Triggered star-formation in the inner filament of Centaurus A

1.4 MB·English

by  R. Mark Crockett

Mon.Not.R.Astron.Soc.000,1–23(2011) Printed18January2012 (MNLATEXstylefilev2.2)
Triggered star-formation in the inner filament of Centaurus A
R. Mark Crockett1, Stanislav S. Shabala2,1⋆, Sugata Kaviraj1,3, Vincenzo Antonuccio-Delogu4,
Joseph Silk1, Max Mutchler5, Robert W. O’Connell6, Marina Rejkuba7, Bradley C. Whitmore5,
2
1 Rogier A. Windhorst8
0 1DepartmentofPhysics,UniversityofOxford,KebleRoad,Oxford,OX13RH,UK
2 2SchoolofMathematics&Physics,UniversityofTasmania,PrivateBag37,Hobart,Tasmania7001,Australia
n 3BlackettLaboratory,ImperialCollegeLondon,LondonSW72AZ,UK
a 4INAF OsservatorioAstrofisicodiCatania,ViaS.Sofia78,CataniaI-95123,Italy
J 5SpaceTelescopeScienceInstitute,Baltimore,MD21218,USA
6DepartmentofAstronomy,UniversityofVirginia,Charlottesville,VA22904-4325,USA
6
7ESO,Karl-Schwarzschild-Strasse2,85748GarchingbeiMu¨nchen,Germany
1
8SchoolofEarthandSpaceExploration,ArizonaStateUniversity,Tempe,AZ85287-1404,USA

]
O
C
18January2012
.
h
p
- ABSTRACT
o
r
t WepresentrecentHubbleSpaceTelescopeobservationsoftheinnerfilamentofCentau-
s
a rusA,usingthenewWideField Camera3(WFC3) F225W,F657N and F814W filters. We
[ findayoungstellarpopulationnearthesouth-westtipofthefilament.CombiningtheWFC3

datasetwitharchivalAdvancedCameraforSurveys(ACS)F606Wobservations,weareable
1
to constrain the ages of these stars to <10 Myrs, with best-fit ages of 1-4 Myrs. No further
v ∼
recentstar-formationisfoundalongthefilament.
9
6 Basedonthelocationandageofthisstellarpopulation,andthefactthatthereisnoradio
3 lobeorjetactivitynearthestar-formation,weproposeanupdatedexplanationfortheorigin
3 of theinner filament.Sutherlandetal. suggestedthat radiojet-inducedshockscan drivethe
. observedopticallineemission.We arguethatsuchshockscannaturallyarise duetoaweak
1
cocoon-drivenbow shock (rather than from the radio jet directly), propagatingthrough the
0
diffuseinterstellarmediumfromalocationneartheinnernorthernradiolobe.Theshockcan
2
1 overruna molecularcloud,triggeringstar-formationin the dense molecularcores. Ablation
: and shock heating of the diffuse gas then gives rise to the observed optical line and X-ray
v
emission.DeeperX-rayobservationsshouldshowmorediffuseemissionalongthefilament.
i
X
Keywords: galaxies:ellipticalandlenticular,cD;ultraviolet:galaxies;galaxies:individual:
r
NGC5128;galaxies:active;intergalacticmedium;galaxies:jets
a
1 INTRODUCTION lobes, potentially indicative of restarting activity (Saxtonetal.
2001; Morgantietal. 1999). A pair of inner lobes at 5 kpc from
ThepeculiarellipticalgalaxyNGC5128isanarchetypalexample
thenucleusco-existwithasingleNorthernMiddleLobeat25-30
ofapost-mergersystem,believedtohaveformedviaamergerbe-
kpc (Fig. 1). On the largest scales, a pair of outer lobes is seen
tweenanearly-typegalaxyandagas-richdiskgalaxy(Israel1998).
at around 100 kpc fromthe nucleus (Feainetal. 2009). The bent
Locatedinapoorgroupatadistanceof3.7Mpc,ithoststheclos-
nature and re-orientation of the jets between these three scales
est powerful extragalactic radio source, Centaurus A. These fac-
suggests interactions between the jets and gaseous environment
tors make Centaurus A the perfect laboratory for studies of star-
areimportant(Gopal-Krishna&Wiita2010).Furtherevidencefor
formationand theinteractionbetween theenergeticoutput of su-
this is provided by observations of X-ray knots along the radio
permassiveblackholesandtheirsurroundings.
jet,mostlikelytriggeredbyjetinteractionwithdensegasclumps
Centaurus A has been extensively studied at wavelengths
(Kraftetal.2009).
ranging from radio all the way through to γ-rays. Radio images
(e.g. Morgantietal. 1999) show multiple synchrotron-emitting It is well-known that interaction between radio sources and
theirenvironment canresultinsignificantquenching of gascool-
ing and star-formation. Such feedback is often invoked to ex-
⋆ Email:[email protected] plain the lack of significant recent star-formation in massive el-
(cid:13)c 2011RAS
2 R. M. Crockettet al.
lipticals(Boweretal.2006;Shabala&Alexander2009).Recently, (NUV)wavelengths,enablesustoisolateandestimatereliableages
Shabalaetal. (2011) have shown that radio sources can suppress forstarsandstellarpopulations.Thelocationandagesofthesestars
star-formation on scales of groups and clusters, by heating and placenewconstraintsonthepossibleoriginandevolutionofthein-
sweepinggasoutofsatellitegalaxies.However,radiojetscanalso nerfilament.
triggerstar-formationbycompressingdensegaswithshortradia- Thepaper isstructured asfollows. InSection2wedescribe
tive cooling times. This phenomenon is modelled in simulations theobservationsandobjectdetection.Identificationofstellarpop-
(e.g.Tortoraetal.2009),andexplainstheso-calledalignment ef- ulationsandderivationofparameters(inparticular,stellarages)is
fectbetweentheradio,UVandopticallineemissionalongthejet presentedinSection3.Wediscusstheimplicationsofourfindings
axisindistantradiogalaxies(McCarthy1993;Bestetal.1996). inthecontextoftriggeredstar-formationandtheAGN-gasinter-
Two filaments, observed as narrow-line optical emission, actioninSection4.WeconcludeinSection5.
