Publications (78)227.98 Total impact
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ABSTRACT: We investigate the evolution of low mass (Md/Mb = 0.005) misaligned gaseous discs around eccentric supermassive black hole (SMBH) binaries. These are expected to form from randomly oriented accretion events onto a SMBH binary formed in a galaxy merger. When expanding the interaction terms between the binary and a circular ring to quadrupole order and averaging over the binary orbit, we expect four nonprecessing disc orientations: aligned or counteraligned with the binary, or polar orbits around the binary eccentricity vector with either sense of rotation. All other orientations precess around either of these, with the polar precession dominating for high eccentricity. These expectations are borne out by smoothed particle hydrodynamics simulations of initially misaligned viscous circumbinary discs, resulting in the formation of polar rings around highly eccentric binaries in contrast to the coplanar discs around circular binaries. Moreover, we observe disc tearing and violent interactions between differentially precessing rings in the disc significantly disrupting the disc structure and causing gas to fall onto the binary with little angular momentum. While accretion from a polar disc may not promote SMBH binary coalescence (solving the `finalparsec problem'), ejection of this infalling lowangular momentum material via gravitational slingshot is a possible mechanism to reduce the binary separation. Moreover, this process acts on dynamical rather than viscous time scales, and so is much faster.  [Show abstract] [Hide abstract]
ABSTRACT: The approximate computation of all gravitational forces between $N$ interacting particles via the fast multipole method (FMM) can be made as accurate as direct summation, but requires less than $\mathcal{O}(N)$ operations. FMM groups particles into spatially bounded cells and uses cellcell interactions to approximate the force at any position within the sink cell by a Taylor expansion obtained from the multipole expansion of the source cell. By employing a novel estimate for the errors incurred in this process, I minimise the computational effort required for a given accuracy and obtain a wellbehaved distribution of force errors. For relative force errors of $\sim10^{7}$, the computational costs exhibit an empirical scaling of $\propto N^{0.87}$. My implementation (running on a 16 core node) outperforms a GPUbased direct summation with comparable force errors for $N\gtrsim10^5$.  [Show abstract] [Hide abstract]
ABSTRACT: The approximate computation of all gravitational forces between N interacting particles via the fast multipole method (FMM) can be made as accurate as direct summation, but requires less than (N) operations. FMM groups particles into spatially bounded cells and uses cellcell interactions to approximate the force at any position within the sink cell by a Taylor expansion obtained from the multipole expansion of the source cell. By employing a novel estimate for the errors incurred in this process, I minimise the computational effort required for a given accuracy and obtain a wellbehaved distribution of force errors. For relative force errors of ∼10−7, the computational costs exhibit an empirical scaling of ∝N 0.87. My implementation (running on a 16 core node) outperforms a GPUbased direct summation with comparable force errors for N ≳ 1 0 5 .  [Show abstract] [Hide abstract]
ABSTRACT: We suggest a new picture of supermassive black hole (SMBH) growth in galaxy centers. Momentumdriven feedback from an accreting hole gives significant orbital energy but little angular momentum to the surrounding gas. Once central accretion drops, the feedback weakens and sweptup gas falls back towards the SMBH on nearparabolic orbits. These intersect near the black hole with partially opposed specific angular momenta, causing further infall and ultimately the formation of a smallscale accretion disk. The feeding rates into the disk typically exceed Eddington by factors of a few, growing the hole on the Salpeter timescale and stimulating further feedback. Natural consequences of this picture include (i) the formation and maintenance of a roughly toroidal distribution of obscuring matter near the hole; (ii) random orientations of successive accretion disk episodes; (iii) the possibility of rapid SMBH growth; (iv) tidal disruption of stars and close binaries formed from infalling gas, resulting in visible flares and ejection of hypervelocity stars; (v) supersolar abundances of the matter accreting on to the SMBH; and (vi) a lower central darkmatter density, and hence annihilation signal, than adiabatic SMBH growth implies. We also suggest a simple subgrid recipe for implementing this process in numerical simulations. 
