In this paper we present the results of two detailed N-body simulations of the interaction of a sample of four massive globular clusters in the inner region of a triaxial galaxy. A full merging of the clusters takes place, leading to a slowly evolving cluster which is quite similar to observed Nuclear Clusters. Actually, both the density and the velocity dispersion profiles match qualitatively, and quantitatively after scaling, with observed features of many nucleated galaxies. In the case of dense initial clusters, the merger remnant shows a density profile more concentrated than that of the progenitors, with a central density higher than the sum of the central progenitors central densities. These findings support the idea that a massive Nuclear Cluster may have formed in early phases of the mother galaxy evolution and lead to the formation of a nucleus, which, in many galaxies, has indeed a luminosity profile similar to that of an extended King model. A correlation with galactic nuclear activity is suggested.
The origin of metal-rich, highly flattened, and rapidly rotating disk globular cluster system in the Galaxy is one of longstanding issues in the context of the Galaxy formation. Our numerical simulations suggest a new ``two-fold'' scenario that the disk globular clusters are firstly formed in the high-pressure, dense central region of a gas-rich dwarf galaxy, as induced during the tidal interaction with the pre-existing, young thin disk of the Galaxy, and then dispersed into the disk region owing to the final tidal destruction of the merging dwarf. We also demonstrate that spatial distribution…
We present results of fully self-consistent N-body simulations of the motion of four globular clusters moving in the inner region of their parent galaxy. With regard to previous simplified simulations, we confirm merging and formation of an almost steady nuclear cluster, in a slightly shorter time. The projected surface density profile shows strong similarity to that of resolved galactic nuclei. This similarity reflects also in the velocity dispersion profile which exhibits a central colder component as observed in many nucleated galaxies.
We present the results of detailed N-body simulations regarding the interaction of four massive globular clusters in the central region of a triaxial galaxy. The systems undergo a full merging event, producing a sort of 'Super Star Cluster' (SSC) whose features are close to those of a superposition of the individual initial mergers. In contrast with other similar simulations, the resulting SSC structural parameters are located along the observed scaling relations of globular clusters. These findings seem to support the idea that a massive SSC may have formed in early phases of the mother galax…
The interpretation of the difference observed between the radial distribution of globular clusters and that of halo-bulge stars in elliptical galaxies is discussed in terms of evolution of their globular cluster systems. I present a short summary of the evidence that dynamical evolution of globular cluster systems is not only able to explain the flattening of their distribution toward the galactic center, but also it may have played an important role on the primordial activity of the parent galaxy.
Different types of observations, together with consistent and physical modelizations, suggest as realistic the hypothesis of enrichement of galactic nuclei by mean of massive globular clusters orbitally decayed and merged in the inner regions of early type galaxies. In this context, the scenario of globular cluster mergers and subsequent formation of a dense Super Star Cluster in the center of a triaxial galaxy is presented and discussed, together with its astrophysical implications, including that of massive black hole feeding and accretion in the center of a triaxial galaxy.