are found in the vicinity of the Northern Middle Lobe (Fig. 1) Throughout the paper, a distance of 3.68±0.43 Mpc (µ =
and oriented more or less in the direction of the inner radio jet 27.83±0.24) to Centaurus A is assumed, giving a scale of 1′ =
(Morgantietal. 1999; Blancoetal. 1975; Petersonetal. 1975). 1.07kpc.ThisdistanceistakenfromNED1andisthemeanofsev-
Theinnerfilamentislocated8.5kpcfromthenucleus,some2kpc eral distance estimates made using a range of methods: Cepheid
awayfromtheradiojets(Morgantietal.1999),whiletheouterone variables(Ferrareseetal.2007);TypeIasupernovae(e.g.Jhaetal.
is18kpcfromthecentralengineandalignedwiththeradioemis- 2007; Ruiz-Lapuenteetal. 1992); surface brightness fluctuations
sion. Broad-band optical observations reveal that both filaments (e.g.Ferrareseetal.2007;Shopbelletal.1993);tipoftheredgi-
contain hot, young stars of <10 Myrs old (Rejkubaetal. 2001, antbranch(e.g.Rizzietal.2007;Rejkuba2004;Soriaetal.1996);
∼
2002, 2004; Fassett&Graham 2000; Graham&Fassett 2002). globularclusterradius(Chattopadhyayetal.2009);globularclus-
However, while the young stars appear to be evenly distributed terluminosityfunction(Harrisetal.1984);planetarynebulalumi-
along the outer filament (Rejkubaetal. 2001; Fassett&Graham nosityfunction(Ciardulloetal.2002;Huietal.1993);Miravari-
2000), those in the inner filament are concentrated at its south- ables(Rejkuba2004).
westerntip,closesttothegalaxynucleus(Rejkubaetal.2002).
HIobservationshighlightanotherdifferencebetweenthefil-
aments. Theouter filamentliesinverycloseproximitytoalarge
HIcloud(Schiminovichetal.1994;Oosterloo&Morganti2005). 2 HUBBLESPACETELESCOPEOBSERVATIONS
Thetransversedimensionofthecloudiscomparabletothefilament
Our recent HST observations of the Cen A inner filament were
length,andtherotationalvelocityofthecloudsupportsitfromcol-
madeusing thenew WFC3aspart of theWFC3SOC(Scientific
lapsingtowardsthenucleus(Schiminovichetal.1994).Moreover,
OversightCommittee)EarlyReleaseScienceProgram(HSTpro-
thestellarageof∼ 10−15Myrs(Rejkubaetal.2002)isconsis-
gram 11360, PI: R. O’Connell). These data, taken 2010 Jul 02,
tentwiththecrossingtimeofthecloudacrosstheradiojet,andthe
wereobtainedusingtheF225W,F657N andF814W filters(NUV,
ionizedgasvelocitieswithinthefilamentarystructurearesimilarto
Hα + [N II], and I-band respectively). Of particular note is the
thecloudvelocity(Schiminovichetal.1994;Graham1983,1998).
F225W (NUV) data. Later in this paper, we exploit the sensitiv-
This scenario led a number of authors (e.g. Rejkubaetal. 2002;
ityof theNUV tostellarage inorder toagedatethevery recent
Gopal-Krishna&Wiita2010)tosuggestthattheouterfilamentisa
star-formationinandaroundtheinnerfilament.
primeexampleofrecentjet-inducedstar-formation,whichisprob-
Earlier HST observations of the Cen A inner filament from
ablystillongoing(Mouldetal. 2000;Rejkubaetal.2004).Onthe
2004 Aug 10&11 were found in the HST archive (HST program
otherhand,nosuchHIcloudisobservedinthevicinityoftheinner
10260, PI:W.Harris).TakenwiththeAdvancedCameraforSur-
filament.
veys(ACS),thesedatacompriseofthreeseparatepointings,allin
Despitethelackofcoldgasanddistributedstellarpopulations,
the F606W filter.Thisverybroad ‘V′-band filterspans anumber
theinnerfilamentappearsbrighterandmoretightlycollimatedin
ofprominent emissionlineswhichdominatetheopticalspectrum
opticalemissionlinesthantheouterfilament.Thisemissionisalso
of the inner-filament – most notably [OIII] 4959Å,5007Å, [OI]
lessuniformacrossthefilament,appearingmuchbrighterattheend
6300Å,6370Å,[NII]6548Å,6584Å,Hα,and[SII]6725Å.Thefil-
nearesttothenucleus(Morgantietal.1991).Twopotentialmech-
amentarystructureisthereforestronglydetectedintheF606Wim-
anismshavebeenproposedasbeingresponsibleforthelineemis-
age.
sion: photoionization by the nucleus (Morgantietal. 1991) and
TheACSdatasetsweredownloadedfromtheHSTarchive2at
shockheating(Sutherlandetal.1993).Thephotoionizationmodel
theSpaceTelescopeScienceInstitute(STScI)viatheon-the-flyre-
requiresCentaurusAtobeablazar,whichisatoddswithradioob-
calibration(OTFR)pipeline,andweadoptedthepipelinedrizzled
servationsofitsjets(Hardcastleetal.2003).Italsofailstoaccount
dataasourscienceimages.TheWFC3datawerebias,darkandflat-
for the highest excitation lines and complex velocity structure in
fieldcorrectedlocallyusingcalwfc3,andsubsequentlydrizzledus-
theinnerfilament(Evans&Koratkar2004;Graham&Price1981;
ing the multidrizzle image reconstruction software. The purpose
Morgantietal.1991).Thejet-drivenshock excitationmodel sug-
ofmultidrizzle(boththepipelineandlocalimplementations)was
gested by Sutherlandetal. (1993) can explain both the high and
to align exposures in each filter, correct for geometric distortion,
low-excitationemissionbyinvokingfastandslow-movingshocks.
remove defects such as cosmic rays and hot pixels, and combine
However,thismodelexplicitlyrequiresthepresenceofaradiojet
theexposuresusingthedrizzleimagereconstructiontechniqueof
inthevicinityofthefilament,whichisnotobserved.Italsofailsto
Fruchter&Hook(2002).Thelatestcalibrationfiles,includingim-
predictthedistributedX-rayemissionfoundbyEvans&Koratkar
agedistortioncoefficienttables(IDCTAB),weredownloadedfrom
(2004).
