Hβ andMgbindexstrengthsfortheseSSPsarerepresented
1
LECTORprogrammeofA.Vazdekis inbandpassesdefined by the grid in Fig. 6, which shows the restricted age range
by Trager et al. (1998). Since the analysis was principally of 1 – 14 Gyr.
aimed at constraining ages of stellar populations, the most The observed spectra yielded Hβ EWs covering the
importantindexusedwasHβ.Thestrongdependenceofthis range0.974–2.334˚A withthefullsetofvaluesbeinglisted
feature on stellar population age is shown in Fig. 5, which incolumn2ofTable3.SuchEWsareonlycompatiblewith
clearlyillustratesthemonotonicdeclineinHβstrengthwith SSPsof age>3 Gyr,regardless of metallicity,as is apparent
increasing age of a simple (single-age and single metallic- from themodelgrid in Fig. 6.The lowest valuesof Hβ EW
ity) stellar population (SSP), over the age range 50 Myr – are formally inconsistent with any model predictions, even
4 Gyr.However, since Hβ strength shows some dependence for the oldest SSPs. Such low values motivate the study of
on metallicity, the Mgb index was also determined for all contamination by emission lines, which can potentialy ‘fill
spectratobreakapossibleage-metallicitydegeneracywhen in’ the Hβ absorption feature, leading to significant over-
interpreting index strengths. estimates of population ages.
4.2 Comparison with simple stellar population 4.3 Correction for line emission within absorption
models features
For the initial comparison, we used SSP models to pro- InJP15, we presentedan analysis of these4 galaxies which
videausefulreferencepoint,whileacknowledgingthatthese showed evidence for low-level diffuse emission lines, includ-
poorly represent the extended and potentially complex SF ingHα,throughouttheirSFDregions.Thustheremustalso
historiesofbrightspiralgalaxies.Themodelsusedassumed be associated Hβ emission, and the filling-in problem men-
tionedabovemustbeoccurringat somelevel.However,the
relative weakness of Hβ emission compared with Hα, cou-
1 Seehttp://www.iac.es/galeria/vazdekis/index.html pled with the strength of Hβ absorption and the relatively
(cid:13)c 2015RAS,MNRAS000,1–9
6 P. A. James & S. M. Percival
poor spectral resolution of our WHT spectroscopy mean
thatnoHβemissionisdirectlyapparentinthereducedspec-
tra of the SFD regions. In addition, the restricted spectral
rangecoveredbyourdataandthelackofcontiguouscover- 50 Myr
agebetweentheHαandHβspectralregionsmakeitdifficult
to fit a model to the continuum spectrum and hence di- 100 Myr
rectly decouple continuum and emission-line contributions,
as is frequently done elsewhere. Instead, we adopt an iter-
250 Myr
ative procedure of corrections to the measured Hα and Hβ
emissionandabsorptionfeaturestrengthstoproduceaself-
consistent solution, as described below. Iteration is needed 500 Myr
as underlyingabsorption affects themeasured Hαline flux,
while the Hβ emission line flux affects the Hβ absorption 1 Gyr
strength.
Thisiterativecorrectionisimplementedasfollows.First
2 Gyr
the Hβ absorption EW is determined by direct integra-
tion on the observed spectrum in the bandpass defined by
4 Gyr
Trager et al.(1998)asdescribedabove.TheMgbEWisde-
rived in the same way, and the resulting index values are
4700 4800 4900 5000
used to identify the closest matching SSP model spectrum.
