by Schneider, Justus and Amaro-Seoane, Pau and Spurzem, Rainer
Submitted to MNRAS, comments welcome

Dense stellar systems such as globular clusters, galactic nuclei and nuclear star clusters are ideal loci to study stellar dynamics due to the very high densities reached, usually a million times higher than in the solar neighborhood; they are unique laboratories to study processes related to relaxation. There are a number of different techniques to model the global evolution of such a system. In statistical models we assume that relaxation is the result of a large number of two-body gravitational encounters with a net local effect. We present two moment models that are based on the collisional Boltzmann equation. By taking moments of the Boltzmann equation one obtains an infinite set of differential moment equations where the equation for the moment of order $latex n$ contains moments of order $latex n+1$. In our models we assume spherical symmetry but we do not require dynamical equilibrium. We truncate the infinite set of moment equations at order $latex n=4$ for the first model and at order $latex n=5$ for the second model. The collisional terms on the right-hand side of the moment equations account for two-body relaxation and are computed by means of the Rosenbluth potentials. We complete the set of moment equations with closure relations which constrain the degree of anisotropy of our model by expressing moments of order $latex n+1$ by moments of order $latex n$. The accuracy of this approach relies on the number of moments included from the infinite series. Since both models include fourth order moments we can study mechanisms in more detail that increase or decrease the number of high velocity stars. The resulting model allows us to derive a velocity distribution function, with unprecedented accuracy, compared to previous moment models.

by Konstantinidis, Simos and Kokkotas, Kostas D.
24 pages, 28 figures

We present a new C++ code for collisional N-body simulations of star clusters. The code uses the Hermite fourth-order scheme with block time steps, for advancing the particles in time, while the forces and neighboring particles are computed using the GRAPE-6 board. Special treatment is used for close encounters, binary and multiple sub-systems that either form dynamically or exist in the initial configuration. The structure of the code is modular and allows the appropriate treatment of more physical phenomena, such as stellar and binary evolution, stellar collisions and evolution of close black-hole binaries. Moreover, it can be easily modified so that the part of the code that uses GRAPE-6, could be replaced by another module that uses other accelerating-hardware like the Graphics Processing Units (GPUs). Appropriate choice of the free parameters give a good accuracy and speed for simulations of star clusters up to and beyond core collapse. Simulations of Plummer models consisting of equal-mass stars reached core collapse at t~17 half-mass relaxation times, which compares very well with existing results, while the cumulative relative error in the energy remained below 0.001. Also, comparisons with published results of other codes for the time of core collapse for different initial conditions, show excellent agreement. Simulations of King models with an initial mass-function, similar to those found in the literature, reached core collapse at t~0.17, which is slightly smaller than the expected result from previous works. Finally, the code accuracy becomes comparable and even better than the accuracy of existing codes, when a number of close binary systems is dynamically created in a simulation. This is due to the high accuracy of the method that is used for close binary and multiple sub-systems.

by Burkert, Andreas and Tremaine, Scott
13 pages, 3 figures, submitted to ApJ

Elliptical, lenticular, and early-type spiral galaxies show a remarkably tight power-law correlation between the mass M_BH of their central supermassive black hole (SMBH) and the number N_GC of globular clusters: M_BH=m*N_GC^(1.11+/-0.04) with m=1.3*10^5 solar masses. Thus, to a good approximation the SMBH mass is the same as the total mass of the globular clusters. Based on a limited sample of 13 galaxies, this relation appears to be a better predictor of SMBH mass (rms scatter 0.2 dex) than the M_BH-sigma relation between SMBH mass and velocity dispersion sigma. The small scatter reflects the fact that galaxies with high globular cluster specific frequency S_N tend to harbor SMBHs that are more massive than expected from the M_BH-sigma relation. A possible explanation is that both large black-hole masses and large globular cluster populations are associated with recent major mergers.

by Vesperini, Enrico and McMillan, Stephen L. W. and D’Ercole, Annibale and D’Antona, Francesca
4 pages, 1 figure, Accepted for publication in The Astrophysical Journal, Letters

Spectroscopic and photometric observations show that many globular clusters host multiple stellar populations, challenging the common paradigm that globular clusters are “simple stellar populations” composed of stars of uniform age and chemical composition. The chemical abundances of second-generation (SG) stars constrain the sources of gas out of which these stars must have formed, indicating that the gas must contain matter processed through the high-temperature CNO cycle. First-generation massive Asymptotic Giant Branch (AGB) stars have been proposed as the source of this gas. In a previous study, by means of hydrodynamical and N-body simulations, we have shown that the AGB ejecta collect in a cooling flow in the cluster core, where the gas reaches high densities, ultimately forming a centrally concentrated subsystem of SG stars. In this Letter we show that the high gas density can also lead to significant accretion onto a pre-existing seed black hole. We show that gas accretion can increase the black hole mass by up to a factor of 100. The details of the gas dynamics are important in determining the actual black hole growth. Assuming a near-universal seed black hole mass and small cluster-to-cluster variations in the duration of the SG formation phase, the outcome of our scenario is one in which the present intermediate-mass black hole (IMBH) mass may have only a weak dependence on the current cluster properties. The scenario presented provides a natural mechanism for the formation of an IMBH at the cluster center during the SG star-formation phase.

