The majority of observed mass-to-light ratios of globular clusters are too low to be explained by `canonical' cluster models, in which dynamical effects are not accounted for. Moreover, these models do not reproduce a recently reported trend of increasing M/L with cluster mass, but instead predict mass-to-light ratios that are independent of cluster mass for a fixed age and metallicity. This study aims to explain the M/L of globular clusters in four galaxies by including stellar evolution, stellar remnants, and the preferential loss of low-mass stars due to energy equipartition. Analytical cluster models are applied that account for stellar evolution and dynamical cluster dissolution to samples of globular clusters in Cen A, the Milky Way, M31 and the LMC. The models include stellar remnants and cover metallicities in the range Z=0.0004-0.05. Both the low observed mass-to-light ratios and the trend of increasing M/L with cluster mass can be reproduced by including the preferential loss of low-mass stars, assuming reasonable values for the dissolution timescale. This leads to a mass-dependent M/L evolution and increases the explained percentage of the observations from 39% to 92%. This study shows that the hitherto unexplained discrepancy between observations and models of the mass-to-light ratios of globular clusters can be explained by dynamical effects, provided that the globular clusters exhibiting low M/L have dissolution timescales within the ranges assumed in this Letter. Furthermore, it substantiates that M/L cannot be assumed to be constant with mass at fixed age and metallicity.
The colors of globular clusters in most of large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. However, here we find that, because of the non-linear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality. The models of cluster colors indicate that the horizontal-branch stars are the main drivers behind the empirical non-linearity. We show that the scenario gives simple and cohesive explanations for all the …
The Large Magellanic Cloud (LMC) has a unique cluster formation history in that nearly all of its globular clusters were formed either $\sim$ 13 Gyr ago or less than $\sim$ 3 Gyr ago. It is not clear what physical mechanism is responsible for the most recent cluster formation episode and thus the mysterious age gap between the LMC clusters. We first present results of gas dynamical N-body simulations of the evolution of the LMC in the context of its Galactic orbit and interactions with the SMC, paying special attention to the effect of tidal forces. We find that the first close encounter betwe…
We investigate the possibility of using globular clusters as targets for microlensing searches. Such searches will be challenging and require more powerful telescopes than now employed, but are feasible in the 0 future. Although expected event rates are low, we show that the wide variety of lines of sight to globular clusters greatly enhances the ability to distinguish between halo models using microlensing observations as compared to LMC/SMC observations alone.
NGC 3311, the central cD galaxy in the Hydra cluster, was previously thought to host the most metal-rich globular cluster system known. Ground-based Washington photometry had indicated the almost complete absence of the population of globular clusters near [Fe/H] ~ -1 dex, normally dominant in the metallicity distribution functions of giant elliptical galaxies. Lacking the normal metal-poor globular cluster population, NGC 3311 was an outstanding exception among galaxies, not easily understood under any of the current globular cluster formation scenarios. Our HST/WFPC2 data yield normal globul…
The most massive elliptical galaxies show a prominent multi-modality in their globular cluster system color distributions. Understanding the mechanisms which lead to multiple globular cluster sub-populations is essential for a complete picture of massive galaxy formation. By assuming that globular cluster formation traces the total star formation and taking into account the radial variations in the composite stellar populations predicted by the Pipino & Matteucci (2004) multi-zone photo-chemical evolution code, we compute the distribution of globular cluster properties as a function of galacto…
We combine near-infrared photometry obtained with the VLT/ISAAC instrument and archival HST/WFPC2 optical images to determine VIK magnitudes and colours of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The combination of near-IR and optical photometry provides a way to lift the age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal two major sub-populations, consistent with previous studies. By comparing the V-I, V-K colours of the NGC 3115 globular clusters with SSP models, we find that the colour difference between the two >10 Gyr old major sub-p…
The discovery of young globular clusters in merging galaxies and other environments provides an opportunity to study directly the process of globular cluster formation. Empirically it appears that globular cluster formation occurs preferentially in regions in which star formation occurs at a high rate and efficiency. Further, the interstellar medium in such regions is likely to be at a higher pressure than less active star-forming environments. An additional observational clue to the globular cluster formation process is that young globular clusters have little or no mass-radius relationship. …