This model spectrum is then used to derive an estimate of
theHαabsorptionEW,whichforpracticalpurposeswede-
Figure 7. Simulated spectra showing the variation in the
fine in a narrow 8 ˚A passband. This EW is then combined strength of the Hβ absorption feature as a function of the time
withthemeasuredcontinuumfluxdensityfromtheobserved sinceSFactivitywastruncated,forsolarmetallicitypopulations.
spectrum to derive a flux correction for the Hα emission
line, which was originally measured in a passband of width
8 ˚A (JP15). The corrected Hα emission line flux is then equal to 50 per cent of the size of the final correction. This
usedtopredictthestrengthoftheHβ emissionline,assum- error is much larger than, for example, the changes that
ing the widely-adopted Case B Hα/Hβ ratio of 2.85 (e.g. wouldresultfromadoptinganyoftheHα/Hβintrinsicemis-
Brocklehurst 1971; Hummer& Storey 1987), although this sionlineratiosresultingfromdifferentstellarandnon-stellar
assumption is examined further below. The Hβ absorption processesinvestigatedbyFerland & Netzer(1983)andlisted
EW is then corrected for the effect of this predicted emis- in their Table 3.
sion line, and the new EW used as the basis for the next The absorption-corrected Hβ indices, combined with
stage ofiteration. Inpractice, theprocess converged after2 Mgbmetallicity-sensitiveindices,areplottedinFig.6.This
or3iterations.Thetotalsizesoftheresultingcorrectionsto shows that the index strengths are matched by SSPs with
themeasuredHβ EWvalueswerebetween0.18and0.62˚A, ages ranging between 3 and 14 Gyr. Fig. 6 also indicates
which increased the strength of the Hβ feature by between thatthestellarpopulationsappeartohavemoderatelysub-
13 and 32 per cent. These corrected Hβ indices are used solar metallicities, with most of the SFD points lying be-
in the discussion in the remainder of this paper, and are tween [Fe/H] of +0.06 and –0.66. This is slightly surpris-
plotted in Fig. 6 and subsequent figures. The measured Hβ ing for regions towards thecentres of intrinsically luminous
absorption indices, after emission-line correction, are listed spiral galaxies, where metallicities similar to solar or some-
in column 3 of Table 3. whathighermightbeexpected.GiventheshapeoftheSSP
TheintrinsicHα/Hβemissionlineratioof2.85thatwas age-metallicity grid, shifting thepointsto somewhat higher
adoptedaboveisusedwidelyintheliterature,butgenerally metallicities would result in somewhat younger mean ages,
for regions ionized by hot stars. Ho, Filippenko, & Sargent but it is still clear that most of the measured points would
(1993), following Ferland & Netzer (1983), prefer a higher indicate old stellar populations in these SFD regions, with
ratioof3.1forLowIonizationNuclearEmission-lineRegion equivalent SSPages of several billion years.
(LINER) typesources, noting that Hα emission strength is
boostedinregionsdominatedbycollisionalexcitation.Given
4.4 Extended SF histories with a defined
thattheSFDregionsinthesebarredgalaxiesshowLINER-
truncation epoch
like[NII]/Hαratios(JP15),thesehigherHα/Hβ ratiosmay
be more appropriate here. Adopting a ratio of 3.1 reduces One limitation of the above analysis was the comparison
the emission-line correction by about 10 per cent, resulting withSSPmodels,whichwillpoorlyrepresenttheproperties
incorrectedEWvaluesthatarelowerbybetween0.014and of any components of a real galaxy, where SF histories are
0.050 ˚A. Extinction will also act to increase the effective likely to be temporally extended and complex. In this sec-
Hα/Hβ ratio, thus weakening this correction and decreas- tion,weattempttoinclude,atleasttofirstorder,theeffects
ing the corrected absorption EW. Extinction in these SFD of a more realistic SFhistory.Wedo thisby calculating in-
regions is largely unconstrained, but we note that they are dex strengths from stellar populations with SF rates that
visuallysmoothandshownoevidenceofpatchydustobscu- are constant in time over an interval starting 13 Gyr ago,
ration, and so we apply no extinction correction. andendingatatruncationepochwhichwevaryfrom4Gyr
Given thesignificant uncertainties in this emission-line to50Myrago.Whilesuchamodelisclearlyasimplification
correction process, we assume that it contributes an error of the true history, this assumption gives a one-parameter
(cid:13)c 2015RAS,MNRAS000,1–9
Age constraints for galactic bars 7
Table 3.MeasuredHβ absorptionlineindices,beforeandafteremission-linecorrection.
