We have conducted an extensive photometric search for dwarf nova (DN) outbursts in 16 Galactic globular clusters (GCs). The survey was based on the rich photometric data collected by the Cluster AgeS Experiment (CASE) team. We have identified two new DNe. Together with previously known systems this gives the total number of 12 known DNe in 7 Galactic GCs. Inserting artificial light curves of “DNe” into frames of investigated clusters allowed us to assess completeness of the search. Our results clearly show that outbursting cataclysmic variables (CVs) are very rare in GCs in comparison to field CVs where half of the systems belongs to DNe. Recent X-ray observations of GCs lead to identification of hundreds of compact binaries. Many of them are promising candidates for CVs. The theory also predicts that dozens of white/red dwarf binaries should form in the cores of GCs via dynamical processes or internal evolution of the binaries. Our results rises the question about possible causes of paucity of outbursts in GCs.

We report on XMM-Newton and Chandra observations of the globular cluster NGC 2808. We detect one quiescent low mass X-ray binary of the 3±1 expected, if these systems are formed through encounters, and we show evidence for the presence of 20±10 bright cataclysmic variables in the core with a luminosity above 4×1031 erg s−1. We also review the specific nature of cataclysmic variables in globular clusters with reference to recent VLT/FORS1 observations of a cataclysmic variable in M 22.

We present a brief update on the ACS Virgo and Fornax Cluster Surveys — Hubble Space Telescope programs to obtain ACS imaging for 143 early-type galaxies in the two galaxy clusters nearest to the Milky Way. We summarize a selection of science highlights from the surveys as including new results on the central structure of early-type galaxies, the apparent continuity of photometric and structural parameters between dwarf and giant galaxies, and the properties of globular clusters, diffuse star clusters and ultra-compact dwarf galaxies.

Explorations of the globular cluster populations in many nearby galaxies are revealing increasing connections to other dense stellar systems such as UCDs, DGTOs, and nuclear star clusters in dwarf galaxies. The nearest giant elliptical, NGC 5128, is now giving us a much-improved delineation of the GC Fundamental Plane of structural parameters, and indicates as well that the known correlation between GC scale size and metallicity is likely to be at least partly a projection effect coupled with the different spatial distributions of the metal-poor and metal-rich clusters. New photometry of the huge cluster populations around the giant Brightest Cluster Ellipticals, which allows us to study samples of many thousands of GCs at once, are now beginning to turn up surprising examples of “sequences” of high-mass GCs leading up to the UCD regime. Lastly, new modelling of cluster formation through a specially tuned semi-analytic galaxy formation code strongly suggests that the mass-metallicity relation now known to affect the blue GC sequence can arise fairly naturally out of such models, if significant numbers of the massive GCs actually represent the remnant nuclei of stripped dwarf satellites.

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 of the globular cluster mass function with distance from the center of the host galaxy. The problem is that a radial trend is expected for isotropic cluster orbits, and while the orbits are observed to be roughly isotropic, no radial trend in the globular cluster system is observed. I review three extant proposals to account for this, and discuss observations and calculations that might determine which of these is most correct. The final subject is the origin of the very weak mass-radius relation observed for globular clusters. I discuss how this strongly constrains how globular clusters form and evolve. I also note that the only viable current proposal to account for the observed weak mass-radius relation naturally effects the globular cluster mass function, and that these two problems may be closely related.

We probe the evolution of globular clusters that could form in giant molecular clouds within high-redshift galaxies. Numerical simulations demonstrate that the large and dense enough gas clouds assemble naturally in current hierarchical models of galaxy formation. These clouds are enriched with heavy elements from earlier stars and could produce star clusters in a similar way to nearby molecular clouds. The masses and sizes of the model clusters are in excellent agreement with the observations of young massive clusters. Do these model clusters evolve into globular clusters that we see in our and external galaxies? In order to study their dynamical evolution, we calculate the orbits of model clusters using the outputs of the cosmological simulation of a Milky Way-sized galaxy. We find that at present the orbits are isotropic in the inner 50 kpc of the Galaxy and preferentially radial at larger distances. All clusters located outside 10 kpc from the center formed in the now-disrupted satellite galaxies. The spatial distribution of model clusters is spheroidal, with a power-law density profile consistent with observations. The combination of two-body scattering, tidal shocks, and stellar evolution results in the evolution of the cluster mass function from an initial power law to the observed log-normal distribution.