At the distance of NGC 5128 (3.6 \pm 0.2 Mpc) it is possible to resolve globular clusters with high resolution imaging from the ground, thus allowing the globular cluster candidate selection primarily through their morphological properties. I report the discovery of 71 globular clusters in NGC 5128 on VLT UT1+FORS1 images, including the faintest members (Mv ~ -5) known to date in this galaxy as well as 5 previously known clusters. U- and V-band photometry has been measured for all the candidates and the luminosity function, spanning -10.1<Mv<-4.9 and -9.3<Mu<-3.3, constructed. These are the de…
The radial distribution of globular clusters in galaxies is always less peaked to the centre than the halo stars'. Extending previous work to a sample of HST globular cluster systems in ellipticals, we evaluate the number of clusters lost to the galactic centre as the integrals of the difference between the observed globular cluster system distribution and the underlying halo light profile. It results that the initial populations of globular clusters were from 25% to 50% richer than now. This significant number of missing globular clusters supports the hypothesis that a large quantity of globu…
Nine presently known "young" Galactic globular clusters, which have ages / 3 Gyr smaller than the oldest clusters of similar metallicity, all have RGC > 15 kpc. Furthermore these young globulars are found to have below-average luminosities. These results suggest that (1) globular cluster formation extended over a considerable period of time in the outermost part of the Galactic halo, and (2) that typical luminosities (masses) of such outer halo clusters decreased with time.
Extremely rich populations of globular clusters, numbering ten thousand or more, surround the central giant galaxies in rich clusters. I discuss some recent spectroscopic and photometric observations of these rich globular clusters populations. In very nearby galaxy clusters, the globular cluster velocities can be used to trace the transition from the galaxy potential to the larger cluster potential. Beyond cz \approx 3000 km/s, the globulars are too faint for spectroscopic studies with even the largest telescopes and cease to be useful dynamical tracers. However, the spatial distribution and …
The radial distribution of globular clusters in our Galaxy, M31 and M87 is studied and compared with that of halo stars. The globular cluster distributions seem significantly flatter than those of the stars bulge. Assuming this is a consequence of an evolution of the globular cluster distribution in these galaxies, a comparison with the (unevolved) stellar distribution allows us to obtain estimates of the number and total mass of clusters lost, which are possibly gone to feed the massive central objects present in these galaxies. It results that the cluster systems studied should have been ini…
The main aspects of the globular cluster luminosity function needing to be explained by a general theory of cluster formation are reviewed, and the importance of simultaneously understanding globular cluster systematics (the fundamental plane) within such a theory is pointed out.
This review focuses on how galaxies and their globular cluster systems form. I first discuss the now fairly convincing evidence that some globular clusters form in galaxy starbursts/mergers. One way these observations are valuable is they place important constraints on the physics of the formation of globular clusters. Moreover, it is natural to associate the typically metal-rich clusters forming in mergers with the substantial metal-rich population of globulars around ellipticals, thereby implying an important role for galaxy mergers in the evolution of elliptical galaxies. I also highlight s…
Globular cluster populations of supergiant elliptical galaxies are known to vary widely, from extremely populous systems like that of UGC 9799, the centrally dominant galaxy in Abell 2052, to globular-cluster-poor galaxies such as NGC 5629 in Abell 2666. Here we propose that these variations point strongly to the existence of a population of globular clusters that are not bound to individual galaxies, but rather move freely throughout the cores of clusters of galaxies. Such intracluster globular clusters may have originated as tidally stripped debris from galaxy interactions and mergers, or al…
CONTEXT: Extragalactic globular clusters have been studied in elliptical galaxies and in a few luminous spiral galaxies, but little is known about globular clusters in low-luminosity spirals. AIMS: Past observations with the ACS have shown that NGC 45 hosts a large population of globular clusters (19), as well as several young star clusters. In this work we aim to confirm the bona fide globular cluster status for 8 of 19 globular cluster candidates and to derive metallicities, ages, and velocities. METHODS: VLT/FORS2 multislit spectroscopy in combination with the Lick/IDS system was used to de…
The majority of the globular clusters associated with the Sagittarius dwarf galaxy are faint. In this respect it differs significantly from the globular cluster systems surrounding typical giant galaxies. The observation that most of globular clusters in the outer halo of the Galaxy are also sub-luminous may be understood by assuming that these clusters once also belonged to faint cluster-rich dwarf systems that were subsequently captured and destroyed by the Milky Way System.