Galaxyandregion Hβ,nocorr. Hβ,corrected
˚A ˚A
NGC2543,SFD1 2.334 2.876
NGC2543,SFD2 1.921 2.544
NGC2712,SFD1 1.778 2.195
NGC2712,SFD2 1.812 2.326
NGC3185,SFD1 0.974 1.237
NGC3185,SFD2 1.233 1.527
NGC3351,SFD1 1.362 1.541
NGC3351,SFD2 1.244 1.429
set of models which is better suited to the specific ques-
tion we address here than are, for example, the mass- and
Continuous SF truncated
luminosity-weighted effective ages of stellar populations re- 50 Myr ago
100 Myr
sulting from the more sophisticated full spectrum fitting of
4.0
S´anchez-Bla´zquez et al. (2011).
Figure 7 shows examples of themodeled spectra, using 250 Myr
theBaSTImodelsandspectrallibrary(Percival et al.2009).
The figure shows the decreasing depth of the Hβ feature as
thetruncationepochispushedbackfrom50Myr(topspec- 3.0 500 Myr
trum) to 4 Gyr (bottom). This is the same trend in Hβ
strength with age as is shown in the SSP models (Fig. 5),
butwithEWvaluesoverallbeinglowerduetotheeffectsof 1 Gyr
theoldest starswhicharepresentinallmodels. Thegrid of 2.0
theHβindexvstheMgbindexshowninFig.8quantifiesthe 2 Gyr
variation inHβ with theepoch of last SF,showing particu- 4 Gyr
larlystrongvariationovertherange0.1-2Gyr.Thisiswell
matchedtotherequirementsfordiscriminatingbetweenthe [Fe/H]=
theoretical possibilities outlined in the introduction; if bars 1.0 -1.27 -0.96 -0.66 -0.35 +0.06(solar) +0.4
typically persist for only a few rotations, the observations
2.0 4.0 6.0
should populate theupperpart of thegrid shown in Fig. 8,
whereasacontinuationofthecurrentSFpatternforseveral
Gyr should lead to an old population in the SFD regions,
Figure8. AgridofindicesfromBaSTIstellarpopulationmod-
populating thebottom of thediagram. els, showing the strong correlation of Hβ EW with time since
These observations have yielded age estimates for the truncationofSFactivity.Thegridpointsandoverlaiddatapoints
twoSFDregionsineachofthefourbarredgalaxies,shownin correspondtoequivalentwidthsoftheappropriatefeaturesusing
Fig.8.Alleightregions indicatethatSFhasbeencurtailed thesamewavelengthranges,definedinTrageretal.(1998). The
foratleastthelast1Gyr,andhencethatbarsarelikelytobe datapointsaredeterminedfromspectratakenintheSFDregions
long-lived features. However, significant differences are evi- onoppositesidesofthenucleusof4barredgalaxies(Solidsquare,
dentintheagesrevealedbyourspectroscopicanalysis,with NGC2543;opensquare,NGC2712;opencircle,NGC3185;solid
circle, NGC 3351), and the Hβ index strengths have been cor-
theSFDregionsintwogalaxies(NGC3185andNGC3351)
rected for the presence of diffuse line emission. The dashed line
havingveryoldpopulations,whileNGC2543andNGC2712
atthebottomoftheplotcorrespondstoSSPsofage14Gyr.
have evidence for more recent truncation of SF, within the
last ∼2 Gyr. We discuss the consequences of these findings
in thenext section. studied. This was done using averaged Hα profiles for un-
barredgalaxiesofthesameHubbleT-type(SborSa),taken
from James, Bretherton, & Knapen (2009). The resulting
fractions are: for NGC 2543, 33 per cent; for NGC 2712,
5 DISCUSSION
26percent;forNGC3185, 37 percent;and forNGC3351,
WhendiscussingtheimplicationsoftheseimpliedSFhisto- 15percent,withthesmallervalueforthelast galaxybeing
ries,itisimportanttostressthatwearestudyingtheradial due to its shorter bar. Thus it is clearly not the case that
rangewhich lies intheheartof thestar-forming disk in un- wearesimplydeterminingtheageoftheoldstellarpopula-
barred galaxies, as illustrated by Fig. 1 of JP15. This is a tionexpectedinbulgeregionsofgalaxies;theseobservations
key point for this analysis, and has not been addressed in cover a radial range that in unbarred but otherwise similar
previous studies of stellar populations and SF histories of galaxies is characterised by disk regions with very vigorous
barred galaxies. To make this point more quantitative, we SFactivity.