Inthiswork,weemployhigh-resolutionHubbleSpaceTele-
scope(HST)WideFieldCamera3(WFC3)observationstoexam-
ine star-formation in the inner filament. The unique spatial res- 1 http://nedwww.ipac.caltech.edu/cgi-bin/nDistance?name=NGC+5128
olution and sensitivity of WFC3, in particular at near-ultraviolet 2 http://archive.stsci.edu/hst
(cid:13)c 2011RAS,MNRAS000,1–23
Triggered star-formationinCentaurusA 3
Figure 1. Reproduction of Fig.1from Oosterloo&Morganti (2005). Optical image (provided byD. Malin) showing the location ofthe inner and outer
filamentsrelativetothenucleusofCenA,andtheradiojet(whitecontours).TheblackcontoursshowthepositionoftheHIcloud,associatedwiththeouter
filament,whichiscrossingtheradiojet.Angularsizeof1′correspondsto1.07kpc.Notethatthelargediffusepatterninthetopleftcornerisanimageartefact
andnotarealobject.
theWFC3referencefilewebsite3.FurtherdetailsofWFC3/UVIS
Table1.HSTobservationsoftheCenAinnerfilament
datareductionaregivenbyWindhorstetal.(2011).
TheCenAinnerfilamentisstronglydetectedinACSF606W
andWFC3F657N asopticallineemissionfromionizedgas.Itis Date Instrument Filter ExposureTime
situated∼8.5kpctothenorth-eastofthegalaxynucleus,andex- (s)
tendsafurther2200pcalongthesamedirection,withawidthof
2004Aug10 ACS/WFC F606W 2370
∼50-100pc.(Notealloftheaboveareprojectedlengths).Thefirst
2004Aug10 ACS/WFC F606W 2370
500pcof thefilament,beginning inthesouth-west closest tothe
2010Jul02 WFC3/UVIS F225W 2232
galaxy nucleus, consists of threemajor groups of extended emis-
2010Jul02 WFC3/UVIS F657N(Hα+[NII]) 1640
sion.Theseappearasunresolved‘blobs’intheseeing-limitedob- 2010Jul02 WFC3/UVIS F814W 1050
servationsofMorgantietal.(1991)(seetheirFig.2a),wholabelthe
regions A, B and C (following Blancoetal. 1975; Osmer 1978).
3 http://www.stsci.edu/hst/observatory/cdbs/SIfileInfo/WFC3/reftablequeryindex TheHSTimagesinFigs.2–5revealsignificantstructureinthese
(cid:13)c 2011RAS,MNRAS000,1–23
4 R. M. Crockettet al.
regions. Shell-like features are visible, which presumably demar- cludethattheseparticularobjectsaremostlikelyforegroundstars
cateregionsofhigherdensitygas(perhapsduetocompressionby inourownMilkyWay.
shocks),andappeartolieperpendiculartothelineextendingfrom ThehighestconcentrationofblueobjectsisfoundattheSW-
thegalaxynucleus,andalongtheinnerfilament. tipof theinner filament,whichalsoharbours thebrightest of the
The filament continues to the north-east with decreased sur- blue objects. It is also the only region that such blue objects are
face brightness, and appears to be split into two further regions, found in association with strong line-emission. The colours and
labelledEandFbyMorgantietal.(1991).4 Themorphologiesof magnitudesoftheseobjects(discussedmorefullyinthefollowing
regions Eand F arelesswell defined intheHST imagerydue to sections) coupled withtheline-emissionsuggeststhepresence of
lowsurfacebrightness.However,therearetentativetracesofshell youngstarsorstarclusters.Figs.4,5&6showprogressivelymag-
structures orthogonal to the main axis of the filament, similar to nified views of the SW-tip in greyscale and in colour, where the
thoseseeninregionsAtoC. RGB colour images have been constructed by assigning F814W,
F606W, and F225W to the red, green and blue channels respec-
tively.Thehot,bluestarsareclearlyseenconcentratedintwore-
gions at the SW-tip. No other blue stars are seen anywhere else
2.1 Objectdetection
alongtheemission-linefilament5.
A unique aspect of theWFC3dataset isthe inclusion of F225W Thereader should notethatthespeckled background visible
observations.ThesediffractionlimitedNUVimagesaremostsen- inthe F606W and F814W imagesinFigs.3&4or inthecolour
sitivetothehottest,youngeststars,andofferusclearinformation imagesshowninFigs.5&6)isnotduetonoise,butratheristhe
astotheirpropertiesandlocationinandaroundtheinnerfilament. myriadofbackgroundstarsdetectedinthebodyofCenA.Interest-
Giventhatotherpurelyopticalstudieshavealreadybeenpublished ingly,thestellarfieldcoincidentwiththeemission-linefilamentis
(e.g.Rejkubaetal.2002)wehavefocussedourattentiononthose indistinguishablefromthisgeneralbackground,exceptattheSW-
objectsthataredetectedinF225W. tipwhereweseethedenseconcentration of(presumably) young,
We used daofind – part of the iraf daophot package NUV-bright stars. This suggests that recent star-formation in the
(Stetson 1987) – to automatically identify point sources in the innerfilamenthasbeenpredominantlyconfinedtotheSW-tip,and
F225W,F606W and F814W data. A 5σ detection threshold was thatno–oratleastnosignificant–recentstar-formationhastaken
applied. The resulting coordinate tables were cross-correlated to placeanywhereelsealongthefilament.
