Halo Globular Clusters Research Articles

Galaxy Evolution Explorer Ultraviolet Photometry of Globular Clusters in M31

We present ultraviolet photometry for globular clusters (GCs) in M31 from 15 square deg of imaging using the Galaxy Evolution Explorer (GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750 - 2750 Angstroms) and far-ultraviolet (FUV; 1350 - 1750 Angstroms) bandpasses, respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV, to an approximate limiting V magnitude of 19. Out of six clusters with [Fe/H]>-1 seen in the NUV, none is detected in the FUV bandpass. Furthermore, we find no candidate metal-rich clusters with significant FUV flux, because of the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which are metal-rich Galactic GCs with hot HB stars. We show that our GALEX photometry follows the general color trends established in previous UV studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in the UV and with population synthesis models, we suggest that the age range of M31 and Galactic halo GCs are similar.

Triggering the Formation of Halo Globular Clusters with Galaxy Outflows

We investigate the interactions of high-redshift galaxy outflows with low-mass virialized (Tvir 20% of the cloud. Such estimates ignore the likely presence of large-scale vortices, however, which would further enhance turbulence generation. Thus quantitative mixing predictions must await more detailed numerical studies.

Self-enrichment of Galactic halo globular clusters: stimulated star formation and consequences for the halo metallicity distribution

We explore the self-enrichment hypothesis for globular cluster formation with respect to the star formation aspect. Following this scenario, the massive stars of a first stellar generation chemically enrich the globular progenitor cloud up to Galactic halo metallicities and sweep it into an expanding spherical shell of gas. This paper investigates the ability of this swept protoglobular cloud to become gravitationally unstable and, therefore, to seed the formation of second-generation stars which may form a globular cluster later on. We use a simple model based on a linear perturbation theory for transverse motions in a shell of gas to demonstrate that the pressures by which the progenitor clouds are bound and the supernova numbers required to achieve Galactic halo metallicities support the successful development of the shell transverse collapse. Interestingly, the two parameters controlling the metallicity achieved through self-enrichment, namely the number of supernovae and the external pressure, also rule the surface density of the shell and thus its ability to undergo a transverse collapse. Such a supernova-induced origin for the globular cluster stars therefore opens the way to the understanding of the halo metallicity distributions. This model is also able to explain the lower limit of the halo globular cluster metallicity.

Secondary Globular Cluster populations

This study is motivated by two facts: 1. The formation of populous star cluster systems is widely observed to accompany violent star formation episodes in gas-rich galaxies as e.g. those triggered by strong interactions or merging. 2. The Globular Cluster (GC) systems of most but not all early-type galaxies show bimodal optical color distributions with fairly universal blue peaks and somewhat variable red peak colors, yet their Luminosity Functions (LFs) look like simple Gaussians with apparently universal turn-over magnitudes that are used for distance measurements and the determination of . Based on a new set of evolutionary synthesis models for Simple (= single burst) Stellar Populations (SSPs) of various metallicities using the latest Padova isochrones I study the color and luminosity evolution of GC populations over the wavelength range from U through K, providing an extensive grid of models for comparison with observations. I assume the intrinsic widths of the color distributions and LFs to be constant in time at the values observed today for the Milky Way or M 31 halo GC populations. Taking the color distributions and LFs of the Milky Way or M 31 halo GC population as a reference for old metal-poor GC populations in general, I study for which combinations of age and metallicity a secondary GC population formed in some violent star formation event in the history of its parent galaxy may or may not be detected in the observed GC color distributions. I also investigate the effect of these secondary GCs on the LFs of the total GC system. Significant differences are found among the diagnostic efficiencies in various wavelength regions. In particular, we predict the NIR to be able to reveal the presence of GC subpopulations with different age – metallicity combinations that may perfectly hide within one inconspicuous optical color peak. If the entire manifold of possible age – metallicity combinations is admitted for a secondary GC population, we find several cases where the resulting LF of the whole GC system is significantly affected and its turn-over could not serve as a reliable distance indicator. If, on the other hand, we assume some age – metallicity relation for GC populations, the second peak of the LFs vanishes and models indicate single-peak GC LFs even in GC systems with bimodal color distributions. A broad but sufficient age – metallicity relation is, for example, obtained if the secondary GC populations form in mergers of various spiral galaxy types from the ISM pre-enriched over the redshift range from to . As a first illustrative example we apply our models to V- and I-band data presented by Larsen et al. ([CITE]) for blue and red peak GCs in three early-type galaxies. We point out the importance of having multi-band information to independently constrain ages and metallicities of different GC subpopulations and again stress the diagnostic potential of K-band data in addition to optical observations. The models presented here will be used for the interpretation of multi-wavelength data on GC systems in galaxies of various types, luminosities and environments as well as on young star cluster systems in interacting galaxies and mergers. By independently constraining ages and metallicities of individual clusters we expect to shed light on both cluster and galaxy formation scenarios.