have estimated the fraction of total SF that might be ex- The first finding from our spectroscopic analysis
pected in an unbarred galaxy in the radial range covered is that the stellar populations in SFD regions can be
by the extracted SFD slit spectra for each of the4 galaxies very old. This is in agreement with the findings of
(cid:13)c 2015RAS,MNRAS000,1–9
8 P. A. James & S. M. Percival
S´anchez-Bla´zquez et al. (2011) for four different barred Wozniak (2007) and Aumer& Sch¨onrich (2015),but this is
galaxies, although their analysis methods were quite dif- an important question for future studies of this type.
ferent. Old stellar populations have also been found within While we confirm the result of JP15 that the SFD re-
bars by some studies (P´erez, S´anchez-Bla´zquez, & Zurita gions of the four galaxies studied have heavily suppressed
2007; P´erez, S´anchez-Bla´zquez, & Zurita SF rates, all four galaxies have strong emission-line peaks
2009; deLorenzo-Ca´ceres et al. 2012; attheirbarcentres.TheobservationspresentedinJP15 in-
deLorenzo-Ca´ceres, Falc´on-Barroso, & Vazdekis 2013), cludethecentralregionsofallfourgalaxies,althoughcentral
in general agreement with ourresults, but notethat in this properties were not discussed in that paper. The observed
case it is not clear how to relate these ages to the bar age [NII]/Hαcentral-regionlineratiosareconsistentwithexpec-
(an old population could have been assembled into a bar tations for SF in the nucleus of one galaxy (NGC 2543), in
morphology very recently). centralringsintwoothers(NGC2712andNGC3185),and
in both nucleus and central ring for the remaining galaxy
However,oneofourgalaxies isconsistentwith SFhav-
(NGC3351).Thisconfirmsthegeneralfindingofotherstud-
ing been truncated only 1 Gyr ago, and another between 1
ies(e.g. P´erez-R´amirez et al.2000;Coelho & Gadotti2011;
and 2 Gyr ago. Thus we find a significant range in stellar
Ellison et al.2011)thatbarscanefficientlyinduceSFinthe
population properties, even within a small sample of four
centralregionsoftheirhostgalaxies,andweshowherethat
early-type barred galaxies. One advantage of the analysis
such SFcan co-exist with theSFD phenomenon.
methodadoptedisthatwegettwomeasurementsoftheSFD
Many studies have discussed the possibility of radial
stellar populations within each galaxy, on opposite sides of
migration of stars. This was first introduced as an effect
thenucleus.Giventhatthereductionofeachregioniscom-
that is driven by spiral arm patterns at their corotation
pletely independent of all the others (so that, for example,
resonances (Sellwood & Binney 2002; Roˇskar et al. 2008;
all of the parameters defining the emission-line correction
Loebman et al. 2011).Otherstudieshaveexplored thepos-
are locally-derived and not averaged across both galaxy re-
sible effects of bar–spiral arm coupling on radial migration
gions),itisreassuringtonotethatthelineindicesmeasured
(Debattista et al. 2006; Brunetti, Chiappini, & Pfenniger
are entirely consistent for all of the pairs of spectra within
2011; Minchev et al. 2011, 2012; DiMatteo et al. 2013), al-
individual galaxies. Thus, we do not find any indications
though the evidence for strong bar-driven migration is not
of variation in stellar population properties within any of
widely accepted (Roˇskaret al. 2012), and S´anchez et al.