We investigate the spatial, kinematic and chemical properties of globular cluster systems formed in merging and interacting galaxies using N-body/SPH simulations. Although we can not resolve individual clusters in our simulation, we assume that they form in collapsing molecular clouds when the local external gas pressure exceeds 10$^5$ $k_B$ (where $k_B$ 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. Gra…
We present wide field-of-view near-infrared imaging from the NTT and very deep optical imaging from the HST of the young merging galaxy NGC 3597. The morphology of the galaxy and the properties of the newly formed proto-globular clusters (PGCs) are examined. Our K band data reveals the presence of a second nucleus, which provides further evidence that NGC 3597 is the result of a recent merger. Combining new K band photometry with optical photometry, we are able for the first time to derive a unique age for the newly formed PGCs of a few Myrs. This is consistent with the galaxy starburst age of…
Prompted by a new catalogue of M31 globular clusters, we have collected together individual metallicity values for globular clusters in the Local Group. Although we briefly describe the globular cluster systems of the individual Local Group galaxies, the main thrust of our paper is to examine the collective properties. In this way we are simulating the dissipationless merger of the Local Group, into presumably an elliptical galaxy. Such a merger is dominated by the Milky Way and M31, which appear to be fairly typical examples of globular cluster systems of spiral galaxies. The Local Group `Ell…
CCD images and spectra of ionized hydrogen in the merging system NGC3256 were acquired as part of a kinematic study to investigate the formation of globular clusters (GC) during the interactions and mergers of disk galaxies. This paper focuses on the proposition that giant H II regions, with an H_alpha luminosity > 1.5 x 10^{40} erg s^{-1}, are birthplaces of young populous clusters (YPC). We estimate 900 30-Doradus-like H II regions formed during the galaxy-galaxy interaction which created NGC 3256. A specific frequency argument for forming a sufficient number of GC, assuming that the NGC3256…
Collisions and mergers of gas-rich galaxies trigger bursts of star and cluster formation. Of the thousands of clusters typically formed during a major merger, only the most massive and compact survive for Gigayears as globular clusters (GCs). In <~1 Gyr old merger remnants, these `second-generation' GCs appear by the hundreds as young halo clusters of about solar metallicity. Their likely descendants, metal-rich GCs of intermediate age (2 - 5 Gyr), have recently been found in ~10 elliptical galaxies, where they appear as slightly overluminous red GCs with a still power-law-like luminosity func…
Based on the earlier work of Gunn and McCrea we model the formation of globular clusters in merging galaxies. Neutral hydrogen observations of dwarf irregular galaxies as well as more luminous systems are used to provide the key parameters of the model. The observations indicate that clusters with the mass of globular clusters should still be forming today. The model is incorporated into a phenomenological picture of galaxy evolution making use of a simple chemical evolution model. These results are compared to recent observations of the metallicity distributions of F and G stars from a recent…
We present VI photometry of the metal-poor inner halo globular clusters NGC6293 and NGC6541 using the planetary camera of the WFPC2 on board HST. Our color-magnitude diagrams of the clusters show well-defined BHB populations, consistent with their low metallicities and old ages. NGC6293 appears to have blue straggler stars in the cluster's central region. We discuss the interstellar reddening and the distance modulus of NGC6293 and NGC6541 and obtain E(B-V) = 0.40 and (m-M)_0 = 14.61 for NGC6293 and E(B-V) = 0.14 and (m-M)_0 = 14.19 for NGC6541. Our results confirm that NGC6293 and NGC6541 are…
We show that the globular cluster luminosity function (GCLF) of the inner halo of the Milky Way is statistically different from the GCLF of the outer halo. We also find a similar difference between the inner and outer halo population of M31. We assert that this difference is evidence for some form of dynamical evolution of the cluster population and/or a dependence of GCLF shape on the environment in which the cluster population formed. We also find that the turnover luminosity of the GCLF is unaffected by these differences and further assert that this stability of the turnover luminosity affi…
The main topic of this paper is the investigation of the modes of interaction of globular clusters (GCs) moving in the inner part of a galaxy. This is tackled by means of high-resolution N-body simulations, whose first results are presented in this article. Our simulations dealt with primordial very massive (order of 10^7 solar masses) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a time much shorter than their internal relaxation time. To check the disruptive role of both tidal forces and GC-GC collisions, we maximised the tidal int…
JHK images obtained with the Canada-France-Hawaii Telescope (CFHT) Adaptive Optics Bonnette (AOB) are used to investigate the near-infrared photometric properties of red giant branch (RGB) and horizontal branch (HB) stars in eight metal-poor globular clusters with R_GC < 2 kpc. The slope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature of these clusters, four of which appear to have metallicities comparable to M92. The luminosity functions of RGB stars in inner spheroid and outer halo clusters have similar slopes, although there is a tendency for core-collapsed clusters to have s…