extract those objects detected in the F225W image and at least ThiscontradictstheobservationsofRejkubaetal.(2002)who
one other filter. Cross correlation of the ACS and WFC3 object reportedthedetectionofyoungstars(<10Myrs)alongtheentire
∼
positionsrequiredatransformationbetweentheACSF606W and lengthoftheinnerfilament(althoughtheyalsonotedtheconcen-
WFC3 F814W coordinate grids, which was calculated using the trationattheSWtip).OurFig.7isapartialreproductionofFig.4
iraftaskgeomapandthepositionsof37pointsourcescommonto fromRejkubaetal.(2002),showingthepositionsofthebluest(U-
bothimages.TheACSF606W coordinateswerethentransformed V)<-0.5stars(blacktriangles)whichtheydetectedalongtheinner
totheWFC3coordinateframeusingthetaskgeoxytran. filament.Wehaveoverlaidacolourcompositeimage,constructed
Afinalvisualinspectionofthedatawascarriedouttocheck fromthesamebroadbandU(blue),V(green)andI(red)Magellan
for any other common sources missed by daofind and cross- TelescopeobservationsasusedbyRejkubaetal.intheiranalysis.
correlation, adding 17 extra objects to the list. Several of these Itisclearthatthemajorityoftheseapparentlybluestarsliealong
weresaturatedobjectsthathadbeenrejectedbydaofindduetotheir thefilament,whichalsoappearsblueinthisfigure,mostlikelydue
shapecharacteristics.Otheraddedobjectswerejustbelowthede- to[OII] 3727Å line-emissiondetectedinthebroad U-band filter.
tectionthresholdand/orpartiallyblendedwithneighbouringbright ApartfromtheSWtipandafewother outlying sources, noneof
objectsinoneormorefilters. these blue stars are detected in our WFC3 F225W image. Given
ThepositionsofallobjectsdetectedinF225Wandatleastone theresolutionanddepthoftheHSTdata,wewouldhaveexpected
otherbroadbandfilterareplottedonthetwoimagesinFig.3.The strongNUVdetectionsifhot,youngstarswerepresent.
positionmarkershavebeencolour-codedinrelationtotheobserved If the dust extinction along the rest of the filament is much
colourofeachoftheobjects,andthisphotometrywillbedescribed higherthanattheSWtip,itmightbepossiblethattheNUVlight
fullyinthefollowingsection.However,itisusefultodiscussnow from the young stars is fully attenuated. However, Rejkubaetal.
therelativemorphologiesofthetwomainpopulationsweseeinthis (2002) find no evidence for such high extinction. Another expla-
figure–thosewithblueNUV-opticalcolours[(F225W−F814W) nationmightbethattheU-bandphotometryfromRejkubaetal.is
<0;bluemarkers]andthosewithredcolours[(F225W−F814W) contaminatedby[OII]3727Ålineemission,leadingtoartificially
>0;redandgreenmarkers]. bluer photometry. Lacking narrow-band imagery, Rejkubaetal.
Themostobviousdifferenceisintheirdistributionsacrossthe had torelyon sky-background subtraction tocorrect for underly-
field-of-view. Red objects appear quite evenly spread, while blue ingline-emissionintheirstellarphotometry,particularlyintheU-
objectsaremostlytotheright(southwest)oftheframe,closestto band. We can see from the Magellan and HST images presented
thegalaxynucleusandtheradiojet. herethatthesurfacebrightnessofthefilamentchangesrapidlyon
Many of the red objects are very bright, or even saturated, shortspatialscales.Thismakesitdifficulttodetermineanappro-
in F606W and F814W and have diffraction spikes or show clear priatesky-backgroundlevelforanyindividualobjectalongthefil-
diffractionpatternsaroundthecentralpeakoftheirPSFs.Wecon- ament.Forexample,whereanobjecthappenstocoincidewithare-
gionofhigheremission-lineflux,thebackgroundleveldetermined
4 Thelackofaregion‘D’inMorgantietal.(1991)relatestothelabeling
schemeofBlancoetal.(1975),whoidentifiedregionsA,BandCofthe 5 5σ detection limits (Vegamag) are: F225W = 25.0; F606W = 27.4
innerfilament,andused‘D’todenotewhatisnowreferredtoastheouter (noemission-linebackground),F606W=26.8(strongemission-lineback-
filament. ground);F814W=25.8
(cid:13)c 2011RAS,MNRAS000,1–23
Triggered star-formationinCentaurusA 5
from the surrounding pixels will be an underestimate, leading to
brightersourcephotometry.
Thismayhavebeen thecase withsome of theU-band pho-
tometryinRejkubaetal.(2002),andwouldnaturallyexplainwhy
themajorityoftheirbluestarcandidatesarefoundassociatedwith
thebrightestregionsofthefilament.Ofcourse,onecouldalsoar-
gue that the association of young stars with regions of brighter
line-emission is a direct consequence of their photoionization of
thesurrounding gas. Nevertheless, without narrow-band data itis
difficulttoruleout contamination intheU-bandphotometry, and
ourWFC3F225W observationstronglysuggeststhatveryyoung
starsarealmostentirelyconfinedtotheSWtip.
InlightofourHSTdata,itseemsreasonabletosuggestthat
photoionization by young stars (<10 Myr old) is contributing to
the strong line-emission at the SW tip. Indeed, Graham&Price
(1981) havealready shown that a spectrum of thisregion is sim-
ilartothatofanormalHIIregionpoweredbyembedded,hotstars.
However,theapparentlackofyoungstarsalongtherestofthefil-
ament, and the appearance of high excitation lines, suggests that
someothermechanismmustbeexcitingthegasintheseregions.As
mentionedintheintroduction,twomechanismshavealreadybeen
proposed to explain the line emission in the inner filament: pho-
toionizationbythegalaxynucleus(Morgantietal.1991)andshock
heating (Sutherlandetal. 1993). Later in this paper we present a Figure7.Partialreproduction ofFig.4ofRejkubaetal.(2002)showing
model, based on the Sutherland et al. shock heating hypothesis, thepositionsofthebluest(U-V)<–0.5stars(blacktriangles)whichthey
whichincorporatestheformationoftheinner filament,theX-ray detectedalongtheinnerfilament.Here,wehaveoverlaidacolourcompos-
fluxandopticallineemission,andtherecentstar-formationatthe iteimage,constructedusingtheU (blue),V (green)andI(red)Magellan
south-western tip. In the following section we focus on the star- Telescopeobservations usedbyRejkubaetal.2002intheiranalysis.The
smallsquaredelineatesKnotAofRejkubaetal.(2002),whichweidentify
formation,andattempttofitagestotheyoungstellarpopulations
withtheSWtip.
usingHSTphotometry.
tudesV >25.5–26mag.TheF606W photometryofthereddish
3 STAR-FORMATIONINTHEINNERFILAMENT objectsisconsistentwiththeirbeingRGBstars,indicatingagesof
>1Gyr.Itisworthstressingthatnoneoftheredobjectsvisiblein
As described above, two regions in the south-western tip of the Fig.6closesttotheSW-tiparelikelysupergiants,astheirabsolute
inner filament (Figs. 2&5) shows signs of young, massive stars; magnitudes(typicallyM >–2)aretoofaint.