Collapse of an inhomogeneous protogalaxy with random motions

The free fall time for a galaxy collapsing inside a dark matter halo is ∼5.10 8 years while the reported age differences between halo globular clusters (even those in eccentric orbits) are of the order of a few Gigayears. The two time scales cannot be reconciled in models where the main body of the galaxy collapsed as a single unit. In this context, we examine the time taken by an inhomogeneous cloud, with gravitationally fed random motions to collapse. The collapse rate depends on the number, length scale and the degree of condensation of the inhomogeneities, and we find that for a range of values for these parameters, the protogalaxy takes significantly longer than a free fall time to collapse. We use the two component dynamic virial equation, supplemented by an evolution equation for the energy to investigate the time taken by an inhomogeneous cloud to dissipate its binding energy and collapse to one-tenth the original size, inside a dark matter halo. We find that the collapse time scales are increased to a few Gigayears and, our model suggests that the clumpy substructure of protogalaxies would have had a filling factor ∼10 −2–10 −3.

Nitrogen Overabundance: Globular Cluster and Halo Formation

Halo globular clusters pose four succinct issues that must be solved in any scenario of their formation: single-age, single metallicity stellar populations; a lower limit ([Fe/H] ~ -2.3) to their average metallicity; comprising only 1% of the stellar halo mass, and being among the oldest stars in our Galaxy. New spectra are presented of Galactic stars and integrated spectra of Galactic globular clusters which extend to 3250 angstroms. These spectra show show that the most metal-poor and among the best-studied Galactic globular clusters show strong NH3360 absorption, even though their spectral energy distributions in the near-UV are dominated by blue horizontal branch, AF-type stars. These strong NH features must be coming from the main sequence stars in these clusters. These new data are combined with existing data on the wide range of carbon and nitrogen abundance in very metal-poor ([Fe/H] < -3.5) halo giant and dwarf stars, together with recent models of zero-metal star formation, to make a strawman scenario for globular cluster formation that can reproduce three of the above four issues, and well as related two of the three issues pertaining to nitrogen overabundance. This strawman proposal makes observational and theoretical predictions that are testable, needing specific help from the modelers to understand all of the elemental constraints on globular cluster and halo formation.

Blue Horizontal-Branch Stars in the Sloan Digital Sky Survey. I. Sample Selection and Structure in the Galactic Halo

We isolate samples of 733 bright (g 18) high Galactic latitude blue horizontal-branch stars with photometry and spectroscopy in the Sloan Digital Sky Survey (SDSS). Comparison of independent photometric and spectroscopic selection criteria indicates that contamination from F and blue straggler stars is less than 10% for bright stars (g 18), and this is qualitatively confirmed by proper motions based on the USNO-A catalog as first epoch. Analysis of repeated observations shows that the errors in radial velocity are ≈26 km s-1. A relation between absolute magnitude and color is established using the horizontal branches of halo globular clusters observed by SDSS. Bolometric corrections and colors are synthesized in the SDSS filters from model spectra. The redder stars agree well in absolute magnitude with accepted values for RR Lyrae stars. The resulting photometric distances are accurate to about 0.2 mag, with a median of about 25 kpc. Modest clumps in phase space exist and are consistent with the previously reported tidal stream of the Sagittarius dwarf galaxy.

Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects

The properties of ~939 star clusters in the Large and Small Magellanic Clouds were determined from ground-based CCD images in UBVR passbands. The areal coverage was extensive, corresponding to 11.0 kpc2 in the LMC and 8.3 kpc2 in the SMC. After corrections for reddening, the colors and magnitudes of the clusters were converted to ages and masses, and the resulting mass distributions were searched for the effects of fading, evaporation, and size-of-sample bias. The data show a clear signature of cluster fading below the detection threshold. The initial cluster mass function (ICMF) was determined by fitting the mass and age distributions with cluster population models. These models suggest a new method to determine the ICMF that is nearly independent of fading or disruption and is based on the slope of a correlation between age and the maximum cluster mass in equally spaced intervals of log age. For a nearly uniform star formation rate, this correlation has a slope equal to 1/(α - 1) for an ICMF of dn(M)/dM ∝ M-α. We determine that α is between 2 and 2.4 for the LMC and SMC using this method plus another method in which models are fitted to the mass distribution integrated over age and to the age distribution integrated over mass. The maximum mass method also suggests that the cluster formation rate in the LMC age gap between 3 and 13 Gyr is about a factor of 10 below that in the period from 0.1 to 1 Gyr. The oldest clusters correspond in age and mass to halo globular clusters in the Milky Way. They do not fit the trends for lower mass clusters but appear to be a separate population that either had a very high star formation rate and became depleted by evaporation or formed with only high masses.

The Metallicity of Pregalactic Globular Clusters: The Observational Consequences of the First Stars

We explore a scenario in which metal-poor globular clusters (GCs) are enriched by the first supernovae in the universe. If the first stars in a 107 M? dark halo were very massive (>180 M?), then a pair-instability supernova from a single massive star could produce sufficient iron to enrich 106 M? of pristine, primordial gas to [Fe/H] ~ -2. In such a scenario, in which a single massive star acts as a seed for halo GCs, an accurate abundance analysis of GC stars would allow us to directly measure the Population III initial mass. Using the latest theoretical yields for zero-metallicity stars in the mass range of 140-260 M?, we find that the metals expelled from an ~230 M? star are consistent with [Si/Fe] and [Ca/Fe] observed in GC stars. However, no single star in this mass range can simultaneously explain all halo GC heavy-element abundance ratios, such as [V/Fe], [Ti/Fe], and [Ni/Fe]. These require a combination of masses for the Population III stellar progenitors. The various observational consequences of this scenario are discussed.

The Age of the Oldest Stars in the Local Galactic Disk from Hipparcos Parallaxes of G and K Subgiants11Part review of the history of the discovery of subgiants and part new results from Hipparcos.

