arXiv subjects

C. M. Boily

Publications and source records attributed to C. M. Boily.

16 recordsLinked to original sources

arXiv2025

Dynamical evolution timescales for the supermassive black hole system in the galaxy NGC 7727 (Arp 222)

Context. A dual active galactic nucleus candidate with a separation of only 500 pc was recently found in NGC 7727. According to the hierarchical merging scenario, such objects would be expected to merge on a timescale of a few hundred Myr. However, estimating the accurate merging timescales for the two nuclei is still a complex challenge. Aims. Using our numerical N-body code, we can trace the full evolution of central black holes during all phases: dynamical friction of unbound black holes, binary black hole formation, hardening of the system due to two-body scattering, and emission of gravitational waves leading to the final merger. Methods. Our model has next components: the bulge contains two dense stellar nuclei, each of which hosts a black hole. The most massive black hole in the center of the galaxy has a mass of 1.54x10^8 Msol and the least massive black hole in the offset second stripped nucleus has a mass of 6.33x10^6 Msol. We followed the dynamical evolution of the system up to a final separation of four Schwarzschild radii. The black holes were added as special relativistic particles and their equation of motion contains a full post-Newtonian approximation - 2.5 term. Results. Initially, the black holes are not gravitationally bound and, thus, the system spends more than 60 Myr in the phase of dynamical friction while tightening the orbit. The two-body scattering phase takes place from 60 Myr up to 120 Myr. In the last 10 Myr, the black hole's separation is seen to be rapidly shrinking due to the gravitational wave emission. Starting from the physical separation observed today, the total merging time in our model is 130 (10) Myr. Conclusions. These results have implications for the statistics of strong sources of gravitational waves at low frequencies, namely, systems engaged in an advanced state of are expected to be prime sources for the LISA mission to observe.

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arXiv2025

Dynamical traction and black hole orbital migration

We investigate the circumstances which allow a black hole to remain put at the galactic centre when the stellar core is anisotropic. We use N-body calculations to study the response of stellar orbit families embedded in a larger, isotropic isochrone (Hénon) background potential. When the BH orbits in an odd f[E,Lz] velocity distribution function, they transfer angular momentum to it. We call this dynamical traction: it takes place whenever the kinetic energy drawn from f[E,Lz] has an excess of streaming motion over its (isotropic) v-dispersion. For a dynamically cold disc, the outcome depends on both the orbit of the BH and that of a Jeans-unstable stellar sub-structures. When the stellar clumps have much binding energy, a BH may scatter off of them after they formed. In the process the BH may be dislodged from the centre and migrate outward due to dynamical traction. When the stellar clumps are less bound, they may still migrate to the centre where they either dissolve or merge with the BH. The final configuration is similar to a nuclear star cluster which may yet be moving at ~10 km/s wrt the barycentre. The angular momentum transferred to a BH by dynamical traction delays the migration to the galactic centre by several hundred million years. The efficiency of angular momentum transfer is a strong function of the fragmented (cold) state of the stellar space density. In a dynamically cold environment, a BH is removed from the central region through a two-stage orbital migration instability. A criterion against this instability is proposed in the form of a threshold in isotropic velocity dispersion compared to streaming motion. For a BH to settle at the heart of a galaxy on time-scales of ~ 300 Myr or less requires that a large fraction of Lz be dissipated, or, alternatively, that the BH grows in situ in an isotropic environment devoid of sub-structures.

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arXiv2010

Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation

(abridged) Predictions of the Concordance Cosmological Model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new most probably irreconcilable problems are uncovered. However, the Local Group properties provide hints that may lead to a solution of the above problems The DoS and bulge--satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. Such events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the sub-structure predictions of CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.

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arXiv2009

Substructures formation induced by gravitational tides?