The age of globular clusters inferred from observations depends sensitively on assumptions such as the initial helium abundance and the mass loss rate. A high helium abundance (e.g., $Y\approx$0.28), as well as an inclusion of helium diffusion and oxygen-enhancement in stellar models, can lower the current age estimate for metal-poor globular clusters from 14$\pm 1.5$ Gyr to about 11$\pm 1$ Gyr, significantly relaxing the constraints on the Hubble constant, allowing values as high as 60km/sec/Mpc for a universe with the critical density and 90km/sec/Mpc for a baryon-only universe. The uncertai…
Data are presently available on the luminosities and half-light radii of 101 globular clusters associated with low-luminosity parent galaxies. The luminosity distribution of globulars embedded in dwarf galaxies having $M_{v} > -16$ is found to differ dramatically from that for globular clusters surrounding giant host galaxies with $M_{v} < -16$. The luminosity distribution of globular clusters in giant galaxies peaks at $M_{v} \sim -7.5$, whereas that for dwarfs is found to increases monotonically down to the completeness limit of the cluster data at $M_{v} \sim -5.0$. Unexpectedly, the power …
We present the results of a search for globular clusters in the surroundings of 15 low surface brightness dwarf galaxies belonging to the Fornax Cluster, which was carried out on CCD images obtained with the C and T1 filters of the Washington photometric system. The globular cluster candidates show an extended and probably bimodal (C-T1) color distribution, which is inconsistent with the presence of a single population of metal-poor clusters detected in several dwarf galaxies. The surface number density of these candidates shows no concentration towards the respective dwarf galaxies, in whose …
We review empirical and theoretical findings concerning white dwarfs in Galactic globular clusters. Since their detection is a critical issue we describe in detail the various efforts to find white dwarfs in globular clusters. We then outline the advantages of using cluster white dwarfs to investigate the formation and evolution of white dwarfs and concentrate on evolutionary channels that appear to be unique to globular clusters. We also discuss the usefulness of globular cluster white dwarfs to provide independent information on the distances and ages of globular clusters, information that i…
The distance to the centroid of the M31 globular cluster system is determined by fitting theoretical isochrones to the observed red-giant branches of fourteen globular clusters in M31. The mean true distance modulus of the M31 globular clusters is found to be 24.47 +/- 0.07 mag. This is consistent with distance modulii for M31 that have been obtained using other distance indicators.