We present a study of globular clusters in the inner region of the giant elliptical galaxy NGC 4472, based on the HST WFPC2 archive data. We have found about 1560 globular cluster candidates at the galactocentric radius r < 4 arcmin. V-(V-I) diagram of these objects shows a dominant vertical structure which consists obviously of two components: blue globular clusters (BGCs) and red globular clusters (RGCs). The luminosity function of the globular clusters is derived to have a peak at V(max)=23.50+/-0.16 from Gaussian fitting. The distance to NGC 4472 is estimated to be d=14.7+/-1.3 Mpc.The pea…
We consider the observational signature of the dynamical effects on the luminosity function of globular clusters. For the three best studied systems, in Milky Way, M31, and M87, there is a statistically significant difference between the inner and outer population of globular clusters. In all cases the inner clusters are on average brighter than the outer clusters ($0.26 < Δm_0 < 0.84$) and have a smaller dispersion in magnitudes ($0.04 < Δσ< 0.53$), with the larger differences for the local, better observed samples. The differences are of the type that would be expected if the inner populatio…
We carry out a large program to classify newly discovered star clusters from Froebrich et al. (2006) in the inner Galaxy. Here, we present a first analysis of a new high-priority globular cluster candidate, FSR1735 at l=339.1879; b=-1.8534, based on new deep near infrared observations from Sofi at the NTT. A significant peak in the K-band luminosity function is found, which is interpreted as the clump of post-He-flash stars. The distance and the reddening of the cluster are determined to 9.1kpc and A_K=0.5mag, respectively, the metallicity is estimated to [M/H]=-0.8. Radial star density profil…
We have identified 26 new globular cluster candidates in the inner 3 kpc of NGC 5128 (Cen A), the nearest known large galaxy being the probable product of a merger. The clusters are selected on the basis of their structural parameters (observed core diameters and ellipticities), as measured from archival WFPC HST images. IR photometry obtained with IRAC2B at the ESO/MPI 2.2-m telescope is combined with the optical HST photometry. Most of these clusters have normal colors typical of old globular clusters like those found in the Milky Way and M31. We estimate their metal abundances based on the …
We studied the relation between the ratio of rotational velocity to velocity dispersion and the metallicity (<v_{rot}>/σ_{v}-metallicity relation) of globular cluster systems (GCS) of disk galaxies by comparing the relation predicted from simple chemo-dynamical models for the formation and evolution of disk galaxies with the observed kinematical and chemical properties of their GCSs. We conclude that proto disk galaxies underwent a slow initial collapse that was followed by a rapid contraction and derive that the ratio of the initial collapse time scale to the active star formation time scale …
We have carried out a detailed abundance analysis for 21 elements in a sample of four RGB stars in the outer halo globular cluster NGC 7492 (R{GC} 25 kpc); we find [Fe/H] = -1.82 dex inferred from FeI lines (-1.79 from FeII) using high dispersion (R=35,000) spectra obtained with HIRES at the Keck Observatory. Most elements show no sign of star-to-star variation within our limited sample. We have, however, detected an anti-correlation between O and Na abundances similar to that seen in our previous analyses of inner halo GCs as well as in studies of relatively nearby GCs by others. We compare t…
The evolution of globular cluster systems in some galaxies can be cause of merging of globulars in the very central regions. This high stellar density favours the growth of a central nucleus via swallowing of surrounding stars. The infall of stars into a nuclear black hole is here shown to be, under certain conditions, not only source of electromagnetic radiation but also a significant source of gravitational waves.
HIRES on the Keck I telescope has been used to measure the first radial velocities for stars belonging to eleven, heavily-reddened globular clusters in the direction of the inner Galaxy. The question of kinematic substructuring among the Galactic globular cluster system is investigated using an updated catalog of globular cluster distances, metallicities and velocities. It is found that the population of metal-rich globular clusters shows significant rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc of the Galactic center, the measured rotation velocity and line-of…
Theoretical considerations and observational data support the idea that mergers were more frequent in the past. At high redshifts, violent interactions and mergers may be implicated in the origin of Lyman-break galaxies, sub-mm starbursts, and active galactic nuclei. Most stars in cluster ellipticals probably formed at redshifts z > 2, as did most of the halo and globular clusters of the Milky Way; these events may all be connected with mergers. But what kind of galaxies merged at high redshifts, and how are these early events connected to present-epoch mergers? I will approach these questions…
Taken together, key latest observations assume that (i) old metal-rich globular cluster populations (MRGCPs) in bulges were able to form due to essentially increased, self(internally)-induced star formation rate (SFR) in the hosts, while galaxy merging played an additional role; (ii) massive star cluster populations (MSCPs) in irregulars may be young, less prominent counterparts of the old MRGCPs in spheroids.