We present a study of the globular cluster (GC) systems of nearby elliptical and S0 galaxies at a variety of wavelengths from the X-ray to the infrared. Our analysis shows that roughly half of the low mass X-ray binaries (LMXBs), that are the luminous tracers of accreting neutron star or black hole systems, are in clusters. There is a surprisingly strong correlation between the LMXB frequency and the metallicity of the GCs, with metal-rich GCs hosting three times as many LMXBs as metal-poor ones, and no convincing evidence of a correlation with GC age so far. In some of the galaxies the LMXB formation rate varies with GC color even within the red peak of the typical bimodal cluster color distribution, providing some of the strongest evidence to date that there are metallicity variations within the metal-rich GC peak as is expected in a hierarchical galaxy formation scenario. We also note that any analysis of subtler variations in GC color distributions must carefully account for both statistical and systematic errors. We caution that some published GC correlations, such as the apparent ‘blue-tilt’ or mass-metallicity effect might not have a physical origin and may be caused by systematic observational biases.

Evidence favouring a Gaussian initial mass function for systems of old globular clusters has accumulated over recent years. We show that a bell-shaped mass function may be the imprint of expulsion from protoclusters of the leftover star forming gas due to supernova activity. Owing to the corresponding weakening of its gravitational potential, a protocluster retains a fraction only of its newly formed stars. The mass fraction of bound stars extends from zero to unity depending on the star formation efficiency achieved by the protoglobular cloud. We investigate how such wide variations affect the mapping of the protoglobular cloud mass function to the initial globular cluster mass function. We conclusively demonstrate that the universality of the globular cluster mass function originates from a common protoglobular cloud mass-scale of about 106 M⊙ among galaxies. Moreover, gas removal during star formation in massive gas clouds is highlighted as the likely prime cause of the predominance of field stars in the Galactic Halo.

We present an analysis of the radial velocities and velocity dispersions for 27 bright globular clusters in the nearby elliptical galaxy NGC 5128 (Centaurus A). For 22 clusters we combine our new velocity dispersion measurements with the information on the structural parameters, either from the literature when available or from our own data, in order to derive the cluster masses and mass-to-light (M/L) ratios. The masses range from 1.2 × 105M⊙, typical of Galactic globular clusters, to 1.4 × 107M⊙, similar to more massive dwarf globular transition objects (DGTOs) or ultra compact dwarfs (UCDs) and to nuclei of nucleated dE galaxies. The average M/LV is 3±1, larger than the average M/LV of globular clusters in the Local Group galaxies. The correlations of structural parameters, velocity dispersion, masses and M/LV for the bright globular clusters extend the properties established for the most massive Local Group clusters towards those characteristic of dwarf elliptical galaxy nuclei and DGTOs/UCDs. The detection of the mass-radius and the mass-M/LV relations for the globular clusters with masses greater than ~ 2 × 106M⊙ provides the link between “normal” old globular clusters, young massive clusters, and evolved massive objects.

The evolution induced by dynamical friction on a spherical shell of rigid satellites interacting directly with the particles sampling the host elliptical galaxy is followed by means of N-body simulations for a variety of shell–galaxy configurations.

We present results from a new wide-field study of the NGC 5128 globular cluster system. We have obtained new high resolution images with the Magellan 6.4m + IMACS camera. Our images cover an area of 1.2x1.2 sq. degrees and have a seeing of 0.45″. This allows us to not only resolve most of the globular clusters (GCs) but also derive their structural parameters. These are combined with existing Washington photometry in order to select by metallicity. We present here results for a subsample of 359 GCs which includes all currently confirmed GC members of the system. Our derived sizes are in very good agreement with those derived from ACS data. We find, as expected, that the metal-rich GCs in the inner regions (r < 10′) are 26% smaller than their metal-poor components, but in the outer region this normal trend is reversed. We compare our GCs to previous results for GCs, UCDs, etc. in the luminosity - size plane and find substantial overlap between different types of objects, indicating more of a continuum in these properties.

Dynamical mass estimates of ultra-compact dwarfs galaxies and massive globular clusters in the Fornax and Virgo clusters and around the giant elliptical Cen A have revealed some surprising results: 1) above ~106M⊙ the mass-to-light (M/L) ratio increases with the objects' mass; 2) some UCDs/massive GCs show high M/L values (4 to 6) that are not compatible with standard stellar population models; and 3) in the luminosity-velocity dispersion diagram, UCDs deviate from the well-defined relation of “normal” GCs, being more in line with the Faber-Jackson relation of early-type galaxies. In this contribution, we present the observational evidences for high mass-to-light ratios of UCDs and discuss possible explanations for them.

We present a Gemini/GMOS program to measure spectroscopic metallicities and ages of globular clusters (GCs) and nuclei in dwarf elliptical galaxies in the Virgo and Fornax Clusters. Preliminary results indicate that the globular clusters are old and metal-poor, very similar to the GCs in the Milky Way halo. The nuclei tend to be more metal-rich than the globular clusters but more metal-poor and older, on average, than the stars in the bodies of the galaxies. The [α/Fe] ratio appears to be solar for the GCs, nuclei, and dEs, but the uncertainties do not exclude some globular clusters from being enhanced in alpha elements.