We studied the properties of the globular cluster systems of five early--type galaxies in the Fornax galaxy cluster: NGC 1374, NGC 1379, NGC 1387, NGC 1427, and the central giant elliptical NGC 1399. From the investigation of their number of globular clusters, specific frequency, color distribution, surface density profile, and spatial distribution, we conclude that our faint ellipticals have no more globular clusters than spirals of the same mass, and neither differ in their other properties. For these galaxies there is no need to imply a different formation scenario of the globular cluster s…
Dynamical friction due to field stars and tidal disruption caused by a central nucleus are crucial in determining the evolution of the globular cluster system in an elliptical galaxy. In this paper I examine the possibility that some of galactic nuclei have been formed by frictionally decayied globular clusters moving in a triaxial potential. The initial rapid growth of the nucleus, due mainly to massive clusters on box orbits falling in a short time scale into the galactic centre, is found to be later slowed by tidal disruption induced by the nucleus itself on less massive clusters in the way…
We discuss the different biases affecting the sample of field subdwarfs selected from the Hipparcos Catalogue, and used in the Main Sequence Fitting technique to derive distances to Galactic Globular Clusters. The adopted average corrections significantly affect the derived distance moduli, explaining the differences among various groups using this technique.
We present the first spectra obtained with the Very Large Telescope for white dwarfs in a globular cluster. Estimates of atmospheric parameters are obtained and compared to evolutionary tracks. We discuss possible implications for the distance scale of globular clusters and white dwarf evolution and demonstrate how white dwarfs might be used to establish an independent distance scale to globular clusters.
We present the results of moderate resolution spectroscopy for a globular cluster in the M81 group dwarf spheroidal galaxy DDO~78. The DDO78 globular cluster, 4 Milky Way globular clusters, spectroscopic and radial velocity standards were observed with the Long-slit spectrograph of the 6-m telescope (SAO RAS, Russia). Lick spectrophotometric indices were determined in the bandpasses adopted by Burstein et al. (1984). We have derived the mean metallicity of the globular cluster in DDO78 to be [Fe/H] = -1.6 +- 0.1 dex by taking the weighted mean of metallicities obtained from the strength of sev…
We discuss the existence of a mass-metallicity relation among galactic halo globular clusters. The lack of any luminosity-metallicity correlation in globular cluster systems has been used as an argument against self-enrichment models of cluster formation. We show that such a relation is statistically present among the galactic Old Halo globulars. This observational correlation implies that the least massive old clusters are the most metal-rich. This is in contradiction with the idea that, if globular clusters were self-enriched systems, the most metal-rich clusters would also be the most massi…
A brief review on globular cluster sub-populations in galaxies, and their constraints on galaxy formation and evolution is given. The metal-poor and metal-rich sub-populations are put in a historical context, and their properties, as known to date, are summarized. We review why the study of these sub-populations is extremely useful for the study of galaxy formation and evolution, but highlight a few caveats with the current interpretations. We re-visit the current globular cluster system formation scenarios and show how they boil down to a single scenario for the metal-poor clusters (namely th…
I present selected results from a wide-field CCD survey of the globular cluster systems of giant galaxies, including showing how measurements of the specific frequency of metal-poor globular clusters can constrain the redshift of their formation.
We have derived ages and metallicities from co-added spectra of 131 globular clusters associated with the giant elliptical galaxy NGC 4472. Based upon a calibration with Galactic globular clusters, we find that our sample of globular clusters in NGC 4472 span a metallicity range of approximately -1.6 <= [Fe/H] <= 0 dex. There is evidence of a radial metallicity gradient in the globular cluster system, which is steeper than that seen in the underlying starlight. Determination of the absolute ages of the globular clusters is uncertain, but formally, the metal-poor population of globular clusters…