Article: Constraints on the Galactic Bar from the Hercules stream as traced with RAVE across the Galaxy
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ABSTRACT: Nonaxisymmetries in the Galactic potential (spiral arms and bar) induce kinematic groups such as the Hercules stream. Assuming that Hercules is caused by the effects of the Outer Lindblad Resonance of the Galactic bar, we model analytically its properties as a function of position in the Galaxy and its dependence on the bar's pattern speed and orientation. Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius, in a manner consistent with our analytical model. This allows us to obtain new estimates of the parameters of the Milky Way's bar. The combined likelihood function of the bar's pattern speed and angle has its maximum for a pattern speed of Omega_b=(1.89 + 0.08) x Omega_0 where Omega_0 is the local circular frequency. Assuming a Solar radius of 8.05 kpc and a local circular velocity of 238 km/s, this corresponds to Omega_b=56 + 2 km/s/kpc. On the other hand, the bar's orientation phi_b cannot be constrained with the available data. In fact, the likelihood function shows that a tight correlation exists between the pattern speed and the orientation, implying that a better description of our best fit results is given by the linear relation Omega_b/Omega_0=1.905 + 0.0044[phi_b(deg)47.7], with standard deviation of 0.02. For example, for an angle of phi_b=30deg the pattern speed is 54.0+0.5km/s/kpc. These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun, and are robust to biases in distance.  [Show abstract] [Hide abstract]
ABSTRACT: We simulate the evolution of onedimensional gravitating collisionless systems from non equilibrium initial conditions, similar to the conditions that lead to the formation of dark matter halos in three dimensions. As in the case of 3D halo formation we find that initially cold, nearly homogeneous particle distributions collapse to approach a final equilibrium state with a universal density profile. At small radii, this attractor exhibits a powerlaw behavior in density, {\rho}(x) \propto x^({\gamma}_crit), {\gamma}_crit \simeq 0.47, slightly but significantly shallower than the value {\gamma} = 1/2 suggested previously. This state develops from the initial conditions through a process of phase mixing and violent relaxation. This process preserves the energy ranks of particles. By warming the initial conditions, we illustrate a crossover from this powerlaw final state to a final state containing a homogeneous core. We further show that inhomogeneous but cold powerlaw initial conditions, with initial exponent {\gamma}_i > {\gamma}_crit, do not evolve toward the attractor but reach a final state that retains their original powerlaw behavior in the interior of the profile, indicating a bifurcation in the final state as a function of the initial exponent. Our results rely on a highfidelity eventdriven simulation technique.  [Show abstract] [Hide abstract]
ABSTRACT: Nonaxisymmetric components, such as spirals and central bars, play a major role in shaping galactic discs. An important aspect of the disc secular evolution driven by these perturbers is the radial migration of stars. It has been suggested recently that migration can populate a thickdisc component from innerdisc stars with high vertical energies. Since this has never been demonstrated in simulations, we study in detail the effect of radial migration on the disc velocity dispersion and disc thickness, by separating simulated stars into migrators and nonmigrators. We apply this method to three isolated barred TreeSPH Nbody galaxies with strong radial migration. Contrary to expectations, we find that as stellar samples migrate, on the average, their velocity dispersion change (by as much as 50%) in such a way as to approximately match the nonmigrating population at the radius at which they arrive. We show that, in fact, migrators suppress heating in parts of the disc. To confirm the validity of our findings, we also apply our technique to three cosmological resimulations, which use a completely different simulation scheme and, remarkably, find very similar results. We believe the inability of migration to thicken discs is a fundamental property of internal disc evolution, irrespective of the migration mechanism at work. We explain this with the approximate conservation of the (average) vertical and radial actions rather than the energy. This "action mixing" can be used to constrain the migration rate in the Milky Way: estimates of the average vertical action in observations for different populations of stars should reveal flattening with radius for older groups of stars. 
Article: The mass distribution of the Fornax dSph: Constraints from its globular cluster distribution
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ABSTRACT: Uniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical friction has not yet dragged any of Fornax's five globular clusters to the centre, and also why there is no evidence that any similar star cluster has been in the past (for Fornax or any other dSph). We set up a suite of 2800 Nbody simulations that sample the full range of globularcluster orbits and mass models consistent with all existing observational constraints for Fornax. In agreement with previous work, we find that if Fornax has a large darkmatter core then its globular clusters remain close to their currently observed locations for long times. Furthermore, we find previously unreported behaviour for clusters that start inside the core region. These are pushed out of the core and gain orbital energy, a process we call 'dynamical buoyancy'. Thus a cored mass distribution in Fornax will naturally lead to a shelllike globular cluster distribution near the core radius, independent of the initial conditions. By contrast, CDMtype cusped mass distributions lead to the rapid infall of at least one cluster within \Delta t = 12Gyr, except when picking unlikely initial conditions for the cluster orbits (\sim 2% probability), and almost all clusters within \Delta t = 10Gyr. Alternatively, if Fornax has only a weakly cusped mass distribution, dynamical friction is much reduced. While over \Delta t = 10Gyr this still leads to the infall of 14 clusters from their present orbits, the infall of any cluster within \Delta t = 12Gyr is much less likely (with probability 070%, depending on \Delta t and the strength of the cusp). Such a solution to the timing problem requires that in the past the globular clusters were somewhat further from Fornax than today; they most likely did not form within Fornax, but were accreted.  [Show abstract] [Hide abstract]