these four galaxies. That is not true for the range of index
(2014) argue against strong radial mixing in their study of
strengths across the full sample of four galaxies, which is
metallicitygradientsinbarredandunbarredgalaxies.Ifsuch
much larger than would be expected from measurement er-
radial mixing does occur, it would complicate the interpre-
rors alone, indicating real differences in population ages or
tationoftheresultspresentedhere.However,anymixingof
truncation epochs between these galaxies.
theyoungstellarpopulationsfromeitherthecentralSFpeak
The galaxy sample studied here is too small for any
or thesurroundingSF ring intothe SFD region would only
formal statistical analysis, but it is instructive to look for
strengthenthegeneralresultwefindhere,oflong-suppressed
correlationsbetweenSFDpopulationagesandothergalaxy
SFactivity within the SFD region itself.
properties.TheyoungestpopulationisfoundfortheSFDof
One obvious extension of the analysis presented here
NGC2543(thetwopointslyingonthe1Gyrtruncationage
would be to look at the stellar populations in the disk out-
inFig.8);thisisclassifiedasanSBb,withquitevigorousSF
side the SFD, but between HII regions, to determine the
in the disk outside the SFD region. The second youngest is
characteristic age of the non-star-forming disk population.
foundinNGC2712, anotherSBbwith verystrongdiskSF,
Thiswasattemptedwiththepresentlong-slit data,butdid
that is morphologically very similar to NGC 2543. There is
not prove feasible, as any line-free regions were very small
then a significant jump to greater SFD age for NGC 3351,
andofsuchlowsurfacebrightnessthatitwasimpossible to
whichisalsoclassified asSBb.Theoldest population ageis
fitanyabsorptionlinefeaturesinanysuchdiskregions.This
foundforNGC3185,theonegalaxyinthissampleclassified
demonstrates the unique nature of the SFD regions as ex-
as SBa, and the one that shows weakest (but still clearly
tensive,relativelyhighsurfacebrightnessareasthatarefree
detected) disk SF in its Hαimage (Fig. 3, lower frame).
ofongoingstarformation.Theanalysisofouterdiskregions
Figures1–4showthatthesespectrasampleregionsthat might be possible using integral field units, through theco-
are significantly offset from the main bar axis. However, addition of spectracovering large areas with theremovalof
there is some variation in this offset, with two galaxies ob- all those showing line emission.
servedwiththeslitperpendiculartothebar(NGC2543and As noted in JP15, the analysis presented here only ap-
NGC 3351) while NGC 2712 and NGC 3185 were observed plies to a particular type of bar, that is characteristic of
◦ ◦
atsmallerangularoffsetsof38 and27 respectively.Thisis early-typespirals and is not found in late types. Futurear-
potentiallysignificant,giventhatthebarmodelsofWozniak eas of study include extending this type of analysis to such
(2007) and Aumer& Sch¨onrich (2015) suggest that signif- later type strongly barred galaxies, to determine whether
icant numbers of young stars may be trapped within the thebimodalityinbartypesnotedby,e.g.,Nair & Abraham
bar. However, there is no sign of such effects in the present (2010) and Hakobyan et al. (2014) is reflected in the asso-
sample, and indeed the youngest SFD population is found ciated stellar populations. It would also be useful to apply
for NGC 2543, one of the cases where the slit was perpen- the methods of the present study to the stellar population
dicular to the bar and no contribution from the bar light foundwithinthebodyofthebaritself.Futurestudiesmight
canbeexpected.Giventhesmallsizeofthepresentsample, also beableto constrain radial migration effectsbylooking
and the lack of spectra taken very close to the bar axis, we forage gradientsin theouterpartsof theSFD,as thestar-
cannotmakeanystrongstatementsaboutthepredictionsof forming ring is approached.
(cid:13)c 2015RAS,MNRAS000,1–9
Age constraints for galactic bars 9
6 CONCLUSIONS EllisonS.L.,NairP.,PattonD.R.,ScudderJ.M.,Mendel
J. T., Simard L., 2011, MNRAS,416, 2182
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