F606W
namelybright,compactobjectsvisibleinalloftheHSTobserva- ItisinstructivetodeterminetheagesoftheNUV-brightstellar
tions(NUVto I-band).Fig.6shows magnifiedcolour composite populationswithintheinnerfilament,particularlyinthecontextof
andgreyscaleF225W imagesofthesefields.Inthecolourimages, theSutherlandetal.(1993)shockheatingframework.Thepossible
theF225W dataisshowninblue,F606W ingreen,andF814W in associationofanAGNrelatedshock withagascloudandyoung
red. starspointstowardsjet-inducedstar-formation–i.e.positiveAGN
The diffuse green glow prominent in the north of Region 1, feedback. In the following sections we carry out photometry on
and concentrated around the NUV-bright star clusters, is strong theHSTACSandWFC3datasets,andsubsequentlycomparethe
line-emission detected inthebroad F606W filter.Incontrast, the observedmeasurementstomodelsofsinglestarsandstellarpopu-
blueobjectsinthesouthofRegion1,inRegion2,andthroughout lationstodetermineages.Althoughwepresentphotometryforall
the rest of the field show little or no sign of such line-emission. sourcesdetectedintheF225W filter,ouranalysisandsubsequent
Thiscouldsuggestoneoracombinationofthefollowing:(1)that discussionisfocussedonthedenseconcentrationofblueobjectsat
the photo-ionizing flux emitted by these starsistoo low; (2) that theSW-tipandthepotentiallinkbetweentheirformationandthat
thegasdensityintheseareasislowerthaninthenorthofRegion oftheemission-linefilament.
1;or(3)thatgashasbeensweptoutbystellarwindsorshocks.
Region2(situatedtothesouth-westofRegion1andcloserto
thegalaxynucleus)containsfewerNUV-brightobjects,andshows 3.1 Photometry
onlytracesofline-emissionarounditstwobrightestNUVcompo-
3.1.1 PSF-fitting
nents. It is detected in [OIII] by Morgantietal. (1991) (see their
Fig.2a),butatmuchlowerintensitythanRegion1.Thisagainsug- Photometrywascarriedoutusingpointspreadfunction(PSF)fit-
gests lower photo-ionizing fluxesand/or gas densities than inthe tingtasks within the iraf daophot package (Stetson1987). Vega-
northernpartofRegion1. mag zeropoints for both the WFC3 and ACS instruments were
Alsovisibleinthecolourimagesisamultitudeofyellowand taken from their respective webpages hosted by STScI6,7. Only
redobjectsthatarenotdetectedintheNUV.Theseareapparently
part of the background stellar population in Cen A as these stars
appear throughout the HSTfield. FromRejkubaetal. (2004),we 6 http://www.stsci.edu/hst/wfc3/phot zplbn
expect thered giant branch (RGB) inCenA to appear at magni- 7 http://www.stsci.edu/hst/acs/analysis/zeropoints
(cid:13)c 2011RAS,MNRAS000,1–23
6 R. M. Crockettet al.
Figure2.CombinedWFC3F657N(Hα+[NII])andF814WimageoftheCenAinnerfilamentanditsimmediateenvironment.Thefilament,observedhere
asHαemission,issplitintofivemajorregionslabelledA,B,C,EandF,followingthelabellingschemeofMorgantietal.(1991).(TheabsenceofaregionD
ishistoricalandrelatestothediscoveryofregionsA,BandCoftheinnerfilamentbyBlancoetal.(1975),whodenotedtheouterfilamentasregionD.)The
filamenthasaprojectedlengthof∼125′′(2200pc),andwidthof∼3−6′′(50-100pc).Itsappears,atleastinprojection,toliealongalineoriginatingatthe
galaxynucleusanddirectedtothenorth-east.Themajorityofbrightsourcestothenorth-westofthefilamentareGalacticstarsalongthelineofsight,although
twoCenAglobularclustersaremarked(GCs1&2).Noyoungstarsarefoundalongthelengthofthefilament,exceptintworegionsinthesouth-westerntip
(labelledregions1&2).
Figure3.ACSF606WandWFC3F814WimagesshowingthelocationsofallobjectsdetectedinF225Wandatleastoneotherfilter.Bluemarkersindicate
objectswithobserved(F225W−F814W)<0,whileredmarkersdenotethosewith(F225W−F814W)>0.Greenmarkersdenoteobjectsthataresaturated
intheWFC3F814Wdata.ThedifferentpointingoftheACSF606Wimageresultsinseveralobjectsfallingoutsidethefield-of-viewtothesoutheast.The
innerfilamentisclearlyvisibleinthecentreoftheF606Wframe,runningrighttoleft(southwesttonortheast).Aconcentrationofblue,NUV-brightobjects
islocatedatitssouthwesterntip,butnoothersuchobjectsarevisiblealongtherestofthefilament.Otherblueobjectsaredetectedintherestofthefield–
althoughnowherewiththesameconcentrationasobservedattheSW-tip–withthevastmajoritylocatedinthewestbetweenthefilamentandtheradiojet.
Theredandsaturatedobjects(redandgreenmarkers)aremoreevenlydistributedacrossthefield-of-view,andmanyofthesearelikelyforegroundstarsinour
ownGalaxy.Some,includingGC1,areconsistentwithbeingglobularclustersinCenA.
(cid:13)c 2011RAS,MNRAS000,1–23
Triggered star-formationinCentaurusA 7
Figure4.WFC3F225W,ACSF606WandWFC3F814WimagesectionsshowingmagnifiedviewofregionsA,BandCoftheinnerfilament,including
theSW-tip.Blackcirclesmarkthepositionoftwolikelyforegroundstars.ItisimmediatelyclearfromtheF225WimagethattheNUV-brightsourcesinthis
figureareconfinedentirelytothetworegionsintheSW-tip.NootherNUVpoint-sourcesaredetectedcoincidentwithanyofthelineemissionanywhereelse
alongthissectionofthefilament.
