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Towards an Understanding of the Globular Cluster Over-Abundance Around the Central -Giant Elliptical NGC 1399 I

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Towards an Understanding of the Globular Cluster Over-Abundance Around the Central -Giant Elliptical NGC 1399 I Towards an Understanding of the Globular Cluster Over-abundance around the Central -Giant Elliptical NGC 1399 I bIarkus Kissier-Patig '92 Uco/Lick observatory, University of California, Santa Cruz, CA 9,5064. USA Electronic mails: mkissler8ucblick.org Carl J. Grillmajr SIRTF Science Center, Mail Stop 100-22, California Instibute of Technology, Pasadena, CA 9112'5, USA Electronicmail: [email protected] Georges Meylan European Southern Observatory, Iiarl-Schwarzschild-Strasse 2, 0-85748 Garching bei i\.lunchen, Germany Electronic mail: [email protected] Jean P. Brodie UCO/Lick observatory, University of California, Santa Cru:, CA 95064, USA Electronic mail: [email protected] Dante Minniti Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Departamento de Astronomia y Astrofisica, P. Universidad Catdlica, Casilla 104, Santiago 22, Chile Electronic mail: [email protected] Paul Goudfrooij Space Telescope Science Institute, 9700 San ,\[artin Drive, Baltimore, MD 21218. lJSA Electronic mail: [email protected] ABSTRACT We investigate the kinematics of a combined sample of 74 globular clusters around NGCl 1399. Their high velocity dispersion, increasing with radius, supports their associ- ation with the gravitational potentialof the ga.laxy cluster rather than with thatof NGC 'Feodor Lynerl Fellow of the Alexander von Hunlholdt Foundation 2Current address: ESO, Karl-Schwanschild-Str. 2, 85748 Garching, Germany. Email: rnkissler'Qeso.org 3Afiliated with the Astrophysics Division, Space Science Department, European Space Agency, ESTEC, Postbus 299, NL-2200 AG Noordwijk, The Netherlands 1 1399 itself. We find no evidence for rotatiod'fnihe fdIsample, although some indication for rotation in the outer regions. The ,data*donoballow us to detect differences between the kinematics of the blue and red sub-populations of globular clusters. A comparison between the globular cluster systems of NGC 1399 and those of NGC 1404 and NGC 1380 indicates that the globular cluster's in all three galaxies are likely to have formed via similar mechanisms and at-similar epochs. The only property which distinguishes the NGC 1399 globular cluster system from these others is that it is ten times more abundant. We summarize the evidence for associating these excess globulars with the galaxy cluster rather than with NGC 1399 itself, and suggest that the over-abundance can be explainedby tidal strippink of neighboring galaxies and subsequent accumulation of globulars in the gravitational pdtential of the galaxy cluster. 'L" Subject headings: globularclusters: general , galaxies:elliptical and lenticular, cD, galaxies: halos , galaxies: kinematicsand dynamics, galaxies: formation, galaxies: evo- lu t lu ion I I J' I I 1. Introduction pectetl to be 011 orbit,sradial innerin the regions and to ~110~solne systemic rotation far out. The kinemat- Extragalactic: globular clusters have in recent years ics of globular clusters that mighthave formed during establishecl tllernseives as potential tracers of the for- d plerger Was not aciclressed in their study; it \vould mation and evolution of galaxies (see Ashmarl & Zepf depend 011 the kinelnatics of the in-falling gas from 1998 for a recent review). Thenumber of detailed dli(:tl theyformed. Other simulations (e.g. bIuzzio photometric studies has rapidly increaed, and these 1987) studied the accretion and stripping of globular studies reveal a number of interesting connections be- clusters in galaxyclusters, but noclear predictions tween globular cluster systems and their host galax- for the kinematics of accreted globular clusters were ies. With the recent commissioning of l0m-class tele- formulated. scopes, measuring absorption line indices of a large Tlie primary goal of the present paper is to corn- numberof individual globular clusters has become bine all existing kinematic data on the globular ~1~s- feasible (Kissler-Patig et al. 1998; Cohen, Blakeslee, Fer system of NGC 1399 to better constrain itsorigin. & Ryzhov 1998). Colors, magnitudes, total numbers, A secondary aim is to use the photometric properties spatial distributions, radial density profiles, ages, and of globular clusters in the brightest Fornax galaxies metallicities, can now be obtained and used to con- .to further constrain formation scenarios. The sample strain the formation history of globular clusters and , of radial velocities for globular clusters around NGC their host galaxies. , 1399 is compiled and presented in Sect. 2. These are Anotheressential source of information fordis- ,;used to investigate the kinematic properties of the criminating between different formation scenarios is full