Towards an Understanding of the Globular Cluster Over--Abundance
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Towards an Understanding of the Globular Cluster Over–abundance around the Central Giant Elliptical NGC 1399 Markus Kissler-Patig 1,2 UCO/Lick observatory, University of California, Santa Cruz, CA 95064, USA Electronic mails: [email protected] Carl J. Grillmair SIRTF Science Center, Mail Stop 100-22, California Institute of Technology, Pasadena, CA 91125, USA Electronic mail: [email protected] Georges Meylan European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei M¨unchen, Germany Electronic mail: [email protected] Jean P. Brodie UCO/Lick observatory, University of California, Santa Cruz, CA 95064, USA Electronic mail: [email protected] Dante Minniti Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Departamento de Astronom´ıa y Astrof´ısica, P. Universidad Cat´olica, Casilla 104, Santiago 22, Chile Electronic mail: [email protected] Paul Goudfrooij 3 arXiv:astro-ph/9811373v1 24 Nov 1998 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Electronic mail: [email protected] ABSTRACT We investigate the kinematics of a combined sample of 74 globular clusters around NGC 1399. Their high velocity dispersion, increasing with radius, supports their associ- ation with the gravitational potential of the galaxy cluster rather than with that of NGC 1Feodor Lynen Fellow of the Alexander von Humboldt Foundation 2Current address: ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany. Email: [email protected] 3Affiliated 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 rotation in the full sample, although some indication for rotation in the outer regions. The data do not allow 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 clusters 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 abun- dant. 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 explained by tidal stripping, at an early epoch, of neighboring galaxies and subsequent accumulation of globulars in the gravitational potential of the galaxy cluster. Subject headings: globular clusters: general , galaxies: elliptical and lenticular, cD, galaxies: halos , galaxies: kinematics and dynamics, galaxies: formation, galaxies: evo- lution 2 1. Introduction pected to be on radial orbits in the inner regions and to show some systemic rotation far out. The kinemat- Extragalactic globular clusters have in recent years ics of globular clusters that might have formed during established themselves as potential tracers of the for- a merger was not addressed in their study; it would mation and evolution of galaxies (see Ashman & Zepf depend on the kinematics of the in-falling gas from 1998 for a recent review). The number of detailed which they formed. Other simulations (e.g. Muzzio photometric studies has rapidly increased, and these 1987) studied the accretion and stripping of globular studies reveal a number of interesting connections be- clusters in galaxy clusters, but no clear predictions tween globular cluster systems and their host galax- for the kinematics of accreted globular clusters were ies. With the recent commissioning of 10m–class tele- formulated. scopes, measuring absorption line indices of a large The primary goal of the present paper is to com- number of individual globular clusters has become bine all existing kinematic data on the globular clus- feasible (Kissler-Patig et al. 1998; Cohen, Blakeslee, ter system of NGC 1399 to better constrain its origin. & 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 Another essential source of information for dis- used to investigate the kinematic properties of the criminating between different formation scenarios is full sample, as well as sub–samples selected 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-light ratio in the outer environs persion for the globular 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 and Huchra 1991; Grillmair et al. 1994; Cohen & the globular cluster systems of the next two brightest Ryzhov 1997; Minniti et al. 1998; Kissler-Patig et early–type galaxies in Fornax, NGC 1380 and NGC al. 1998). Mould et al. (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. Grillmair et al. (1994) suggested that the 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- rather than just that of the host galaxy. In NGC mair (1992), Grillmair et al. (1994), Minniti et al. 5128, Harris et al. (1988) and Hui et al. (1995) re- (1998), and Kissler-Patig et al. (1998). Briefly, the ported rotation in the globular cluster system, though data from Grillmair 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 (Kissler-Patig & Gebhardt 1998) in which the tem in the wavelength range 3800–4800 A˚ with ≃ 13 A˚ metal–poor globular clusters seem to dominate the resolution. Minniti et al. obtained their data with the rotation. New Technology Telescope at the European Southern Unfortunately, few models exist to compare with Observatory, using the ESO Multi–Mode Instrument the (still sparse) data. Predictions for the kinematic in the wavelength range 6000–9000 A˚ with a resolu- signature in globular cluster systems after spiral– tion of 7.5 A.˚ Kissler-Patig et al. observed with the spiral mergers have been presented by Hernquist & Low Resolution Imaging Spectrograph on the Keck 1 Bolte (1992). They studied the kinematics of glob- Telescope; their spectra covered a wavelength range ular clusters already present in the progenitors and from about 4000 A˚ to 6100 A˚ with 5.6 A˚ resolution. found that in the merger product these clusters are ex- We refer the reader to the original papers for a more 3 detailed description of the observations, the data re- may be contamination of the sample by globulars be- duction, and the velocity measurements. longing to NGC 1404 or NGC 1380. We discuss this Hereafter, we will refer to the respective samples possibility further below. as the AAT sample (Grillmair 1992, Grillmair et al. 1994), the NTT sample (Minniti et al. 1998), and 3.2. Spatial distribution of the velocities the Keck sample (Kissler-Patig et al. 1998). The In Figure 3 we plot the cluster velocities against combined sample of 74 globular clusters is listed projected radius from the center of NGC 1399. The in Table 1. For each globular cluster we give the mean velocity does not change significantly with ra- ID number (taken from the papers with the prefix 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 measurements were beyond 5′ the globular clusters seem to have either available), as well as the available photometric infor- significantly higher or lower velocities than the mean. − mation. For 52 globular clusters V I colors accu- 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 Bj 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 −1 velocities greater than 500 km s and less than 2500 ring are consistent at the 98% confidence level with −1 km s (i.e. within 3σ 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 in in electronic form is available from the first author. the outer ring at the 90% confidence level.