We present a chemical abundance study of three inner old halo clusters NGC 6287, NGC 6293, and NGC 6541, finding [Fe/H] = -2.01 +/- 0.05, -1.99 +/- 0.02, and -1.76 +/- 0.02, respectively, and our metallicity measurements are in good agreement with previous estimates. The mean alpha-element abundances of our program clusters are in good agreement with other globular clusters, confirming previous results. However, the individual alpha-elements appear to follow different trends. The silicon abundances of the inner halo clusters appear to be enhanced and the titanium abundances appear to be deplet…
Images recorded through J, H, K, 2.2 micron continuum and CO filters have been obtained of a sample of metal-poor ([Fe/H] < -1.3) globular clusters in the inner spheroid of the Galaxy. The shape and color of the upper giant branch on the (K, J-K) CMD, combined with the K brightness of the RGB-tip, are used to estimate the metallicity, reddening, and distance of each cluster. CO indices are used to identify bulge stars, which will bias metallicity and distance estimates if not removed from the data. Bulge fields were also observed near each cluster. The slope of the bulge giant branch luminosit…
HST (V,I) photometry has been obtained for the inner halo globular cluster NGC 6652. The photometry reaches approximately 4 mag below the turn-off and includes a well populated horizontal branch. This cluster is located close to the Galactic center at a galactocentric distance of approximately 2.0 kpc with a reddening of E(V-I) = 0.15 +/- 0.02 and has a metallicity of [Fe/H] approximately -0.85. Based upon Delta(V) between the point on the sub-giant branch which is 0.05 mag redder than the turn-off and the horizontal branch, NGC 6652 is 11.7 +/- 1.6 Gyr old. Using this same Delta(V), precise d…
Globular clusters are stellar dynamical systems which evolve on stellar evolutionary and both internal and external dynamical timescales. Quantitative comparison of cluster properties with realistic evolutionary dynamical models is becoming feasible, and will underpin the subject in the next few years. A few examples of the types of analysis becoming possible are presented.
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. …
We have investigated the formation of globular cluster (GC) systems in the fiducial semi-analytic galaxy formation model of Cole et al. We assume that GCs are formed at high-redshift (z > 5) in proto-galactic fragments, and during the subsequent gas-rich merging of these fragments. We find that the mean ages of the metal-rich GCs are dependent upon host galaxy luminosity and halo circular velocity, whereas the mean ages of the metal-poor GCs are not. We find that gaseous merging of the proto-galactic fragments leads to a significant age-spread amongst the metal-rich GCs, which increases for lo…
We describe a scheme for the formation of globular cluster systems in early-type galaxies using a semi-analytic model of galaxy formation. Operating within a Lambda CDM cosmology, we assume that metal-poor globular clusters are formed at high-redshift in pre-galactic fragments, and that the subsequent gas-rich merging of these fragments leads to the formation of metal-rich clusters. We compare our results with contemporary data, and look at the particular case of the globular cluster and stellar metallicity distribution function of the nearby elliptical galaxy Centaurus A.