ABSTRACTWe review the history of the discovery of field subgiant stars, the role that they played in the development of the early understanding of stellar evolution, and their importance in the age dating of the Galactic disk. We use the cataloged data from the Hipparcos satellite in this latter capacity.Based on Hipparcos parallaxes with relative accuracies of σπ/π≤0.10, the absolute magnitude of the lower envelope of the nearly horizontal subgiant sequence for field stars in the H‐R diagram for B−V colors between 0.85 and 1.05 is measured to be MV = 4.03 ± 0.06. New stellar evolutionary tracks calculated for metal abundances in the range -0.29≤[Fe/H ]≤ +0.37 are fitted to the main‐sequence, subgiant, and giant‐star distributions in the Hipparcos H‐R diagram. Isochrones for [Fe/H ] = +0.37 provide the best fit to the reddest giants between +3≳MV≳0, as well as to the envelope of the reddest main‐sequence stars at MV≳5. The red edge of the densest part of the distribution of field giants is, however, most readily matched by isochrones having [Fe/H ] ≈ +0.23. Such high metal abundances are evidently confirmed by the spectroscopically observed high metallicity (between [Fe/H ] = +0.2 and +0.4) of the old thick‐disk Galactic cluster NGC 6791, whose color‐magnitude diagram can be made to fit either of these red boundaries by adopting suitable values for the reddening and distance modulus (from within the observed ranges of uncertainties of these quantities).The age of the field stars in the solar neighborhood is found to be 7.9 ± 0.7 Gyr (or 7.4 ± 0.7 Gyr if the stellar models allow for the effects of diffusive processes) by fitting the theoretical isochrones for [Fe/H ] = +0.37 to the lower envelope of the Hipparcos subgiants. However, this age is a function of metallicity. The models show a dependence of δt = -3.99([Fe/H] - 0.37) at MV∼ +4 for metallicities between 0.00 and +0.37. The same grid of isochrones yields ages, in turn, of 4.0 ± 0.2, 6.2 ± 0.5, and 7.5 to 10 Gyr (depending on the assumed reddening) for the old Galactic clusters M67, NGC 188, and NGC 6791 using metal abundances, distance moduli, and reddenings adopted in the text. Although the distance (and hence age) of NGC 6791 is somewhat uncertain, the ages of both the Galactic disk in the solar neighborhood and of NGC 6791 are, nevertheless, likely between 3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of ∼13.5 Gyr. The conclusion is the same as that reached earlier by Liu & Chaboyer, who used Hipparcos parallax stars that were mainly near the main‐sequence turnoff rather than at the lower bound of the subgiant luminosity that we determine here. The most significant results are (1) the supermetallicity of the oldest local disk stars and (2) the large age difference between the most metal poor component of the halo and the thick and thin disk in the solar neighborhood, confirming Liu & Chaboyer. These facts are undoubtedly related and pose again the problem of the proper scenario for the timing of events in the formation of the halo and the Galactic disk in the solar neighborhood.

Photometry of the Globular Cluster NGC 3201 and Its Variable Stars

We present time series BVI photometry of the brighter stars in the halo globular cluster NGC 3201. We derive high-quality light curves for 58 RR Lyrae stars, all of which appear to be members of the cluster. We determine the foreground reddening of each RR Lyrae star from the star's color and confirm that the reddening varies significantly across the face of the cluster. We obtain a mean value of E(B-V) = 0.25 ± 0.02 mag for stars within 20 of the cluster center. Using the reddening values of the individual RR Lyrae stars, we made dereddened color-magnitude diagrams that reach from the red giant branch tip to ~2 mag below the main-sequence turnoff. Analysis of these diagrams indicates that NGC 3201 has an age of 13–14 Gyr, typical of other globular clusters of its metallicity. We detect low-level brightness variations in most of the cluster's brighter red giant stars and tentatively associate this variation with the small-amplitude red variables seen in the field. We also discovered several δ Scuti variable candidates, two of which appear to be cluster members. The distances derived from the δ Scuti and RR Lyrae stars are in good agreement, indicating d = 4.7 ± 0.2 kpc.

Globular cluster formation from gravitational tidal effects of merging and interacting galaxies

We investigate the spatial, kinematic and chemical properties of globular cluster systems formed in merging and interacting galaxies using N-body–smoothed particle hydrodynamics (SPH) simulations. Although we cannot resolve individual clusters in our simulation, we assume that they form in collapsing molecular clouds when the local external gas pressure exceeds 105kB (where kB is the Boltzmann constant). Several simulations are carried out for a range of initial conditions and galaxy mass ratios. The input model spirals are given a halo globular cluster system similar to those observed for the Milky Way and M31. Gravitational tidal effects during galaxy merging and interaction lead to a dramatic increase in gas pressure, which exceeds our threshold and hence triggers new globular cluster formation. We investigate the properties of the globular cluster system in the remnant galaxy, such as the number density, the specific frequency, kinematic properties and the metallicity distribution. Different orbital conditions and mass ratios give rise to a range in globular cluster properties, particularly for the interaction models. Our key results are the following: the newly formed metal-rich clusters are concentrated at the centre of the merger remnant elliptical, whereas the metal-poor ones are distributed to the outer parts because of strong angular momentum transfer. The dissipative merging of present-day spirals, including chemical evolution, results in metal-rich clusters with a mean metallicity that is super-solar, i.e. much higher than is observed in elliptical galaxies. If elliptical galaxies form by dissipative major mergers, then they must do so at very early epochs when their discs contained low-metallicity gas. Our simulations show that the specific frequency can be increased in a dissipative major merger. However, when this occurs it results in a ratio of metal-poor to metal-rich clusters that is less than one, contrary to the ratio observed in many elliptical galaxies.