We explore the constraints on globular cluster formation provided by the observed conditions in starbursts where globulars are currently forming, and by the observed properties of young and old globular clusters. We note that the pressure in the ISM of starbursts and mergers implies that molecular clouds in these environments have radii similar to those of globular clusters. Such molecular clouds are therefore viable precursors to globular clusters if the star formation efficiency in the clouds is high. A high star formation efficiency may be a consequence of the high density and associated hi…
We present a progress report of our wide field CCD survey of globular clusters in M31. We have covered 3 deg X 3 deg area centered on M31, using the KPNO 0.9m and Washington CMT1 filters. Our survey is much deeper and more sensitive than previous surveys. We have found several hundred new globular cluster candidates in M31 in addition to confirming previously known globular clusters, and also have found a number of interlopers among previous globular cluster catalogs. We have also obtained spectra of about 500 objects among these candidates using the HYDRA at the WIYN 3.5m telescope, which are…
We compare the properties of the globular clusters around NGC 1399, NGC 1404 and NGC 1380, the three brightest early-type galaxies in the center of the Fornax galaxy cluster. The globular cluster systems are qualitatively very similar, pointing to similar mechanisms of formation. However, NGC 1399 host a factor of 10 more globular clusters, while being only 0.3-0.4 mag brighter, than the other two galaxies. Further, the kinematics of the globular clusters around NGC 1399 favor the association of a large fraction of them with the galaxy cluster rather than with the giant elliptical. Globular cl…
This paper reviews some of the observational properties of globular cluster systems, with a particular focus on those that constrain and inform models of the formation and dynamical evolution of globular cluster systems. I first discuss the observational determination of the globular cluster luminosity and mass function. I show results from new very deep HST data on the M87 globular cluster system, and discuss how these constrain models of evaporation and the dynamical evolution of globular clusters. The second subject of this review is the question of how to account for the observed constancy…
We have obtained spectra for 103 published globular cluster candidates in the Sculptor Group galaxies NGC 253 and NGC 55. On the basis of radial velocities and digitized plate images, 14 globular clusters are identified in NGC 253 and one probable globular cluster is identified in NGC 55. The majority of the objects in the sample appear to be background galaxies. We have obtained and analysed COSMOS plate scans of NGC 253 and NGC 55 and use these along with the spectroscopically identified clusters to define new samples of globular cluster candidates in the two galaxies which should have reduc…
We investigate the role of dissolution of low-concentration clusters due to mass loss through stellar evolution on the evolution of the properties of globular cluster systems (GCSs) in elliptical galaxies. Our simulations show that, for an initial mass-concentration relationship based on that inferred from Galactic globular clusters, dissolution of low-concentration clusters leads to the disruption of a large number of clusters. A power-law initial globular cluster system mass function (GCMF) similar to that observed in young cluster systems in merging galaxies is transformed by this dissoluti…
Despite the rapid observational progress in the study of young massive star clusters, the formation of globular clusters still remains poorly understood. Yet, it is emerging that globular cluster formation is intimately linked to the formation of the Galaxy. I discuss a generic scenario of the cluster formation within progenitor galaxies, based on the available observational constraints. The oldest clusters formed around redshift z = 7, but the process continued at least until z = 3. Because of their high density, globular clusters survived when their progenitor galaxies were disrupted by the …
Blue horizontal branch and UV bright stars in several globular clusters are analysed spectroscopically and the results are compared with predictions of stellar evolutionary theory. We find that the distribution of temperatures and surface gravities of the blue HB stars may be explained by the effects of deep mixing. The masses derived for these stars are too low unless one uses the long distance scale for globular clusters. First results on blue HB stars in metal rich clusters are presented. Analyses of hot UV bright stars in globular clusters uncovered a lack of genuine post-asymptotic giant …
We present a photometric study of the globular clusters in the giant elliptical galaxy M60 in the Virgo cluster, based on deep, relatively wide field Washington CT_1 CCD images. The color-magnitude diagram reveals a significant population of globular clusters in M60, and a large number of young luminous clusters in NGC 4647, a small companion spiral galaxy north-west of M60. The color distribution of the globular clusters in M60 is clearly bimodal, with a blue peak at (C-T_1)=1.37, and a red peak at (C-T_1)=1.87. We derive two new transformation relations between the (C-T_1)_0 color and [Fe/H]…