We have measured the absolute proper motions of four low-latitude, inner-Galaxy globular clusters. These clusters are: NGC 6266 (M62), NGC 6304, NGC 6316 and NGC 6723. The proper motions are on the Hipparcos system, as no background extragalactic objects are found in these high-extinction regions. The proper-motion uncertainties range between 0.3 and 0.6 mas/yr. We discuss the kinematics of these clusters and of three additional bulge clusters -- NGC 6522, NGC 6528 and NFC 6553 -- whose proper motions with respect to bulge stars had been determined previously. We find that all of the clusters …
The globular cluster system of a typical spheroidal galaxy makes up about 0.25% of the total galaxy mass (McLaughlin 1999). This is roughly the same mass fraction as contained in the nuclear star clus- ter (or stellar nucleus) present in most nearby low-mass galaxies. Motivated by this "coincidence", this Letter discusses a scenario in which globular clusters of present-day galaxies are the surviving nuclei of the dwarf galaxies that - according to the hierarchical merging paradigm of galaxy forma- tion - constitute the "building blocks" of present-day massive galaxies. This scenario, which wa…
Using V and I images obtained with WFPC2/HST, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic ``Toomre Sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921, and NGC 7252 (``Atoms for Peace'') have been studied in order to explore how the star formation depends upon the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging fro…
We present preliminary results from our on-going survey of Galactic globular clusters relative ages. The investigation is based on V,I images obtained at the ESO and La Palma telescopes, which make up the largest homogenous photometric catalog to date. A second, independent sample of globulars observed in the B, V bands provides an independent check of the results based on the groundbased data. Age-dependent morphological parameters are measured on the CMDs and are compared with two sets of independent models. We find that the so-called ``vertical'' and ``horizontal'' methods give compatible r…
Hubble Space telescope (HST) images of merging galaxy system VV29 reveal the 0.3 Mpc baryonic-dark-matter (BDM) halo composed of primordial protoglobularstarcluster (PGC) clumps of planets. Star-cluster-wakes trace the merger by formation of stars from the planets. Aligned young globular star clusters (YGCs), star-wakes and dust-trails show the frictional, spiral passage of galaxy fragments VV29cdef in a long tail-like galaxy (VV29b) as the fragments merge on the accretion disk plane of the central spiral galaxy VV29a. The observations confirm the hydro-gravitational-dynamics (HGD) prediction …
The radial distribution of luminous L_X>10^{36} erg/s X-ray point sources in the bulge of M31 is investigated using archival Chandra observations. We find a significant increase of the specific frequency of X-ray sources, per unit stellar mass, within 1 arcmin from the centre of the galaxy. The radial distribution of surplus sources in this region follows the density squared law, suggesting that they are low-mass X-ray binaries formed dynamically in the dense inner bulge. We investigate dynamical formation of LMXBs, paying particular attention to the high velocity regime characteristic for gal…
The study of cluster populations as tracer of galaxy evolution is now quite possible with 8 m class telescopes and modern instrumentation. The cluster population can be used as a good tracer of the star forming episodes undergone by the merging system. We present two young galaxies mergers NGC3256 and NGC4038, and the studies about the young cluster population on those system. We found that the clusters ages are agree with the mergers age and their metallicities are consistent with them being the progenitors of the old metal rich globulars in ellipticals.
AM-1, at ~120 kpc, and Pal 14, at ~70 kpc, are two of the most distant Galactic globular clusters known. We present Hubble Space Telescope WFPC2 photometry of AM-1 and Pal 14 that reveals unprecedented depth and detail in the color-magnitude diagrams of these two clusters. Absolute and relative age measurements confirm that both are younger than the inner halo globular cluster M 3 by 1.5-2 Gyr, assuming all three clusters have similar compositions. Thus AM-1 and Pal 14 join Pal 3, Pal 4, and Eridanus (studied by Stetson et al.) as distant Galactic globular clusters with red horizontal branches…
Globular clusters (GCs) constitute a system which is evolving because of various interactions with the galactic environment. Evolution may be the explanation of many observed features of Globular Cluster Systems (GCSs); the different radial distribution of the GCS and the stellar component of early type galaxies is explained by dynamical friction and tidal effects, this latter acting both on the large scale (that of the bulge-halo stars) and on the small scale (that of the nucleus, often containing a central massive black hole). Merging of quickly orbitally decayed massive GCs leads to formati…