Chemical Abundances of Three Metal-poor Globular Clusters (NGC 6287, NGC 6293, and NGC 6541) in the Inner Halo

We present a chemical abundance study of three inner clusters, NGC 6287, 6293, and 6541, finding [Fe/H] = -2.01 ± 0.05, -1.99 ± 0.02, and -1.76 ± 0.02 (internal), respectively, and our metallicity measurements are in good agreement with previous estimates. We also present the radial velocity measurements of the clusters. Our radial velocity measurements for NGC 6293 and NGC 6541 are in good agreement with previous measurements; however, our radial velocity measurement for NGC 6287 is almost 80 km s-1 larger than the previous measurement. The mean α-element abundances of our program clusters are in good agreement with other globular clusters, confirming previous results. However, the individual α-elements appear to follow different trends. The silicon abundances of the inner clusters appear to be enhanced, and the titanium abundances appear to be depleted compared to the intermediate clusters. Our results also appear to oppose to those of metal-rich bulge giants studied by McWilliam & Rich, who found that bulge giants are titanium-enhanced and silicon-deficient. In particular, [Si/Ti] ratios appear to be related to Galactocentric distances, in the sense that [Si/Ti] ratios decrease with Galactocentric distance. We propose that contributions from different masses of the Type II supernovae progenitors that enriched proto–globular cluster clouds' elemental abundances and the different initial physical environments surrounding the proto–globular clusters clouds are responsible for this gradient in [Si/Ti] ratios versus Galactocentric distances of the old halo globular clusters. On the other hand, our program clusters' enhanced s-process elemental abundances suggest that the formation timescale of our program clusters might be as short as a few times 108 yr after the star formation is initiated in the Galaxy's central regions, if the s-process site is intermediate-mass asymptotic giant branch stars.

Extragalactic globular clusters in the near infrared

We compare optical and near infrared colours of globular clusters in M87, the central giant elliptical in Virgo, and NGC 4478, an intermediate luminosity galaxy in Virgo, close in projection to M87. Combining V and I photometry obtained with the WFPC2 on HST and Ks photometry obtained with the NIRC on Keck 1, we find the broad range in colour and previously detected bi-modality in M87. We confirm that NGC 4478 only hosts a blue sub-population of globular clusters and now show that these clusters' and colours are very similar to those of the halo globular clusters in Milky Way and M31. Most likely, a metal-rich sub-population never formed around this galaxy (rather than having formed and been destroyed later), probably because its metal-rich gas was stripped during its passage through the centre of the Virgo cluster. The , colours are close to the predicted colours from SSP models for old populations. However, M87 hosts a few red clusters that are best explained by intermediate ages (a few Gyr). Generally, there is evidence that the red, metal-rich sub-population has a complex colour structure and is itself composed of clusters spanning a large metallicity and, potentially, age range. This contrasts with the blue, metal-poor population which appears very homogeneous in all galaxies observed so far.

The White Dwarf Cooling Sequence of the Globular Cluster Messier 4

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I = 28. The white dwarf luminosity function rises sharply for I >25.5, consistent with the behaviour expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 magnitudes fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7 +/- 0.7 Gyr for M4 (2-sigma statistical errors).

Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift

The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population—all of the stars being old and metal-poor—similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi dSph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to ∼0.3 M ⊙. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.

SED, metallicity and age of halo globular clusters in M 33

In this paper we study the properties of ten halo globular clusters in the nearby spiral galaxy M 33. CCD images of M 33 were obtained as a part of the BATC Colour Survey of the sky in 13 intermediate-band filters from 3800 to 10 000 A. By aperture photometry, we obtain the spectral energy distributions (SEDs) of these globular clusters. We estimate the ages of our sample clusters by comparing the photometry of each object with theoretical stellar population synthesis models for different values of metallicity. Our results suggest that eight of the ten sample halo globular clusters have “intermediate” ages, i.e. between 1 and 8 Gyrs.

