1,721,091 research outputs found
Globular cluster-massive black hole interactions in galactic centers
Full text linkMany, if not all, galaxies host massive compact objects at their centers. They are present as singularities (super massive black holes) or high density star clusters (nuclear tar clusters). In some cases they coexist, and interact more or less strongly. In this short paper I will talk of the merger globular cluster scenario, which has been shown in the past to be an explanation of the substantial mass accumulation in galactic centers. In particular, I will present the many astrophysical implications of such scenario pointing the attention on the mutual feedback of orbitally decaying globular clusters with massive and super massive black holes
Dynamics of a superdense cluster of black holes and the formation of the Galactic supermassive black hole
Full text linkThe centre of our Galaxy is known to host a massive compact object, Sgr A∗, which is commonly considered as a supermassive black hole of {sim} 4 imes 106 , ext{{
m M}{odot } , }. It is surrounded by a dense and massive nuclear star cluster, with a half-mass radius of about 5 pc and a mass larger than 10{7} , ext{{
m M}{odot } , }. In this paper, we studied the evolutionary fate of a very dense cluster of intermediate-mass black holes, possible remnants of the dissipative orbital evolution of massive globular cluster hosts. We performed a set of high-precision N-body simulations taking into account deviations from pure Newtonian gravitational interaction via a post-Newtonian development up to 2.5 order, which is the one accounting for energy released by gravitational wave emission. The violent dynamics of the system leads to various successive merger events to grow a single object containing ∼25 per cent of the total cluster mass before partial dispersal of the cluster, and to generate, in different bursts, a significant quantity of gravitational wave emission. If generalized, the present results suggest a mechanism of mass growth up to the scale of a supermassive black hole
Star clusters and super massive black holes: High velocity stars production
Full text linkOne possible origin of high velocity stars in the Galaxy is that they are the product of the interaction of binary systems and supermassive black holes. We investigate a new production channel of high velocity stars as due to the close interaction between a star cluster and supermassive black holes in galactic centres. The high velocity acquired by some stars of the cluster comes from combined effect of extraction of their gravitational binding energy and from the slingshot due to the interaction with the black holes. Stars could reach a velocity sufficient to travel in the halo and even overcome the galactic potential well, while some of them are just stripped from the cluster and start orbiting around the galactic centre
A new method to integrate newtonian N-body dynamics
No full textIn this note we approach the classical, Newtonian, gravitational N-body problem by mean of a new, original numerical integration method. After a short summary of the fundamental characteristics of the problem, including a sketch of some of its mathematical and numerical issues, we present the new algorithm, which is applied to a set of sample cases of initial conditions in the 'intermediate' N regime (N = 100). This choice of N is not due to algorithm limitation but just for computational convenience, in what this preliminary work aims mainly to the presentation of the new method and so we wanted just to provide an acceptable although statistical significant comparison with other integration schemes. The proposed algorithm seems to be fast and precise at the same time and so promising for further, more realistic, tests and scientific applications
The MEGaN project II. Gravitational waves from intermediate-mass and binary black holes around a supermassive black hole
No full textWe investigate the evolution of intermediate-mass (IMBHs), stellar (BHs), and binary black holes (BHBs), deposited near a supermassive black hole (SMBH) by a population of massive star clusters. Stellar BHs rapidly segregate around the SMBH, driving the formation of extreme-mass ratio inspirals that coalesce at a rate Γ = 0.02-0.2 yr-1 Gpc-3 at redshift z = 0. A few IMBHs orbiting the SMBH favour the formation of massive pairs that coalescence within a Hubble time, being the merger rate for this channel Γ = 0.03 yr-1 Gpc-3. Recoiling kicks post-merger can eject the remnant from the galaxy centre, especially in dwarf galaxies. Our results suggest that this mechanism can lead to up to 105 ejected SMBH within 1 Gpc. An IMBH co-existing with a few single and binary BHs in the same cluster can affect significantly their evolution, either driving binary disruption, yielding to intermediate-mass ratio inspirals (merger rate Γ = 9.5 yr-1 Gpc-3), or boosting BHBs coalescence (Γ = 2-8 yr-1 Gpc-3). In a few simulations, the SMBH boosts BHBs coalescence, leading this process to a merger rate Γ = 1 yr-1 Gpc-3. We note that BHBs experiencing a merger in a galactic nucleus can be erroneously estimated {̃ } 30{{ per cent}} heavier than it really is because of the Doppler shift of the wave frequency as caused by the rapid motion around the SMBH. All our simulations are carried out using an N-body code tailored to treat close encounters and post-Newtonian dynamics, that includes also the galaxy field and dynamical friction in the particles equation of motion
Effect of Binarity in Star Cluster Dynamical Mass Determination