Hubble Space Telescope (HST/ACS) images of the galaxy merger Tadpole are interpreted using the hydro-gravitational theory of Gibson 1996-2000 (HGT) that predicts galaxy masses within about 100 kpc are dominated by dark halos of planetary mass primordial-fog-particles (PFPs) in dark proto-globular-star-clusters (PGCs). According to our interpretation, stars and young-globular-clusters (YGCs) appear out of the dark as merging galaxy components VV29cdef move through the baryonic-dark-matter halo of the larger galaxy VV29a creating luminous star-wakes. Frozen PFP planets are evaporated by radiatio…
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 …
1057 globular cluster candidates have been identified in a WFPC2 image of the inner region of M87. The Globular Cluster Luminosity Function (GCLF) can be well fit by a Gaussian profile with a mean value of m_V^0=23.67 +/- 0.07 mag and sigma=1.39 +/- 0.06 mag (compared to m_V^0=23.74 mag and sigma=1.44 mag from an earlier study using the same data by Whitmore it et al. 1995). The GCLF in five radial bins is found to be statistically the same at all points, showing no clear evidence of dynamical destruction processes based on the luminosity function (LF), in contradiction to the claim by Gnedin …
In the high density cores of globular clusters, multibody interactions are expected to be common, with the result that black holes in binaries are hardened by interactions. It was shown by Sigurdsson & Hernquist (1993) and others that 10 solar mass black holes interacting exclusively by three-body encounters do not merge in the clusters themselves, because recoil kicks the binaries out of the clusters before the binaries are tight enough to merge. Here we consider a new mechanism, involving four-body encounters. Numerical simulations by a number of authors suggest that roughly 20-50% of binary…
[Abridged] We present the results of a Hubble Space Telescope snapshot program to survey the inner region of the globular cluster NGC 6441 for its variable stars. A total of 57 variable stars was found including 38 RR Lyrae stars, 6 Population II Cepheids, and 12 long period variables. Of the RR Lyrae stars observed in this survey, 26 are pulsating in the fundamental mode with a mean period of 0.753d and 12 are first-overtone mode pulsators with a mean period of 0.365d. These values match up very well with those found in ground-based surveys. Combining all the available data for NGC 6441, we f…
The core of the Galactic Globular Cluster M3 (NGC 5272) has been observed with the WFPC2 through the filters F255W, F336W, F555W, and F814W. Using these observations along with a thorough reanalysis of earlier catalogs, we have produced a catalog of blue straggler stars (BSS) spanning the cluster. Earlier studies and the fainter part of our sample suffer severe selection biases. Our analysis is based on a more reliable bright global sample of 122 BSS. We confirm earlier suggestions that the radial BSS distribution in M3 is bimodal. It is strongly peaked in the center, has a clear dip 100--200"…
From spectrophotometric and chemical evolutionary modelling of interacting and merging galaxies we predict abundances of burst stars and of globular clusters (GCs) that may be formed in strong bursts. Observations of young GCs in merger remnants confirm the predicted metallicities. We investigate their formation as a result of a strong star burst with a high star formation efficiency over a large volume of the merger remnant. The evolution of GCs of various metallicities in terms of broad band colours and stellar metal indices are modeled. Comparing our results to the V-I, Mgb and Fe5270 obser…
We present the results of a meta-analysis of Keck spectra of extragalactic globular clusters (GCs) in a sample of eight galaxies, ranging from dwarfs to massive ellipticals. We infer ages for the metal-poor and metal-rich GCs in these galaxies through comparisons to Galactic GCs. Both subpopulations appear to be no younger than their Galactic counterparts, with ages ~> 10 Gyr. This is the largest sample of galaxies for which ages have been constrained spectroscopically. Our results support the formation of most GCs in massive galaxies at high redshift. We propose a scenario for the formation o…
We present a high-resolution simulation of globular cluster formation in a galaxy merger. For the first time in such a simulation, individual star clusters are directly identified and followed on their orbits. We quantitatively compare star formation in the merger to that in the unperturbed galaxies. The merging galaxies show a strong starburst, in sharp contrast to their isolated progenitors. Most star clusters form in the tidal features. With a mass range of $5\times10^{5}$--$5\times 10^{6} M_{\odot}$, they are identified as globular clusters. The merger remnant is an elliptical galaxy. Clus…