Estimating the Metallicity of Old Star Clusters in M33

Based on the method proposed by Kong et al. and on the multi-color spectrophotometry by Ma et al., we estimate the metallicities of 16 old star clusters in the nearby spiral galaxy M33, ten of which are halo globular clusters. These old clusters are all metal poor, the range of metallicity ((Fe/H)) is from 0.14 to 2.12. In general, our results are consistent with those derived by other methods, such as integrated spectra and photometry, and our study confirms the reliability of the method of Kong et al. Globular clusters are thought to be among the oldest radiant objects in the Universe. The studies of these systems have played a key role in the development of our understanding of the Universe, including the fundamental question of the age of the Universe. The globular clusters of Milky Way can be used to probe the way in which our Galaxy formed. Studies of similar clusters in other galaxies can also provide us the properties of those galaxies in the early period

On the Distribution of Orbital Poles of Milky Way Satellites

In numerous studies of the outer Galactic halo some evidence for accretion has been found. If the outer halo did form in part or wholly through merger events, we might expect to find coherent streams of stars and globular clusters following orbits similar to those of their parent objects, which are assumed to be present or former Milky Way dwarf satellite galaxies. We present a study of this phenomenon by assessing the likelihood of potential descendant "dynamical families" in the outer halo. We conduct two analyses: one that involves a statistical analysis of the spatial distribution of all known Galactic dwarf satellite galaxies (DSGs) and globular clusters, and a second, more specific analysis of those globular clusters and DSGs for which full phase space dynamical data exist. In both cases our methodology is appropriate only to members of descendant dynamical families that retain nearly aligned orbital poles today. Since the Sagittarius dwarf (Sgr) is considered a paradigm for the type of merger/tidal interaction event for which we are searching, we also undertake a case study of the Sgr system and identify several globular clusters that may be members of its extended dynamical family. In our first analysis, the distribution of possible orbital poles for the entire sample of outer (Rgc > 8 kpc) halo globular clusters is tested for statistically significant associations among globular clusters and DSGs. Our methodology for identifying possible associations is similar to that used by Lynden-Bell & Lynden-Bell, but we put the associations on a more statistical foundation. Moreover, we study the degree of possible dynamical clustering among various interesting ensembles of globular clusters and satellite galaxies. Among the ensembles studied, we find the globular cluster subpopulation with the highest statistical likelihood of association with one or more of the Galactic DSGs to be the distant, outer halo (Rgc > 25 kpc), second-parameter globular clusters. The results of our orbital pole analysis are supported by the great circle cell count methodology of Johnston, Hernquist, & Bolte. The space motions of the clusters Pal 4, NGC 6229, NGC 7006, and Pyxis are predicted to be among those most likely to show the clusters to be following stream orbits, since these clusters are responsible for the majority of the statistical significance of the association between outer halo, second-parameter globular clusters and the Milky Way DSGs. In our second analysis, we study the orbits of the 41 globular clusters and six Milky Way-bound DSGs having measured proper motions to look for objects with both coplanar orbits and similar angular momenta. Unfortunately, the majority of globular clusters with measured proper motions are inner halo clusters that are less likely to retain memory of their original orbit. Although four potential globular cluster/DSG associations are found, we believe three of these associations involving inner halo clusters to be coincidental. While the present sample of objects with complete dynamical data is small and does not include many of the globular clusters that are more likely to have been captured by the Milky Way, the methodology we adopt will become increasingly powerful as more proper motions are measured for distant Galactic satellites and globular clusters, and especially as results from the Space Interferometry Mission (SIM) become available.

The Outer Halo Cluster System of NGC 1399

We investigate the outer halo globular cluster population of NGC 1399. This study uses wide-field imaging of this cluster system, which covers the largest area studied with CCD photometry until now. The cluster system of NGC 1399 is found to extend further than 100 kpc from the galaxy. A population of metal-rich, as well as metal-poor clusters has been identified at these large radii. At radii smaller than 55 kpc the specific frequency of the red cluster system remains constant, while that of the blue clusters increases proportional tor0.8±0.2. For larger radii, the uncertainty of the galaxy light profile does not permit any reliable statement.