Recent HST observations have revealed that compact sources exist at the centers of many galaxies across the Hubble sequence. These sources are called “nuclear star clusters” (NCs), because their structural properties and scaling relationships are similar to those of globular clusters (GCs). It has been also found that the relationship between the masses of NCs and that of the host galaxies is similar to that obeyed by supermassive black holes (SBHs). In this observational frame, the hypothesis that galactic nuclei may be the remains of GCs driven inward to the galactic center by dynamical friction and there merged, finds an exciting possible confirm. In this short paper we report of our recent results on GC mergers obtained by mean of detailed N-body simulations.

Using the most advanced anisotropic (2D) Fokker-Planck (FP) models, we calculate the evolution of the mass functions of the Galactic globular cluster system (GCMF). Our models include two-body relaxation, binary heating, tidal shocks, dynamical friction, stellar evolution, and realistic cluster orbits. We perform 2D-FP simulations for a large number of virtual globular clusters and synthesize these results to study the relation between the initial and present GCMFs. We found two probable IGCMFs that eventually evolve into the Milky Way GCMF : truncated power-law, and log-normal model with higher initial low mass limit and peak mass than the earlier studies.

We describe efforts over the last six years to implement regularization methods suitable for studying one or more interacting black holes by direct N-body simulations. Three different methods have been adapted to large-N systems: (i) Time-Transformed Leapfrog, (ii) Wheel-Spoke, and (iii) Algorithmic Regularization. These methods have been tried out with some success on GRAPE-type computers. Special emphasis has also been devoted to including post-Newtonian terms, with application to moderately massive black holes in stellar clusters. Some examples of simulations leading to coalescence by gravitational radiation will be presented to illustrate the practical usefulness of such methods.

Since we cannot put stars in a laboratory, astrophysicists had to wait till the invention of computers before becoming laboratory scientists. For half a century now, we have been conducting experiments in our virtual laboratories. However, we ourselves have remained behind the keyboard, with the screen of the monitor separating us from the world we are simulating. Recently, 3D on-line technology, developed first for games but now deployed in virtual worlds like Second Life, is beginning to make it possible for astrophysicists to enter their virtual labs themselves, in virtual form as avatars. This has several advantages, from new possibilities to explore the results of the simulations to a shared presence in a virtual lab with remote collaborators on different continents. I will report my experiences with the use of Qwaq Forums, a virtual world developed by a new company (see http://www.qwaq.com).

I'll summarize the current status of GRAPE project. GRAPE-6, completed in 2002, has been used by a number of people, for a wide variety of problems such as planet formation, star cluster dynamics, galactic nuclei, and cosmology. In 2004, we started the development of the next-generation machine, GRAPE-DR. GRAPE-DR has a architecture radically different from that of previous GRAPEs. It does not have hardwired pipeline for gravitational force calculation but a large number of small and simple programmable processors. This change made it possible to apply GRAPE-DR to a wide range of problems to which GRAPE was not efficient, and at the same time it helps us to explore new algorithms for N-body simulations. The GRAPE-DR chip was completed in 2006, and second prototype board was completed in May 2007. We hope to have full production-level board commercially available by the end of year 2007. A single board will offer the theoretical peak speed of 2 Tflops, about 20 times as that of a single PCI card version of GRAPE-6.

We have developed a new tree-direct hybrid algorithm, “Bridge”. It can simulate small scale systems embedded within large-N systems fully self-consistently. Using this algorithm, we have performed full N-body simulations of star clusters near the Galactic center (GC) and compared the orbital evolutions of the star cluster with those obtained by “traditional” simulations, in which the orbital evolution of the star clusters is calculated from the dynamical friction formula. We found that the inspiral timescale of the star cluster is shorter than that obtained with traditional simulations. Moreover, we investigated the eccentricities of particles escaped from the star cluster. Eccentric orbit of the star cluster can naturally explain the high eccentricities of the observed stars.

In the numerical simulations of evolution of star clusters, binary-single star interactions frequently take place. Since the direct integration of them is time consuming, distant interactions between binaries and field stars are often integrated by using some approximations. Traditionally the effect of the error caused by the approximated treatment is regarded as small enough to be ignored. However, if we have a binary-dominated core, the energy drift is large. In this study, we perform numerical experiments to evaluate the effect of neglecting the weak perturbation from distant single particles. We developed an N-body integrator which can manipulate multiple precision floating point numbers.