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The Outer Cluster System of NGC1399: Preliminary Results 5 0 P 0 2 ∗ o YlvaSchuberth n SternwartederUniversitätBonn,AufdemHügel71,D-53121Bonn,Germany S a J E-mail: [email protected] ( 0 TomRichtler 1 B UniversidaddeConcepción,DepartamentodeFísica,Casilla160-C,Concepción,Chile 1 E-mail: [email protected] D v M 6 BorisDirsch 6 UniversidaddeConcepción,DepartamentodeFísica,Casilla160-C,Concepción,Chile H 1 1 E-mail: [email protected] 2 0 5 MichaelHilker 0 0 SternwartederUniversitätBonn,AufdemHügel71,D-53121Bonn,Germany / 0 h E-mail: [email protected] p 4 - SørenLarsen o ) EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,D-85748Garching,Germany r t E-mail: [email protected] 0 s a : 6 v We present preliminary results of our dynamical study of the outer globular cluster system of i 5 X NGC1399,thecentralgalaxyintheFornaxcluster. About160newradialvelocitiesforglobular r clustersatprojectedgalactocentricdistancesbetween8′ and18′ indicatethattheconstantveloc- a itydispersionof∼276kms−1(forallclusters)alreadyfoundfortheinnerregioncanbetracedout to80kpc.Wefindthatthekinematicalpropertiesoftheblue(metal-poor)andthered(metal-rich) globularclustersubpopulationsappeartobedifferent: Whilethevelocitydistributionofthered clustersissymmetricwithrespecttothesystemicvelocityofNGC1399,theblueclustersshowa somewhatasymmetricdistribution,withmorevelocitiesabovethesystemicvelocity. BaryonsinDarkMatterHalos 5-9October2004 Novigrad,Croatia ∗ Speaker. PublishedbySISSA http://pos.sissa.it/ TheOuterClusterSystemofNGC1399:PreliminaryResults YlvaSchuberth 1. Introduction NGC1399, the central giant elliptical in the Fornax Cluster has long since been known to host a very populous and extended globular cluster system (GCS). In the Southern hemisphere it is the prime target when using globular clusters as probes for the gravitational potential of the host galaxy. Recent wide-field photometry in the Washington system (Dirsch et al. 2003) provided the database to select GC candidates for the kinematical and dynamical study of the NGC1399 GCS presented by Richtler et al. (2004) and Dirsch et al. (2004). They used the VLT with FORS2/MXUto obtain asample of 470 GC velocities in a radial range of 2′ <R<9′ (11 to 50kpc in a distance of 19 Mpc), the largest sample of GCvelocities measured until then. Briefly, P their main findings are: For the entire sample, they find a radially constant velocity dispersion of o σ = 325±11kms−1. Omitting the extreme velocities, i.e. taking into account only the clusters S withinthevelocityinterval800<3<2080kms−1 (seeFig.1),theyfindσ =276±11kms−1. In- all ( specting blue (metal-poor) and red (metal-rich) clusters separately, the corresponding dispersions read: σ =291±14kms−1 andσ =255±13kms−1. Usingadynamicalmodelonthebasisof B blue red the spherical Jeans equation, they derive – under the assumption of isotropy – a radially constant D circular velocityof3circ=415±30kms−1 outtoaradiusof50kpc. M Itisofgreatinteresttoextendthisstudytolarger galactocentric distances, inordertoaddress H the several key questions that can put constraints on galaxy formation scenarios: Out to what ra- 2 dius can the constant circular velocity be followed? Does the outer GCS rotate? Can one detect 0 substructure inthe darkmatter distribution oftheFornax cluster (e.g. Ikebe etal. 1996)? Herewe present the firstpreliminary results from astudy measuring cluster velocities atprojected galacto- 0 centric radiiofupto100kpc. 4 ) 2. The Data Set 0 The data have been obtained with FORS2 and the Mask Exchange Unit (MXU) at the Very 6 Large Telescope of the European Southern Observatory at Cerro Paranal, Chile. The observing 5 period wasDecember 2/3, 2002 (ESOprogram ID70.B-0174). Twelvemasksin7different fields have been observed. The spectral resolution provided by the grism 600B is about 3Å. The data structure and the reduction procedure are identical to those in our previous study of the inner cluster system (Richtler et al. 2004, Dirsch et al. 2004). The data are not yet completely reduced. Sofar,wehavedetermined160newGCvelocitieswithtypicaluncertainties of30kms−1. For139 oftheseobjects, colours areavailablefromtheWashingtonphotometry ofDirschetal.(2003). 