A populations of stellar mass black hole binaries may exist in globular clusters. The dynamics of globular cluster evolution imply that there may be at most one black hole binary is a globular cluster. The population of binaries are expected to have orbital periods greater than a few hours and to have a thermal distribution of eccentricities. In the LISA band, the gravitational wave signal from these binaries will consist of several of the higher harmonics of the orbital frequency. A Monte Carlo simulation of the galactic globular cluster system indicates that LISA will detect binaries in 10 %…
The present-day globular cluster populations of galaxies reflect the cumulative effects of billions of years of galaxy evolution via such processes as mergers, tidal stripping, accretion, and in some cases the partial or even complete destruction of other galaxies. If large galaxies have grown by consuming their smaller neighbors, or by accreting material stripped from other galaxies, then their observed globular cluster systems are an amalgamation of the globular cluster systems of their progenitors. Careful analysis of the globular cluster populations of galaxies can thus allow astronomers t…
The globular clusters of the LMC are found to be significantly more elliptical than Galactic globular clusters, but very similar in virtually all other respects. The ellipticity of the LMC globular clusters is shown not be correlated with the age or mass of those clusters. It is proposed that the ellipticity differences are caused by the different strengths of the tidal fields in the LMC and the Galaxy. The strong Galactic tidal field erases initial velocity anisotropies and removes angular momentum from globular clusters making them more spherical. The tidal field of the LMC is not strong eno…
The radial velocities of the 13 globular clusters in the Large Magellanic Cloud have a dispersion of 28 km s$^{-1}$ relative to the HI rotation curve of the LMC, compared to a dispersion of 30 km s$^{-1}$ with regard to the mean globular cluster velocity. This shows that, contrary to a suggestion by Schommer et al. (1992), one cannot yet rule out the possibility that the LMC globular clusters formed in a pressure supported halo, rather than in rotating disk. The globular clusters in the LMC may therefore, after all, exhibit a relationship between age and kinematics that is similar to that of t…
The Whipple 10m Very High Energy gamma-ray telescope has been used to search for indications of WIMP annihilation in the direction of the globular cluster M15. The upper limits derived constrain the amount of super-symmetric dark matter that may reside in globular clusters.
This paper is the third in the series of papers on M81 globular clusters. In this paper, we present spatial and metal abundance properties of 95 M81 globular clusters, which comprise nearly half of all the M81 globular cluster system. These globular clusters are divided into two M81 metallicity groups by a KMM test. Our results show that, the metal-rich clusters did not demonstrate a centrally concentrated spatial distribution as ones in M31, and metal-poor clusters tend to be less spatially concentrated. In other words, the distribution of the metal-rich clusters in M81 is not very similar to…
A velocity dispersion has been measured for the luminous globular cluster M31 037 - B327, claimed to be the most massive star cluster in the Local Group and to be a young ``super star cluster'' that has survived to an old age. M31 037 - B327 has a mass comparable to that of M31 G1, but not significantly larger. Although near the upper end for the mass distribution of globular clusters, it is not an unprecedented extraordinary object.
In this paper we briefly review the properties of the dwarf galaxy population at the core of the Fornax cluster, as well as the properties of the extraordinary rich globular cluster system and the cD halo around the central galaxy NGC 1399. In turn, the question whether a scenario in which dwarf galaxies have been accreted and dissolved in the cluster center can explain the observed properties is addressed. The possibility of accretion of a certain number of dwarf galaxies, the stripping of their globular clusters and gas, and the formation of new globular clusters from stripped infalling gas …
Globular clusters in galaxies have a mutual feedback with the environment, which is tuned by their dynamical evolution. This feedback may be the explanation of various features of both the globular cluster system and the host galaxy. Relevant examples of these are the radial distribution of globulars and the violent initial activity of galaxies as AGN.