by Miocchi, P.
LateX, 5 pages, 5 figures. Accepted for publication by Astronomy & Astrophysics

Context. The problem of the existence of intermediate-mass black holes (IMBHs) at the centre of globular clusters is a hot and controversial topic in current astrophysical research with important implications in stellar and galaxy formation.

Aims. In this paper, we aim at giving further support to the presence of an IMBH in omega Centauri and at providing an independent estimate of its mass.

Methods. We employed a self-consistent spherical model with anisotropic velocity distribution. It consists in a generalisation of the King model by including the Bahcall-Wolf distribution function in the IMBH vicinity.

Results. By the parametric fitting of the model to recent HST/ACS data for the surface brightness profile, we found an IMBH to cluster total mass ratio of M_BH/M = 5.8(+0.9-1.2) x 10^(-3). It is also found that the model yields a fit of the line-of-sight velocity dispersion profile that is better without mass segregation than in the segregated case. This confirms the current thought of a non-relaxed status for this peculiar cluster. The best fit model to the kinematic data leads, moreover, to a cluster total mass estimate of M = (3.1 +/- 0.3) x 10^6 Msol, thus giving an IMBH mass in the range 13,000 < M_BH 12′) is required to match the outer surface brightness profile.

by Zwart, Simon Portegies and McMillan, Steve and Gieles, Mark
Only 88 pages. To be published in ARAA. Final version to be submitted on Friday 12 February

Young massive clusters are dense aggregates of young stars that form the fundamental building blocks of galaxies. Several examples exist in the Milky Way Galaxy and the Local Group, but they are particularly abundant in starburst and interacting galaxies. The few young massive clusters that are close enough to resolve are of prime interest for studying the stellar mass function and the ecological interplay between stellar evolution and stellar dynamics. The distant unresolved clusters may be effectively used to study the star-cluster mass function, and they provide excellent constraints on the formation mechanisms of young cluster populations. Young massive clusters are expected to be the nurseries for many unusual objects, including a wide range of exotic stars and binaries. So far only a few such objects have been found in young massive clusters, although their older cousins, the globular clusters, are unusually rich in stellar exotica. In this review we focus on star clusters younger than $latex \sim100$ Myr, more than a few current crossing times old, and more massive than $latex \sim10^4$ \Msun, irrespective of cluster size or environment. We describe the global properties of the currently known young massive star clusters in the Local Group and beyond, and discuss the state of the art in observations and dynamical modeling of these systems. In order to make this review readable by observers, theorists, and computational astrophysicists, we also review the cross-disciplinary terminology.

by Kong, A. K. H. and Heinke, C. O. and Di Stefano, R. and Barmby, P. and Lewin, W. H. G. and Primini, F. A.
5 pages, 1 figure, submitted to ApJL

We report the most accurate X-ray position of the giant globular cluster G1 in M31 by using the Chandra X-ray Observatory, Hubble Space Telescope (HST), and Canada-France-Hawaii Telescope (CFHT). G1 is clearly detected with Chandra and by cross-registering with HST and CFHT images, we derive a 1sigma error radius of 0.15″, significantly smaller than the previous measurement by XMM-Newton. We conclude that the X-ray emission of G1 comes from within the core radius of the cluster. There are two possibilities for the origin of the X-ray emission: it could be due to either accretion of a central intermediate-mass black hole, or ordinary low-mass X-ray binaries. Based on the ratio of X-ray to the Eddington luminosity, an intermediate-mass black hole accreting from the cluster gas seems unlikely and we suggest that the X-rays are due to accretion from a companion. We also find that the X-ray emission may be offset from the radio emission. Future high-resolution and high-sensitivity radio imaging observations will reveal whether there is an intermediate-mass black hole at the center of G1.

by Downing, J. M. B. and Benacquista, M. J. and Giersz, M. and Spurzem, R.
18 pages, 5 Tables, 10 Figures, Submitted to MNRAS