Figure5.RGBcolourcompositeimageofregionsA,BandCintheCenAinnerfilament.ThisimagewascreatedusingtheWFC3F225W (blue),ACS
F606W(green)andWFC3F814W(red)observations.TheHαemissionofthefilamentfallswithintheF606Wpassbandandisthereforevisibleinthisfigure
asdiffusegreenemission.AtHSTresolution,regionsA,BandCrevealashell-likestructure,withtheshellsorientedperpendiculartothemainaxisofthe
filament.Noyoungstarsareobservedcoincidentwiththefilament,exceptatthesouth-westtipintheregionsmarked1and2.Inthesefieldsweseesignificant
NUV-flux,consistentwiththepresenceofyoung,massivestars.
(cid:13)c 2011RAS,MNRAS000,1–23
8 R. M. Crockettet al.
Figure6.Magnifiedcolourcomposite(left)andgreyscaleF225W(right)imagesofRegions1and2inthesouth-westerntipoftheCenAinnerfilament.All
imagesareorientedsuchthatnorthisupandeastistotheleft.NUVbrightsourcesareidentifiedintheright-handpanels.TheIDnumberscorrespondtothose
usedinFigures11&12(seeSection3.2.2).Thereadershouldnotethatobject1isnotcoincidentwiththebrightyellowobjectvisibleinthecolourimage–it
isthefainterblueobjectimmediatelytothenorth-east.
those sources detected in F225W and at least one other filter dominatetheopticalspectrumoftheinnerfilament.Thestrongest
wereanalyzed,andthisphotometryispresentedinTableA1.The oftheseare[OIII]5007Å,andHα6563Å.
matched coordinate lists (as described in §2.1) were input to the WehaveusedtheWFC3F657N narrowband datatocorrect
allstar task, along with model PSFs for each of the broadband theF606W imageforHαand[NII]6548Å,6584Åemission. First,
filters.EmpiricalPSFmodelswereconstructedusingseveral,rel- wealignedandre-sampledtheF657N datatothecoordinategrid
atively bright stars in each image. Ideally, isolated stars should of the F606W image using the iraf tasks geomap and gregister.
be used and this was possible in the F225W image. The highly Theratiobetween the F606W and F657N continuum fluxes was
crowded F606W and F814W images required a multi-stage pro- calculated using synphot – part of the iraf stsdas package – and
cess,inwhichaninitialPSFmodelwascreatedandsubsequently thespectrumofaK-typemainsequencestar.Suchstarshaveonly
usedtosubtractallobjectsclosetothePSFstars.Asecondmodel veryweakHαabsorptionlinesandwecouldthereforeassumethat
was then generated from the cleaned image and the process re- the calculated fluxes in both filters represented stellar continuum
peateduntilthephotometryofthesubtractedobjectsconverged. only.AppropriatescalingoftheF606W datathereforeallowedus
The crowded nature of the F606W and F814W images in toapproximatethecontinuumemissionofallstellarsourcesinthe
general, and of Regions 1 & 2 in all filters, necessitated the use F657Nimage.Here,wederivedascalefactorof0.04.TheF606W
of smallfittingradii inorder toproperly measurephotometry for imagewasscaledandsubtractedfromthenarrowbanddataresult-
close or partially blended objects. These radii were 1.8 pixels in inginanF657N imagefreefromstellarcontinuum.Thisemission
WFC3/UVISand1.5pixelsinACS/WFC.Medianskybackground lineimagewasrescaledbyafactorof1.68(again,calculatedusing
levelsweremeasuredinconcentricannuliwithinternalandexter- synphot)totakeintoaccountthehigherthroughputoftheF606W
nalradiiof1.5′′and2′′respectively. filterattheHαand[NII]wavelengths,andfinallysubtractedfrom
ContaminationoftheACSF606Wimagebystronglineemis- the original F606W image. The final F606W image was thereby
sionposedaparticularproblemforthephotometryofsomeobjects correctedforcontaminatingHαand[NII]lineemission.
inRegions1.Crowdingandtherapidvariationofthelineemission OurcorrectedF606Wimagestillcontainsemissionfromsev-
onsmallspatialscalesmadeitimpossibletoaccuratelymeasurethe eralotherlines,butwithoutadditionalnarrowbandobservations–
lineemissionusingannularapertures.Asmentionedpreviouslyin inparticularF502N whichtargetsthe[OIII]5007Åline–itisdif-
§2,theF606Wfilterspansseveralprominentemissionlineswhich ficulttoremoveallofthecontamination.Inanattempttoestimate
(cid:13)c 2011RAS,MNRAS000,1–23
Triggered star-formationinCentaurusA 9
thefluxcontributionsoftheremainingemissionlines,wecreateda 3.2 Ageestimation
syntheticspectrumusingthelineratiosrecordedbyMorgantietal.
Wehavetakentheconservative approach ofcomparing allof the
(1991) for the area closest to our Region 1. synphot was used to
photometrytomodelsofbothstellarpopulationsandsinglestars.
calculatethecountratesinthe F606W and F657N filtersforthis
Profile fitting suggested that most of the objects were consistent
syntheticspectrum,fromwhichweestimatedthattheemissionline
with the stellar PSF, but one cannot rule out the possibility that
countsinthebroadbandimageshouldbearound4.2timeshigher
some of these might be close multiple systems (binaries etc.) or
thaninthenarrowband.However,thishighscalefactorleadtoan
low-mass,compactstellarclustersbasedonprofilecharacteristics
over-subtractioninthevicinityofthebrighteststars/starclustersin
alone. While it is true that neither model isparticularly suited to
Region1,observedasnegativecountvaluesinthepixelssurround-
thesespecificpossibilities,thecomparison ofphotometry toboth
ingtheseobjects.Thissuggeststhatthelineratiosintheemitting
modelsallowedustoidentifythoseobjectsthataremoreconsistent
gascoincidentwiththesestars/starclustersmightbedifferentfrom
withoneortheother,thosethatareconsistentwithboth,andthose
thosemeasuredelsewhereinthefilament,andindeedvaryingline
thatarenotwellfittedbyeithermodel.
ratiosareobservedbyMorgantietal.(1991)–e.g.[OIII]5007Å/
Ulitimately,ourmainaimwastodeterminereliableandrobust
Hα.
ageestimatesfortheNUV-brightobjectsintheSWtipoftheinner
Sincenoreliablefull-correctionwasdeemedpossiblewithout
filament.Thiswasachievedthroughtheconvergenceoftheresults
furtherobservationswehaveusedtheF606W imagecorrectedfor
fromstellarpopulationandsinglestarmodelfittingasdescribedin
Hαand[NII]emissiononly.Asaresultobjectphotometryinareas
thefollowingparagraphs.
ofstronglineemission–thenorthofRegion1(Fig.6)–willmost
likelybeartificiallybrightenedduetouncorrectedflux.