sample, as well as sub-samplesselected on the the kinematics of globular cluster systems, as deter- basis of radius and color in Sect. 3. In Section 4 we mined from the measured radial velocities of individ- compare the kinematics of the globular clusters with ual clusters. For example,the studies of M87 and those of the stars, cluster galaxies and X-ray gas, and NGC' 1399 found a significantly higher velocity dis- derive the mass-to-lightratio in theouter environs persion for theglobular clusters than for the stars of the galaxy. In Sect. 5 we compare the properties (Huchra and Brodie 1987; Mould et al. 1990; Brodie of the globular clusters in NGC 1399 with those of andHuchra 1991; Grillmair et al. 1994;Cohen & the globular cluster systems of the next two brightest Ryzhov1997; Minniti et al. 1998;Kissler-Patig et early-type galaxies in Fornax, NGC 1380 and KGC al. 1998). Mould etal. (1990) demonstrated that, 1404. We then discuss the implications for different in M87, this was consistent with the the surface den- formation scenarios. A summary and our conclusions sity distribution of globular clusters being more ex- are given in Sect. 6. tended than the surface density distribution of stars. Grillmairet al. (1994)suggested thatthe globular 2. The data clusters around NGC 1399 were reacting to the grav- itational potential of the Fornax cluster as a whole Our sample is based on the compilations of Grill- ratherthan just that of thehost galaxy. In NGC nlair (1992), Grillmairet al. (1994), Minnitiet al. 5128,Harris et al. (1988)and Huiet al. (1995) re- (1998),and IGissler-Patig et al. (1998). Briefly, the ported rotation in the globular cluster system, though data from Grilllnair et al. were obtained at the Anglo- only for the metal-rich clusters. This is contrary to Australian Telescope with the Low-Dispersion Survey the findings in NGC 4472 (Sharples et al. 1998) and Spectrograph and the Image Photon Counting Sys- M87 (Iiissler-Patig & Gebhardt1998) in which the tem in the wavelength range 3800-4800 A with 2: 13 metal-poor globular clusters seem todominate the resolution. Minniti et al. obtained their data with the rotation. New Technology Telescope at the European Southern Unfortunat,ely, few models exist to compare with Observatory, using the ESO Multi-i\.lode Instrument the (still sparse) data. Predictions for the kinematic in the wavelength range 6000-9000 .-\ with a resolu- signature in globularcluster systems after spiral- tion of 7.q X. Iiissler-Patig et al. observed with the spiral mergershave been presented by Hernquist & LOW Reso1ution Imaging Spectrograph on the Keck 1 Bolte(1992). They studied the kinematics of glob- Telescope; their spectra covered a wavelength range rlIar clusters already present in the progenitorsand from about 4000 .-I to 6100 A with 5.6 A resolution. follllcl that, in the merger product these clusters areex- We refer the reader to the original papers for a more I I: debailed description of the observations, the data re- duction, and the velocity measurements. Hereafter, we will refer to the respective sanlples as the AAT sample(Grillmair 1092, C;rillniair et 3.2. Spatial distribution of the velocities al. 1994), the NTT sample (5Iinniti et al. 1991)), and t,he Keck sample(Iiissler-Patig et aI. 1098).The In Figure 3 we plot the cluster velocities against combinedsample of 74 globularclusters is listed projected radius from the center of NGC 1399. The in Table 1. For each globularcluster we give the mean velocity does not change significantly with ra- ID number(taken from the papers with theprefix dius (see also Table 3). However, we note that within aat/ntt/keck added respectively), the equatorial coor- 4’ the velocities seem to cluster around the mean ve- dinates (B1950), the heliocentric radial velocity (with locity of the stellar component of NGC 1399, whereas the weighted mean when multiple measurementswere beyond 5’ the globular clusters seem to have either available), as well as the availablephotometric infor- . significantly higher or lower velocities than the mean. mation. For 52 globularclusters V-I colorsaccu- This is better illustrated in Fig. 4, where we have plot- rate to 0.035 mag where obtained from the work of ted histograms of the velocities in concentric annuli. Kissler-Patig et al. (1997a). The B, magnitude ,and Using the ROSTAT package (Beers et al. 1990, Bird & Bj-R color were taken from Grillmair (1992). We in- Beers 1993), we tested the samples for normality and cluded in our combined sample all objects with radial unimodality. While the globular clusters in the inner velocities greater than 500 km s-l and less than 2500 ring are consistent at the 98% confidence level with km s-l (i.e. within 3a of the mean). A list of posi- a normal distribution, the statistics are inconclusive tions, velocities and colors of the 74 globular clusters in the middle ring and inconsistent with normality
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