We study the compact binary population in star clusters, focusing on binaries containing neutron stars and black holes, using a self-consistent Monte Carlo treatment of dynamics and full stellar evolution. We find that the black holes experience strong mass segregation and become centrally concentrated. In the core the black holes interact strongly with each other and black hole-black hole binaries are formed very efficiently. The strong interactions, however, also destroy or eject the black hole-black hole binaries. We find no black hole-black hole mergers within our simulations but produce many hard escapers that will merge in the galactic field within a Hubble time. We also find two highly eccentric black hole-black hole binaries that are potential LISA sources, suggesting that star clusters are interesting targets for space-based detectors. We conclude that star clusters must be taken into account when predicting compact binary population statistics.

by Amaro-Seoane, Pau and Santamaria, Lucia
Submitted to ApJ; abstract abridged, figure 1 has a lower resolution

Even though the existence of intermediate-mass black holes has not yet been corroborated observationally, these objects are of high interest for astrophysics. Our understanding of formation and evolution of supermassive black holes (SMBHs), as well as galaxy evolution modeling and cosmography would dramatically change if an IMBH was observed. The prospect of detection and, possibly, observation and characterization of an IMBH has good chances in lower-frequency gravitational-wave (GW) astrophysics with ground-based detectors such as LIGO, Virgo and the future Einstein Telescope (ET). We present an analysis of the signal of a system of a binary of IMBHs based on a waveform model obtained with numerical relativity simulations coupled with post-Newtonian calculations at the highest available order so as to extend the waveform to lower frequencies. We find that initial LIGO and Virgo are in the position of detecting IMBHs with a signal-to-noise ratio (SNR) of $latex \sim 10$ for systems with total mass between 100 and $latex 500 M_{\odot}$ situated at a distance of 100 Mpc. Nevertheless, the event rate is too low and the possibility that these signals are mistaken with a glitch is, unfortunately, non-negligible. When going to second- and third-generation detectors, such as Advanced LIGO or the proposed ET, the event rate becomes much more promising (tens per year for the first and thousands per year for the latter) and the SNR at 100 Mpc is as high as 100 — 1000 and 1000 — $latex 10^{5}$ respectively. The prospects for IMBH detection and characterization with ground-based GW observatories would not only provide us with a robust test of general relativity, but would also corroborate the existence of these systems. Such detections would be a probe to the stellar environments of IMBHs and their formation.

by Baumgardt, Holger and Parmentier, Genevieve and Gieles, Mark and Vesperini, Enrico
8 pages, 4 figures, MNRAS in press

We investigate the ratio between the half-mass radii r_h of Galactic globular clusters and their Jacobi radii r_J given by the potential of the Milky Way and show that clusters with galactocentric distances R_{GC}>8 kpc fall into two distinct groups: one group of compact, tidally-underfilling clusters with r_h/r_J<0.05 and another group of tidally filling clusters which have 0.1 < r_h/r_J<0.3. We find no correlation between the membership of a particular cluster to one of these groups and its membership in the old or younger halo population. Based on the relaxation times and orbits of the clusters, we argue that compact clusters and most clusters in the inner Milky Way were born compact with half-mass radii r_h < 1 pc. Some of the tidally-filling clusters might have formed compact as well, but the majority likely formed with large half-mass radii. Galactic globular clusters therefore show a similar dichotomy as was recently found for globular clusters in dwarf galaxies and for young star clusters in the Milky Way. It seems likely that some of the tidally-filling clusters are evolving along the main sequence line of clusters recently discovered by Kuepper et al. (2008) and are in the process of dissolution.

by Amaro-Seoane, Pau and Eichhorn, Christoph and Porter, Ed and Spurzem, Rainer
21 pages, 12 figs, to appear in MNRAS, abstract abridged for arxiv

The dynamical evolution of binaries of intermediate-massive black holes (IMBHs, massive black holes with a mass ranging between $latex 10^2$ and $latex 10^4 M_{\odot}$) in stellar clusters has recently received an increasing amount of attention. This is at least partially due to the fact that if the binary is hard enough to evolve to the phase at which it will start emitting gravitational waves (GWs) efficiently, there is a good probability that it will be detectable by future space-borne detectors like LISA. We study this evolution in the presence of rotation in the cluster. The eccentricity is strongly connected to the initial IMBHs velocities, and values of $latex \sim 0.7$ up to 0.9 are reached for low initial velocities, while almost circular orbits result if the initial velocities are increased. A Monte Carlo study indicates that these sources will be detectable by a detector such as LISA with median signal to noise ratios of between 10 and 20 over a three year period, although some events had signal to noise ratios of 300 or greater. Furthermore, one should also be able to estimate the chirp-mass with median fractional errors of $latex 10^{-4}$, reduced mass on the order of $latex 10^{-3}$ and luminosity distance on the order of $latex 10^{-1}$. Finally, these sources will have a median angular resolution in the LISA detector of about 3 square degrees, putting events firmly in the field of view of future electromagnetic detectors such as LSST.

by Gnedin, Oleg Y. and Maccarone, Thomas J. and Psaltis, Dimitrios and Zepf, Stephen E.