3.2.1 Simplestellarpopulations
InFig.8weplotourHSTphotometryincolour-vs-colourdiagrams
alongwithmodelcoloursofsimplestellarpopulations(SSPs).Syn-
3.1.2 Partiallyresolvedobjects
theticWFC3andACSphotometryforagridofSSPsbasedonthe
Whilethe majority of objects are relatively well fit by the stellar Padovastellarmodels(Girardietal.2000;Marigoetal.2008)was
PSF,afewpossessprofilesthataresignificantlybroaderthanthat obtained from the CMD web-interface, hosted by the Osservato-
of a point source. In the case of objects 81 (also labelled GC 1 rioAstronomicodiPadova8.Modelphotometrywascalculatedfor
in Figs. 2&3) and 109 (see Table A1) the partially resolved ap- agesof106to2×109yrswithincrementsof∆log(Age[yr])=0.05,
pearance is obvious to the naked eye, and they are identified as metallicitiesofZ=0.002to0.030(0.1to1.5Z )withincrements
⊙
globular cluster candidates within Cen A. Photometry was mea- of∆Z=0.002,andassumingaKroupa(2001)initialmassfunction
sured inlargeapertures withradiiof 0.80′′ and 0.47′′ for objects (IMF).Theuncertaintiesinthesyntheticphotometryweretakento
82and109respectively,theaperturesizeschosentoberoughly3 be0.05magsintheopticalfilters,and0.1magsfortheNUV(Yi
timestheFWHMofeachobjectintheF606W image.Theseradii 2003).TheSSPmodelsshownplottedinFig.8areofroughlyso-
reflectanapproximatecut-offbetweeneachclusterandthestellar larmetallcity(Z=0.020)withextinctionsofE(B−V)=0.0,0.115
background. Thehigh density of background point sources made and0.345 (zero, 1×and 3×foreground extinction; Schlegeletal.
itimpossibletoreliablyisolateobjects82and109inF606W and 1998).
F814W,whichsubsequentlypreventedtheaccuratedetermination ThephotometryinFig.8isshowndividedintothreegroups;
of total brightnesses. However, the measured colours are consid- Regions1and2,bothsituatedattheSWtipoftheinnerfilament,
eredreliable. andtherestof theWFC3/ACSfield(seealsoTableA1).Bydef-
Objects12,15and16–locatedinthethenorthernpartofRe- initiononly thoseobjectswithphotometry inallthreebroadband
gion1(seeFig.6)–werealsofoundtobepoorlyfitbythestellar filtersareplottedinthesediagrams,therebyexcludingobjectsthat
PSF,suggestingtheyarelikelystarclusters.Weemployedamodi- werefoundtobesaturatedorfelloutsidethefield-of-view ofthe
fiedversionofishape(Larsen1999)tomodeleachasaconvolution ACSF606Wimage.Thiseffectivelyremovedallofthebrightfore-
ofthestellarPSFandanintrinsicsourcefunctionofvariablesize groundstars.Visualinspectionofobjects25and29inRegion2re-
andshape. vealedthattheF814Wdetectionswerenotexactlycoincidentwith
Thestandardimplementationofishapedoesnotfunctionwell thoseinF225W,suggestingthattwodifferentobjects–oneredand
whentargetsarepartiallyblended,whichwasthecaseforobjects one blue – had been observed in each case. Whether the F606W
15and16.Despiteapixelweightingprocedurethatwasdesignedto detections were of one, the other, or a blend of both the red and
mask-outneighbouringobjects,thefittedprofilesfortheseobjects blueobjectswasunclear,duemostlytothetransformationrequired
invariablyfellbetweentheirrespectivecentroids.Ourcrudemodi- tomapthe F606W imagetotheWFC3coordinateframeandthe
ficationwastofixthecentroidofthefittedprofileatauser-defined lowerresolutionoftheACSinstrument.Thephotometryofobjects
position,andtolimittheradiuswithinwhichχ2-minimizationoc- 25and29hasthereforenotbeenplotted.Object24,alsoinRegion
curstoonesmallerthantheradiususedtoextractthefittedmodel 2,fallsoutsidetherangesplottedinFig.8havingcoloursinconsis-
– analogous to the fitrad and psfrad parameters in daophot PSF- tentwithSSPsandsinglestarmodels(§3.2.2)–againsuggesting
fittingtasks. Theresidual images for objects 15 and 16 from our thedetectionofdifferentstarsindifferentfilters.
modifiedcodeshowedamarkedimprovementoverthoseproduced ThephotometryofRegion1iscolour-codeddependingonthe
bythestandardversionofishape. location of each object. Objects coincident with strong levels of
Theishapemodelswereusedtodefinefilter-dependentaper- line-emissioninthenorthareplottedingreen, whilethoseinthe
turecorrections,whichweresubsequentlyappliedtosmallaperture souththatareassociatedwithlittleornodiscernibleline-emission
photometryofobjects12,15and16.Radiiof1.8WFC3/UVISpix-
elsand1.5ACS/WFCpixelswerechosentopreventoverlappingof
aperturesduringphotometryofobjects15and16. 8 http://stev.oapd.inaf.it/cgi-bin/cmd
(cid:13)c 2011RAS,MNRAS000,1–23
10 R. M.Crockett et al.
Figure8.PhotometryofdetectedNUV-brightobjects(reddiamonds)overlaidonmodelSSPs.Themetallicityofeachmodelgridissolar.Threeextinction
levels areshown:zero(dot-dashedline),foreground(solidline)and3xforeground(dottedline).Thebluecrossesineachpanel,goingfromlefttoright,
indicatethelocationsof5,10,100and1000Myrpopulationsontheforeground-extinguised modelgrid.ThephotometryofobjectsinRegion1hasbeen
splitintotwogroups:northernobjectsassociatedwithstrongline-emission(greendiamonds);southernobjectsassociatedwithlittleornoline-emission(red
diamonds).Thenorthernobjectsappearsignificantlyredderthanthoseinthesouth,mostprobablyduetohigherextinction.Seetextfordetaileddiscussion.