Physics lectures always refer to the tides as a disruptive effect. However, tides can also be compressive. When the potential of two galaxies overlap, as happens during a merger, fully compressive tides can develop and have a strong impact on the dynamics of substructures such as star clusters or tidal dwarf galaxies. Using N-body simulations of a large set of mergers, we noticed the importance of these tidal modes at cluster scale. With a model of the Antennae galaxies, we conclude that the positions and timescales of these tidal modes match the actual distribution of young clusters. A detailed study of the statistics of the compressive tides shows a stunning correlation between this purely gravitational effect and the observed properties of the star clusters. In this contribution, we introduce the concept of compressive tide and show its relevance in the special case of the Antennae galaxies. We extend our conclusions to a broad range of parameters and discuss their implications on several critical points such as the infant mortality, multiple star formation epochs in clusters or the age distribution.

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arXiv2008

Star cluster survival and compressive tides in Antennae-like mergers

Gravitational tides are widely understood to strip and destroy galactic substructures. In the course of a galaxy merger, however, transient totally compressive tides may develop and prevent star forming regions from dissolving, after they condensed to form clusters of stars. We study the statistics of such compressive modes in an N-body model of the galaxy merger NGC 4038/39 (the Antennae) and show that ~15% of the disc material undergoes compressive tides at pericentre. The spatial distribution of observed young clusters in the overlap and nuclear regions of the Antennae matches surprisingly well the location of compressive tides obtained from simulation data. Furthermore, the statistics of time intervals spent by individual particles embedded in a compressive tide yields a log-normal distribution of characteristic time ~10 Myr, comparable to star cluster formation timescales. We argue that this generic process is operative in galaxy mergers at all redshifts and possibly enhances the formation of star clusters. We show with a model calculation that this process will prevent the dissolution of a star cluster during the formation phase, even for a star formation efficiency as low as ~10%. The transient nature of compressive tides implies that clusters may dissolve rapidly once the tidal field switches to the usual disruptive mode.

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arXiv2008

On time-dependent orbital complexity in gravitational N-body simulations

We implement an efficient method to quantify time-dependent orbital complexity in gravitational $N$-body simulations. The technique, which we name DWaTIM, is based on a discrete wavelet transform of velocity orbital time series. The wavelet power-spectrum is used to measure trends in complexity continuously in time. We apply the method to the test cases N=3 Pythagorean- and a perturbed N=5 Caledonian configurations. The method recovers the well-known time-dependent complexity of the dynamics in these small-$N$ problems. We then apply the technique to an equal-mass collisional N=256 body simulation ran through core-collapse. We find that a majority of stars evolve on relatively complex orbits up to the time when the first hard binary forms, whereas after core-collapse, less complex orbits are found on the whole as a result of expanding mass shells.

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arXiv2007

Black Hole Motion as Catalyst of Orbital Resonances

The motion of a black hole about the centre of gravity of its host galaxy induces a strong response from the surrounding stellar population. We treat the case of a harmonic potential analytically and show that half of the stars on circular orbits in that potential shift to an orbit of lower energy, while the other half receive a positive boost and recede to a larger radius. The black hole itself remains on an orbit of fixed amplitude and merely acts as a catalyst for the evolution of the stellar energy distribution function f(E). We show that this effect is operative out to a radius of approx 3 to 4 times the hole's influence radius, R_bh. We use numerical integration to explore more fully the response of a stellar distribution to black hole motion. We consider orbits in a logarithmic potential and compare the response of stars on circular orbits, to the situation of a `warm' and `hot' (isotropic) stellar velocity field. While features seen in density maps are now wiped out, the kinematic signature of black hole motion still imprints the stellar line-of-sight mean velocity to a magnitude ~18% the local root mean-square velocity dispersion sigma.

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arXiv2007

Satellite survival in cold dark matter cosmology

We study the survival of substructures (clumps) within larger self-gravitating dark matter halos. Building on scaling relations obtained from N-body calculations of violent relaxation, we argue that the tidal field of galaxies and halos can only destroy substructures if spherical symmetry is imposed at formation. We explore other mechanisms that may tailor the number of halo substructures during the course of virialization. Unless the larger halo is built up from a few large clumps, we find that clump-clump encounters are unlikely to homogenize the halo on a dynamical timescale. Phase mixing would proceed faster in the inner parts and allow for the secular evolution of a stellar disk.