Large extra dimensions have been proposed as a possible solution to the hierarchy problem in physics. One of the suggested models, the RS2 braneworld model, makes a prediction that black holes evaporate by Hawking radiation on a short timescale that depends on the black hole mass and on the asymptotic radius of curvature of the extra dimensions. Thus the size of the extra dimensions can be constrained by astrophysical observations. Here we point out that the black hole, recently discovered in a globular cluster in galaxy NGC 4472, places the strongest constraint on the maximum size of the extra dimensions, L < 0.003 mm. This black hole has the virtues of old age and relatively small mass. The derived upper limit is within an order of magnitude of the absolute limit afforded by astrophysical observations of black holes.

by Forbes, Duncan A. and Kroupa, Pavel and Metz, Manuel and Spitler, Lee
3 pages, published in Mercury vol. 38, No. 2, page 24. See http://astronomy.swin.edu.au/dforbes/mw.pdf for a full colour version with figures

Our Milky Way galaxy is host to a number of companions. These companions are gravitationally bound to the Milky Way and are stellar systems in their own right. They include a population of some 30 dwarf satellite galaxies (DSGs) and about 150 globular clusters (GCs). Here we discuss the relationship between GCs and DSGs using an interactive 3D model of the Milky Way.

by Ernst, Andreas and Just, Andreas and Spurzem, Rainer
18 pages, 20 figures; accepted by MNRAS

We present N-body simulations of dissolving star clusters close to galactic centres. For this purpose, we developed a new N-body program called nbody6gc based on Aarseth’s series of N-body codes. We describe the algorithm in detail. We report about the density wave phenomenon in the tidal arms which has been recently explained by Kuepper et al. (2008). Standing waves develop in the tidal arms. The wave knots or clumps develop at the position, where the emerging tidal arm hits the potential wall of the effective potential and is reflected. The escaping stars move through the wave knots further into the tidal arms. We show the consistency of the positions of the wave knots with the theory in Just et al. (2009). We also demonstrate a simple method to study the properties of tidal arms. By solving many eigenvalue problems along the tidal arms, we construct numerically a 1D coordinate system whose direction is always along a principal axis of the local tensor of inertia. Along this coordinate system, physical quantities can be evaluated. The half-mass or dissolution times of our models are almost independent of the particle number which indicates that two-body relaxation is not the dominant mechanism leading to the dissolution. This may be a typical situation for many young star clusters. We propose a classification scheme which sheds light on the dissolution mechanism.

by Barnard, R. and Kolb, U.
Accepted for publication in MRAS letters. Four pages, three figures

We use arguments developed in previous work to identify a second black hole candidate associated with a M31 globular cluster, Bo 144, on the basis of X-ray spectral and timing properties. The 2002 XMM-Newton observation of the associated X-ray source (hereafter XBo 144) revealed behaviour that is common to all low-mass X-ray binaries (LMXBs) in the low-hard state. Studies have shown that neutron star LMXBs exhibit this behaviour at 0.01-1000 keV luminosities <=10% of the Eddington limit (L_Edd). However, the unabsorbed 0.3-10 keV XBo 144 luminosity was ~0.30 L_Edd for a 1.4 M_sun neutron star, and the expected 0.01-1000 keV luminosity is 3-7 times higher. We therefore identify XBo 144 as a black hole candidate. Furthermore, it is the second black hole candidate to be consistent with formation via tidal capture of a mean sequence donor in a GC; such systems were previously though non-existent, because the donor was thought to be disrupted during the capture process.

by Amaro-Seoane, Pau and Miller, Cole and Freitag, Marc
Accepted for publication by ApJ Letts

If binary intermediate-mass black holes (IMBHs; with masses between 100 and $latex 10^4 \Msun$) form in dense stellar clusters, their inspiral will be detectable with the planned Laser Interferometer Space Antenna (LISA) out to several Gpc. Here we present a study of the dynamical evolution of such binaries using a combination of direct $latex N$-body techniques (when the binaries are well separated) and three-body relativistic scattering experiments (when the binaries are tight enough that interactions with stars occur one at a time). We find that for reasonable IMBH masses there is only a mild effect on the structure of the surrounding cluster even though the binary binding energy can exceed the binding energy of the cluster. We demonstrate that, contrary to standard assumptions, the eccentricity in the LISA band can be in {\em some} cases as large as $latex \sim 0.2 – 0.3$ and that it induces a measurable phase difference from circular binaries in the last year before merger. We also show that, even though energy input from the binary decreases the density of the core and slows down interactions, the total time to coalescence is short enough (typically less than a hundred million years) that such mergers will be unique snapshots of clustered star formation.