areplottedinred.ItisclearthatobjectsinthenorthofRegion1 Cervin˜o&Luridiana(2004)calculatethatU-bandphotometryofa
aresystematicallyredderin(F225W−F814W)thanobjectsinthe 4-10MyrclustercanreachMmin of10,000-3,000M ,dropping
⊙
south.Assumingthetwopopulationsareofsimilarage,withsim- aslowas300M fora100Myrcluster.Assumingacontinuation
⊙
ilarintrinsiccolours,suggeststhatthenorthernpopulationismore ofthetrendtoshorterwavelengths,ourF225W(NUV)photometry
heavily extinguished – to be expected given its association with shouldenableevenlowerMmin,butherewehaveoptedtoassume
significantamountsof(line-emitting)gas.Itshouldalsobenoted theU-bandderivedvaluesofCervin˜o&Luridiana(2004)ascon-
that the (F606W −F814W) colour of some objects in Region 1 servativelimits.
maybesignificantlybluerthantheiractualvaluesduetothepartial AssuminganobjecttobeaSSP,wecanmakecrudeestimates
correctionofemissionlinefluxasdescribedin§3.1.1. ofitsmassbytakingroughageandextinctionvaluesfromFig.8
Several points are immediately obvious from Fig. 8: (i) the andcomparingtheabsolutephotometrytomodelSSPs.Usingages
coloursofthevastmajorityoftheNUV-detectedobjectsarecon- of 2to 5Myr and extinctionof E(B-V)=0.35 weestimated ob-
sistentwithstellarpopulationslessthan10Myrold;(ii)allofthe ject15–oneofthebrightestinRegion1–toberoughly1,800-
objects in Regions 1 and 2 of theinner filament have (F225W − 2,700 M .Thisisaround 2 to3timeslower thanour Mmin limit
⊙
F814W) colours consistent with such young stellar populations; andwecanthereforeconcludethat,formostobjects,ageandmass
(iii)objectsinthenorthernpartofRegion1aremoreheavilyextin- estimatesderivedfromSSPswillhavelargeuncertainties10.
guishedthaninthesouth;(iv)assumingforegroundextinction,the InanattempttobetterutilizeSSPstoestimateagesinRegions
bluestobjectsintherestoftheWFC3/ACSfieldmaybeonaverage 1 and 2, we combined the individual object photometry within
slightlyolderthanthoseinRegions1and2;(v)thereddestobjects each region and compared the resultant colours tothe entire grid
[(F225W−F814W)>1.5]arepotentiallyglobularclustersofinter- ofmodel SSPs.Bycombiningthephotometrywithineachregion
mediateage(∼1Gyr).Thisincludesobject81(alsolabelledGC1 we effectively increased the mass of the fitted stellar population.
inFig.2)andobject109,thevisiblyextendedclustercandidates9. However,wealsoimplicitlyassumedthatalltheobjectswithina
It is important at this point to recognize the limitations of region were coeval. This assumption might be reasonable if star-
SSPsinmodelingrealstellarpopulations.SSPscanbeusedtoac- formation was triggered by an event which traversedeach region
curatelymodel massiveclusters(∼ 106M ),butarelesseffective onashorttimescale;e.g.ashock.TheestimatedmassforRegion
⊙
when considering clusters of lower mass. Model SSPs assume a 1–usingagesof2-5Myr andextinctionofE(B−V)=0.115to
fully populated IMF which isonly truein the limit of an infinite 0.35–wasfoundtoberoughly2,000to11,000M ,therangein
⊙
number(andhencemass)ofstars.StochasticsamplingoftheIMF massduemostlytothatinextinction.TheestimatedmassforRe-
inlowmassclusters,particularlywithreferencetothemostmas- gion2wasmuchlowerataround500to1,000M ,assumingonly
⊙
siveconstituentstars,canleadtosignificantdiscrepanciesbetween foregroundextinction(E(B−V)=0.115;Schlegeletal.1998).In
modelcoloursandobservedphotometry(e.g.Dolphin&Kennicutt eithercasethecombinedmassestimatesstillfallcloseto,orbelow
2002; Cervin˜o&Luridiana 2004; Jametetal. 2004; Bastianetal. thevaluesofMmindiscussedearlier,andwemustthereforeexpect
2005).Cervin˜o&Luridiana(2004)havecalculatedminimumclus- largeuncertaintiesinourcomparisonwithmodelSSPs.Toatleast
ter masses (Mmin) below which cluster colours are expected to partiallyaccountforthisinourSSPmodelfittingweincludedan
showsignificantdispersionincomparisontomodelSSPs.AtMmin uncertaintyof±0.35magineachphotometricband–equivalentto
thisdispersionisreportedtobeatleast0.35maginanygivenfil- thedispersionnotedbyCervin˜o&Luridiana(2004)forphotome-
ter.Mmin isdependentonboththeageandmetallicityoftheclus- tryofclusterswithmassesofMmin.
ter, and also the photometric band in which it is observed, with The observed colours were compared to the entire grid of
shorterwavelengthobservationspermittinglowervaluesofMmin.
10 Incontrast,wederiveamassof2.5×106M⊙forthelargeglobularclus-
9 GC2 was not detected in F225W and is therefore not included in our tercandidateobject81(alsolabelledGC1inFig.2),assuminganageof1
photometry. GyrandforegroundextinctionofE(B-V)=0.115
(cid:13)c 2011RAS,MNRAS000,1–23