3. Results Figure1showsaplotofthenewradialvelocities versusgalactocentric distance together with the older data. Indicated are the systemic velocity and the boundaries within which we determine the velocity dispersion to be consistent with the previous selection. The outermost data point has ′ a projected radial distance of 18 , corresponding to 100kpc. Given that more velocities will soon follow,thevelocitydispersions reported heremightstillchangesomewhatinthefuture. A peculiarity is apparent in the velocity distribution of the outer clusters shown in Fig. 2. Whiletheredclusters showareasonably symmetricdistribution, theouterblueclusters exhibitan 065/2 TheOuterClusterSystemofNGC1399:PreliminaryResults YlvaSchuberth 0 0 5 2 1]00 -m s20 k0 velocity [1000150 0 0 5 0 5 10 15 radial distance [arcmin] Figure1: Radialvelocityversusprojectedgalactocentricradiusforthecombinedsample. Thesolidlineat P 1441kms−1indicatesthesystemicvelocityofNGC1399.Thedottedlinesat800and2080kms−1showthe o velocityselectionweadoptedfromRichtleretal.(2004). S 20 20 ( 96 red GCs 118 blue GCs B 15 15 D 10 10 M H 5 5 2 0 0 0 400 800 1200 1600 2000 2400 400 800 1200 1600 2000 2400 0 radial velocity [km s- 1] 4 Figure2:Histogramsofthevelocitydistributionfortheouter(R>5.5′)redandblueclusters.Thesolidline ) marksthesystemicvelocityofNGC1399.Thedottedlineat1950kms−1 indicatesthesystemicvelocityof 0 NGC1404. 6 asymmetry,withapreferenceforhighervelocities. Apeakat1800kms−1wasalreadyfoundforthe 5 blue clusters at smaller radii in the sample ofRichtler et al. (2004). These findings areinteresting in the context of the simulations presented by Bekki et al. (2003) who considered an interaction between the nearby elliptical NGC1404 and NGC1399, an idea firstbrought forward by Kissler– Patig et al. (1997). In the scenario of Bekki et al., where NGC1404 is on a bound orbit around the Fornax cluster centre (i.e. around NGC1399), the low specific frequency (the total number of clustersnormalisedtothehostgalaxy’sluminosity)ofNGC1404isexplainedbythetidalstripping of NGC1404 clusters which subsequently form an additional GC population around NGC1399. ThismostlyaffectstheblueclustersinNGC1404duetotheirshallowernumberdensityprofile. If true, this would complicate the use of the blue clusters as tracers for the gravitational potential of NGC1399. However,morevelocities areneeded toarriveatsaferconclusions. Theprojectedvelocitydispersionsderivedforthetotal,theblue,andtheredsampleareshown inFig.3. Thehorizontal dotted linesindicate themeanvalue oftherespective dispersions andare inexcellentagreementwiththepreviouslyderivedvaluesfortheinnerregion. Apparentdissimilar- ities,forinstancetheslightriseofthevelocitydispersionoftheblue(andthetotal)samplemaybe due to differences in the binning. The highest velocity dispersion values (at 10′) are probably not 065/3 TheOuterClusterSystemofNGC1399:PreliminaryResults YlvaSchuberth 602 clusters (800 < v < 2080 km/s) 350 300 250 200 203 209 80 35 46 22 7 283 blue clusters (800 < v < 2080 km/s) 350 ] 1 -s 300 m k 250 [ )R 200 95 103 33 22 20 10 ( 273 red clusters (800 < v < 2080 km/s) P s 350 300 o 250 S 200 108 94 37 9 17 8 ( 0 5 10 15 radial distance (arcmin) B D Figure 3: Velocity dispersion as function of projected radius. The upper panel shows the values for all selectedclusters.Theresultsforblueandredclustersareshowninthemiddleandbottompanel,respectively. M Inallpanels,thedottedlineindicatesthemeanofthecorrespondingdispersionmeasurements.Thenumber H ′ ofclustersenteringthedispersionmeasurementsforagivenbinof2.5 widthisindicatedaswell. 2 0 reliablesincetheytracearadialintervalwhere,asacomparisonwithFig.1shows,thedistribution ofvelocities ishighlyasymmetric. 0 Atpresent,theglobalpictureisthatofaconstantvelocitydispersionoverthefullradialrange. 4 The total sample has a velocity dispersion of 276kms−1. The dispersion for the red clusters is ) 254kms−1 andfor the blue clusters 295kms−1. Italso seemsthat the difference between the blue 0 andtheredclustersfoundfortheinnerregion,remainsintheouterregion. Thisisremarkablesince ′ 6 the number density profiles of both populations are indistinguishable for radii beyond 8 (Dirsch et al. 2003). In case of isotropy, the blue clusters should assume the same velocity dispersion 5 as the red clusters beyond this radius. Summarising, the present data support the presence of an isothermal darkhalowhichextends toatleast80kpc. References [1] Bekki,K.,Forbes,D.A.,Beasley,M.A.,&Couch,W.J.2003,MNRAS,344,1334 [2] Dirsch,B.,Richtler,T.,Geisler, D.,etal.2003,AJ,125,1908 [3] Dirsch,B.,Richtler,T.,Geisler,D.,etal. 2004,AJ,127,2114 [4] Ikebe, Y.,Ezawa,H.,Fukazawa,Y.etal. 1996,Nature,379,427 [5] Kissler-Patig, M.,Grillmair,C.J.,Meylan,G.,etal. 1999,AJ,117,1206 [6] Richtler, T.,Dirsch,B.,Gebhardt, K.,etal.2004, AJ,127,2094 065/4