We present preliminary results obtained from the comparison of the specific frequencies of Blue Straggler Stars (BSS) detected so far in a sample of 26 Galactic globular clusters. The number of BSS seems to increase almost linearly with increasing the amount of sampled light in loose clusters, while it drops abruptly for clusters having intermediate-high central densities. In particular, a simple interpretative scenario where the BSS in loose clusters are produced from primordial binaries and those detected in high density globulars are due to star interactions leading to binary formation, mer…
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…
In dense stellar systems the frequent dynamical interactions between stars play a crucial role in the formation and evolution of compact binaries. We study these processes using a novel approach combining a state-of-the-art binary population synthesis code with a simple treatment of dynamical interactions in dense star cluster cores. Here we focus on the dynamical and evolutionary processes leading to the formation of compact binaries containing white dwarfs in dense globular clusters. We demonstrate that dynamics can increase by factors ~ 2 - 100 the production rates of interesting binaries s…
Mergers of globular clusters (GCs) once associated with dwarf spheroidal (dSph) galaxies have recently been suggested as an explanation for the bimodal horizontal branches (HBs) of some Galactic GCs, most notably NGC 1851, NGC 2808, and NGC 6229. Through analysis of the available color-magnitude diagrams for the GCs in the Fornax and Sagittarius dSph satellites of the Milky Way, as well as their metallicity distributions, we argue that the merger of two GCs would most likely produce a bimodal distribution in red giant branch (RGB) colors, or at least a significant broadening of the RGB, due to…
We have used high resolution WFPC2-HST and ground based Wide Field images to determine the center of gravity and construct an extended radial density and brightness profile of the cluster NGC 6752 including, for the first time, detailed star counts in the very inner region. The barycenter of the 9 innermost X-ray sources detected by Chandra is located only 1.9" off the new center of gravity. Both the density and the brightness profile of the central region are best fitted by a double King model, suggesting that NGC 6752 is experiencing a post-core collapse "bounce". Taking advantage from our n…
Merging binary systems consisting of two collapsed objects are among the most promising sources of high frequency gravitational wave, GW, signals for ground based interferometers. Double neutron star or black hole/neutron star mergers are also believed to give rise to short hard bursts, SHBs, a subclass of gamma ray bursts. SHBs might thus provide a powerful way to infer the merger rate of two-collapsed object binaries. Under the hypothesis that most SHBs originate from double neutron star or black hole/neutron star mergers, we outline here a method to estimate the incidence of merging events …
The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep high-resolution images of the giant early-type galaxy NGC 1316 which is an obvious merger remnant. These observations supersede previous, shallower observations which revealed the presence of a population of metal-rich globular clusters of intermediate age (~ 3 Gyr). We detect a total of 1496 cluster candidates, almost 4 times as many as from the previous WFPC2 images. We confirm the bimodality of the color distribution of clusters, even in V-I, with peak colors 0.93 and 1.06. The large number of d…
The first locations of short gamma-ray bursts (GRBs) in elliptical galaxies suggest they are produced by the mergers of double neutron star (DNS) binaries in old stellar populations. Globular clusters, where the extreme densities of very old stars in cluster cores create and exchange compact binaries efficiently, are a natural environment to produce merging NSs. They also allow some short GRBs to be offset from their host galaxies, as opposed to DNS systems formed from massive binary stars which appear to remain in galactic disks. Starting with a simple scaling from the first DNS observed in a…
The dynamical evolution of the globular cluster systems in galaxies is predicted, based on the standard dynamical theory normalized to the example of the Milky Way. The major processes varying with the galactocentric distance are the tidal shocks and dynamical friction. Our simple model explains, on a quantitative basis, the observed differences of the inner and outer populations of globular clusters. We can thus calculate corrections for dynamical evolution for the luminosity function of globular clusters with the assumption that the initial luminosity function is identical in all galaxies (a…
In the most prominent current scenario of galaxy formation, galaxies form hierarchically through the merger of smaller systems. Such mergers could leave behind dynamical signatures which may linger long after the event. In particular, the globular cluster system (GCS) of a merging satellite galaxy may remain as a distinct sub-population within the GCS of a massive galaxy. Using the latest available globular cluster velocities and metallicities, we present the results of a search for grouping in the GCS of our nearest large spiral galaxy neighbor, M31. A modified friends-of-friends algorithm is…