No full textIn this paper we explore the effects that the presence of a fraction of binary stars has in the determination of a star cluster mass via the virial theorem. To reach this aim in an accurate and consistent way, we run a set of simulations using the direct summation, high precision, code NBODY7. By means of this suite of simulations we are able to quantify the overestimate of open-star-cluster-like models' dynamical masses when making a straight application of the virial theorem using available position and radial velocity measurements. The mass inflation caused by the binary "heating"contribution to the measured velocity dispersion depends, of course, on the initial binary fraction, f b0 and its following dynamical evolution. For an f b (evolved up to 1.5 Gyr) in the range 8%-42% the overestimate of the mass done using experimentally sounding estimates for the velocity dispersion can be up to a factor of 45. We provide a useful fitting formula to correct the dynamical mass determination for the presence of binaries, and underline how neglecting the role of binaries in stellar systems might lead to erroneous conclusions about their total mass budget. If this trend remains valid for larger systems like dwarf spheroidal galaxies, which are still far out of reach for high-precision dynamical simulations taking their binaries into account, it would imply an incorrect overestimation of their dark matter content, as inferred by means of available velocity dispersion measurements
Properties of self-gravitating quasi-stationary states
Full text linkInitially far out-of-equilibrium, self-gravitating systems form quasi-stationary states (QSS) through a collisionless relaxation dynamics. These may arise from a bottom-up aggregation of structures or in a top-down frame; their quasi-equilibrium properties are well described by the Jeans equation and are not universal. These QSS depend on initial conditions. To understand the origin of such dependence, we present the results of numerical experiments of initially cold and spherical systems characterized by various choices of the spectrum of initial density fluctuations. The amplitude of such fluctuations determines whether the system relaxes in a top-down or bottom-up manner. We find that statistical properties of the resulting QSS mainly depend upon the amount of energy exchanged during the formation process. In particular, in the violent top-down collapses the energy exchange is large and the QSS show an inner core with an almost flat density profile and a quasi Maxwell-Boltzmann (isotropic) velocity distribution, while their outer regions display a density profile (r) / r (0) with radially elongated orbits. We show analytically that = 4, in agreement with numerical experiments. In the less violent bottom-up dynamics, the energy exchange is much smaller, the orbits are less elongated, and 0 (r) 4, where the density profile is well fitted by the Navarro-Frenk-White behavior. Such a dynamical evolution is shown by both nonuniform spherical isolated systems and by halos extracted from cosmological simulations. We consider the relation of these results with the core-cusp problem and conclude that this can be solved naturally if galaxies form through a monolithic collapse
Stability of planetary systems within the S-star cluster: The Solar system analogues
No full textA dynamically relaxed dense cluster comprised of about 40 stars (the so-called S-stars) inhabits the central region of our Galaxy. Their stars revolve around the Sgr A∗ massive object. To understand the dynamical evolution of planetary systems in a particular environment like that around Sgr A∗, we carry out direct N-body simulations of planetary systems embedded in the S-star cluster. In this work, we investigate the short-term stability of the planets orbiting around S-stars after their close interactions with the central massive black hole of our Galaxy. We find that planetary systems go through encounters with the supermassive black hole (SMBH) and the nearby stars. We determine the frequency and the strength of planetary systems' encounters with the nearby stars as these encounters remarkably increase for systems assigned to S-stars closer to the SMBH. The SMBH severely destabilizes the planetary systems, though we noted that the small oscillations in the mutual eccentricity and inclination of the planetary system could be caused by the planet-planet coupling and the near-resonance effect between the two planets. We obtain estimates of the fraction of survivor planets (∼ 51 per cent), and find that planets stripped from their hosting star are generally captured on close orbits around Sgr A∗. We notify while gas giants are tidally disrupted, terrestrial planets do not. We estimate that Sgr A∗ flares can be due to the tidal disruption events of starless giant planets
On the stellar content of the galactic globular cluster M5
No full textNew IUE observations of the innermost region of the galactic globular cluster M5 (NGC 5904) are presented and discussed. Addition of IUE spectra and identification of spectral features were obtained from the present and previous IUE observations of M5. Synthetic models of the cluster were constructed by taking into account optical and UV data. A comparison between theory and observation allowed for some estimates of the stellar content of M5. However, attention is called to the limits of validity of synthetic models, at least when they are compared with IUE observations
The Galactic star cluster NGC 4337: estimates of its photometric and dynamical mass
No full textIn this contribution we discuss various estimates of the mass of NGC 4337, an old open cluster located in the inner Galactic disk. We derive its mass in different ways. First, we obtain a lower estimate of the cluster mass using the surface density profile of the cluster and its luminosity and mass function by means of star counts out of a photometric data set in the UBVI passbands. This data set is also used to derive fundamental cluster parameters. Second, we obtain dynamical estimates of the cluster mass as based on a large survey of cluster star radial velocities. The dynamical estimates correspond to significantly larger values than those from star count estimates. We can roughly match these two estimate sets taking into account the contribution of invisible mass in the form of both low mass stars and remnants of high mass stars in the cluster
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