Here we present HST WFPC2 imaging of 14 kinematically--distinct core ellipticals to examine their globular cluster systems. In particular, we probe the galaxy central regions, for which we might expect to see the strongest signatures of some formation and destruction processes. These data substantially increase the number of extragalactic globular cluster systems studied to date. We find that the globular cluster luminosity, and color, vary only weakly, if at all, with galactocentric distance. The mean color of globular clusters are constant with globular cluster magnitude. Several clear trend…
In this second paper in our series, we continue to test primordial scenarios of globular cluster formation which predict that globular clusters formed in the early universe in the potential of dark matter minihalos. In this paper we use high-resolution N-body simulations to model tidal stripping experienced by primordial dark-matter dominated globular clusters in the static gravitational potential of the host dwarf galaxy. We test both cuspy Navarro-Frenk-White (NFW) and flat-core Burkert models of dark matter halos. Our primordial globular cluster with an NFW dark matter halo survives severe …
Some of the properties of the globular clusters in NGC 5128 are compared to those of Galactic globular clusters. Assuming the color- metallicity relations that hold for Galactic globular clusters then the metal-poor clusters in NGC 5128 that have [Fe/H] < -1.80 are significantly fainter than are the more metal-rich globulars in that galaxy. No such metallicity dependent luminosity difference is observed among the globular clusters associated with the Milky Way. Furthermore the NGC 5128 cluster sample contains two objects that, on the basis of their observed colors, appear to be super metal-poo…
The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the effects of a cluster environment on globular cluster systems. Here we present new optical imaging of these two galaxies from both the Hubble Space Telescope's Wide Field and Planetary Camera 2 and the Cerro Tololo Inter-American Observatory's 1.5m telescope. The combination of both data sets provides unique insight on the spatial and colour distribution of globular clusters. From B-I colours, we find that both galaxies have a broad globular cluster metallicity distribution that is inconsistent with a single population…
We investigated the globular cluster system of the S0 galaxy NGC 1380 in the Fornax cluster with deep BVR photometry. We identified two, presumably old, populations of globular clusters. The blue globular clusters seem to be counterparts to the halo globular clusters in our Milky Way. They have comparable colors and magnitudes, are spherically distributed around the elliptical galaxy but have a very flat surface density profile. The red population follows the stellar light in ellipticity and position angle, has a similar magnitude distribution to the blue clusters, and a surface density profil…
Space-based gravitational wave detectors will have the ability to observe continuous low frequency gravitational radiation from binary star systems. They can determine the direction to continuous sources with an angular resolution approaching tens of arcminutes. This resolution should be sufficient to identify binary sources as members of some nearby globular clusters. Thus, gravitational radiation can be used to determine the population of hard binaries in globular clusters. For particularly hard binaries, the orbital period may change as a result of gravitational wave emission. If one of the…
We propose a self-consistent model for globular cluster formation in, but not limited to, our Galaxy, based on the merger model of Mathews and Schramm (1993). Stars and star clusters form in bursts at the merging interfaces as protogalactic clouds collide. We describe the formation of those star clusters with a simple schematic ansatz which takes into account the thermal and Kelvin-Helmholtz instabilities. It is shown that this model is consistent with many observational properties such as the age and metallicity distributions of globular clusters, the overall number of globular clusters, and …
This review presents the results of ongoing studies of the formation histories of metal-poor and metal-rich globular clusters and their host galaxies. I first discuss the strong observational evidence that the globular cluster systems of most elliptical galaxies have bimodal metallicity distributions. I then focus on new results for metal-poor and metal-rich globular cluster systems. Metal-poor globular clusters are often associated with early structure formation, and I review new constraints on their formation epoch based on the ``bias'' of the number of metal-poor clusters with host galaxy m…
A large number of X-ray sources in globular clusters of galaxies other than the Milky Way has been found with Chandra. We discuss three issues relating to these sources. The X-ray luminosity function (XLF) of the sources in globular clusters of M31 is marginally compatible with the XLF of globular clusters of the Milky Way. The individual XLFs of a dozen elliptical galaxies, after correction for incompleteness, are compatible with one another and show no break; however, the XLF found by adding the individual XLFs of elliptical galaxies has a break at L_x about 5x10(38) ergs/s. For the moment t…