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arXiv2007

On the equilibrium morphology of systems drawn from spherical collapse experiments

We present a purely theoretical study of the morphological evolution of self-gravitating systems formed through the dissipationless collapse of N-point sources. We explore the effects of resolution in mass and length on the growth of triaxial structures formed by an instability triggered by an excess of radial orbits. We point out that as resolution increases, the equilibria shift, from mildly prolate, to oblate. A number of particles N ~= 100000 or larger is required for convergence of axial aspect ratios. An upper bound for the softening, e ~ 1/256, is also identified. We then study the properties of a set of equilibria formed from scale-free cold initial mass distributions, ro ~ r^-g with 0 <= g <= 2. Oblateness is enhanced for initially more peaked structures (larger values of g). We map the run of density in space and find no evidence for a power-law inner structure when g <= 3/2 down to a mass fraction <~0.1 per cent of the total. However, when 3/2 < g <= 2, the mass profile in equilibrium is well matched by a power law of index ~g out to a mass fraction ~ 10 per cent. We interpret this in terms of less-effective violent relaxation for more peaked profiles when more phase mixing takes place at the centre. We map out the velocity field of the equilibria and note that at small radii the velocity coarse-grained distribution function (DF) is Maxwellian to a very good approximation.

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arXiv2006

On the Mass of Dense Star Clusters in Starburst Galaxies from Spectro-Photometry

The mass of unresolved young star clusters derived from spectro-photometric data may well be off by a factor of 2 or more once the migration of massive stars driven by mass segregation is accounted for. We quantify this effect for a large set of cluster parameters, including variations in the stellar IMF, the intrinsic cluster mass, and mean mass density. Gas-dynamical models coupled with the Cambridge stellar evolution tracks allow us to derive a scheme to recover the real cluster mass given measured half-light radius, one-dimensional velocity dispersion and age. We monitor the evolution with time of the ratio of real to apparent mass through the parameter eta. When we compute eta for rich star clusters, we find non-monotonic evolution in time when the IMF stretches beyond a critical cutoff mass of 25.5 solar mass. We also monitor the rise of color gradients between the inner and outer volume of clusters: we find trends in time of the stellar IMF power indices overlapping well with those derived for the LMC cluster NGC 1818 at an age of 30 Myr. We argue that the core region of massive Antennae clusters should have suffered from much segregation despite their low ages. We apply these results to a cluster mass function, and find that the peak of the mass distribution would appear to observers shifted to lower masses by as much as 0.2 dex. The star formation rate (SFR) derived for the cluster population is then underestimated by from 20 to 50 per cent.

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arXiv2005

The M/L ratio of massive young clusters

We point out a strong time-evolution of the mass-to-light conversion factor ηcommonly used to estimate masses of dense star clusters from observed cluster radii and stellar velocity dispersions. We use a gas-dynamical model coupled with the Cambridge stellar evolution tracks to compute line-of-sight velocity dispersions and half-light radii weighted by the luminosity. Stars at birth are assumed to follow the Salpeter mass function in the range [0.15--17 M_\sun]. We find that $η$, and hence the estimated cluster mass, increases by factors as large as 3 over time-scales of 20 million years. Increasing the upper mass limit to $50 M_\sun$ leads to a sharp rise of similar amplitude but in as little as 10 million years. Fitting truncated isothermal (Michie-King) models to the projected light profile leads to over-estimates of the concentration par ameter c of $δc\approx 0.3$ compared to the same functional fit applied to the proj ected mass density.

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arXiv2002

The impact of mass loss on star cluster formation. II. Numerical N-body integration & further applications

We subject to an N-body numerical investigation our analysis of Paper I on the survival of stellar clusters undergoing rapid mass loss. We compare analytical tracks of bound mass-fraction {\it vs} star formation efficiency $ε$ to those obtained with N-body integration. We use these to argue that stellar clusters must develop massive cores of high-binding energy if they are to remain bound despite a star formation efficiency as low as 30% or lower suggested by observations. The average local virial ratio $<σ^2/|ϕ|>$ is introduced to classify bound clusters as function of their critical $ε$ for dissolution. Clusters dissolving at lower $ε$ achieve the lowest ratio. We applied this classification parameter successfully to Michie-King and Hernquist-type distribution functions. The Plummer sphere is exceptional in that it defies this and other classification parameters we tried. The reasons for the discrepancy include less effective energy redistribution during the expansion phase for this case.

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arXiv2002

The impact of mass loss on star cluster formation. I. Analytic results

We study analytically the disruptive effect of instantaneous gas removal from a cluster containing O stars. We setup an iterative calculation based on the stellar velocity distribution function to compute the fraction of stars that remain bound once the cluster has ejected the gas and is out of equilibrium. We show that the stellar bound fraction is a function of the initial cluster distribution function as well as the star formation efficiency, $ε$, taken constant throughout the cluster. The case of the Plummer sphere is dealt with in greater details. We find for this case that up to ~ 50% of the stars may remain bound when $ε$ assumes values < 1/2, contrary to expectations derived from the virial theorem. The fraction of bound stars is expressed algebraically for polytropic distribution functions.

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arXiv2002

Scaling up Tides in Numerical Models of Galaxy- and Halo-Formation

The purpose of this article is to show that when dynamically cold, dissipationless self-gravitating systems collapse, their evolution is a strong function of the symmetry in the initial distribution. We explore with a set of pressure-less homogeneous fluids the time-evolution of ellipsoidal distributions and map the depth of potential achieved during relaxation as function of initial ellipsoid axis ratios. We then perform a series of $N$-body numerical simulations and contrast their evolution with the fluid solutions. We verify an analytic relation between collapse factor ${\cal C}$ and particle number $N$ in spherical symmetry, such that ${\cal C} \propto N^{1/3}$. We sought a similar relation for axisymmetric configurations, and found an empirical scaling relation such that ${\cal C} \propto N^{1/6}$ in these cases. We then show that when mass distributions do not respect spherical- or axial-symmetry, the ensuing gravitational collapse deepens with increasing particle number $N$ but only slowly: 86% of triaxial configurations may collapse by a factor of no more than 40 as $N\to\infty$. For $N\approx 10^5$ and larger, violent relaxation develops fully under the Lin-Mestel-Shu instability such that numerical $N$-body solutions now resolve the different initial morphologies adequately.

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arXiv2001

Efficient N-body Realisations of axisymmetric Galaxies and Haloes

We present an efficient method for building equilibrium multi-component galaxies with non-spherical haloes and bulges. The gist of our approach is to tailor the velocity ellipsoid directly to the geometry of the mass distribution. Thus we avoid computing the anisotropic velocity dispersions which leads to large savings in the cpu budget. The computational time of the algorithm for triaxial equilibria scales linearly with the number of particles, N. The approximate solution to the velocity field causes structural relaxation: tests with N = 50,000 show that fluctuations of the inertia tensor (not exceeding the 10 per cent level) disappear within one half of a revolution at the half-mass radius. At later times equilibrium properties settle to values close to those sought from the initial conditions. A disc component is then added as described by Hernquist (1993). Incorporating the above algorithm to his code BuildGal, test runs show that the stability of the disc against vertical heating is not substantially modified by using our method. The code, MaGali, is made generally available.

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arXiv2000

Superbox - An Efficient Code for Collisionless Galactic Dynamics

We present Superbox, a particle-mesh code with high resolution sub-grids and an NGP (nearest grid point) force-calculation scheme based on the second derivatives of the potential. Superbox implements a fast low-storage FFT-algorithm, giving the possibility to work with millions of particles on desk-top computers. Test calculations show energy and angular momentum conservation to one part in 10^5 per crossing-time. The effects of grid and numerical relaxation remain negligible, even when these calculations cover a Hubble-time of evolution. As the sub-grids follow the trajectories of individual galaxies, the code allows a highly resolved treatment of interactions in clusters of galaxies, such as high-velocity encounters between elliptical galaxies and the tidal disruption of dwarf galaxies. Excellent agreement is obtained in a comparison with a direct-summation N-body code running on special-purpose Grape3 hardware. The orbital decay of satellite galaxies due to dynamical friction obtained with Superbox agrees with Chandrasekhar's treatment when the Coulomb logarithm is approximately 1.5.

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