ABSTRACT: The numerical convergence of smoothed particle hydrodynamics (SPH) can be severely restricted by random force errors induced by particle disorder, especially in shear flows, which are ubiquitous in astrophysics. The increase in the number NH of neighbours when switching to more extended smoothing kernels at fixed resolution (using an appropriate definition for the SPH resolution scale) is insufficient to combat these errors. Consequently, trading resolution for better convergence is necessary, but for traditional smoothing kernels this option is limited by the pairing (or clumping) instability. Therefore, we investigate the suitability of the Wendland functions as smoothing kernels and compare them with the traditional Bsplines. Linear stability analysis in three dimensions and test simulations demonstrate that the Wendland kernels avoid the pairing instability for all NH, despite having vanishing derivative at the origin (disproving traditional ideas about the origin of this instability; instead, we uncover a relation with the kernel Fourier transform and give an explanation in terms of the SPH density estimator). The Wendland kernels are computationally more convenient than the higherorder Bsplines, allowing large NH and hence better numerical convergence (note that computational costs rise sublinear with NH). Our analysis also shows that at low NH the quartic spline kernel with NH ~= 60 obtains much better convergence then the standard cubic spline.  [Show abstract] [Hide abstract]
ABSTRACT: Disc nonaxisymmetrc components, such as spirals and central bars, are nowadays known to play an important role in shaping galactic discs. Here we use TreeSPH Nbody simulations to examine the effect of these perturbers on two aspects: the occurrence of multiple patterns in discs and the effects of radial migration on disc thickening. We find that, in addition to a central bar, multiple spiral patterns and lopsided modes develop in all models. Interaction among these asymmetric features results in a large scale stellar migration. However, we show that, despite the strong radial mixing, discs cannot be thickened sufficiently to match observed thick discs. We relate this to the adiabatic cooling as stars migrate radially outwards. We also find that the bulge contribution to a thickdisc component for an Satype galaxy at ~2.5 disc scalelengths is less than 1% and zero in the case of a Milky Waylike, Sbtype. Our findings cast doubt on the plausibility of thick disc formation via stellar radial migration.  [Show abstract] [Hide abstract]
ABSTRACT: We consider the infall of a massive clump into a dark matter halo as a simple and extreme model for the effect of baryonic physics (neglected in gravityonly simulations of largescale structure formation) on the dark matter. We find that such an infalling clump is extremely efficient in altering the structure of the halo and reducing its central density: a clump of 1 per cent the mass of the halo can remove about twice its own mass from the inner halo and transform a cusp into a core or weaker cusp. If the clump is subsequently removed, mimicking a galactic wind, the central halo density is further reduced and the mass removed from the inner halo doubled. Lighter clumps are even more efficient: the ratio of removed mass to clump mass increases slightly towards smaller clump masses. This process becomes more efficient the more radially anisotropic the initial dark matter velocities are. While such a clumpy infall may be somewhat unrealistic, it demonstrates that the baryons need to transfer only a small fraction of their initial energy to the dark matter via dynamical friction to explain the discrepancy between predicted dark matter density profiles and those inferred from observations of darkmatterdominated galaxies.  [Show abstract] [Hide abstract]
ABSTRACT: We describe the astrophysical and numerical basis of N body simulations, both of collisional stellar systems (dense star clusters and galactic centres) and collisionless stellar dynamics (galaxies and largescale structure). We explain and discuss the stateoftheart algorithms used for these quite different regimes, attempt to give a fair critique, and point out possible directions of future improvement and development. We briefly touch upon the history of N body simulations and their most important results. 
Article: Chemodynamics of the Galaxies: From Cuspy to Dark Matter Density Profiles and Metallicity Gradients
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ABSTRACT: We follow the dynamical and chemical evolution of isolated systems tuned in their properties on selfgravitating spherical three component (gas+stars+dark matter) dwarf spheroidal galaxies (dSph). The system starts as a strongly dark matter dominated cuspy density profile. It is shown that after few Gyr of evolution the star formation processes are naturally able to reshape the cuspy dark matter profile into a flatter one. The same family of models is then evolved by orbiting in a Milky Way external potential in order to investigate the history of the Carina dwarf galaxy. Structural parameters and the star formation history of Carina are selfconsistently determined in good agreement with the observations.  [Show abstract] [Hide abstract]
ABSTRACT: In smoothed particle hydrodynamics (SPH), artificial viscosity is necessary for the correct treatment of shocks, but often generates unwanted dissipation away from shocks. We present a novel method of controlling the amount of artificial viscosity, which uses the total time derivative of the velocity divergence as shock indicator and aims at completely eliminating viscosity away from shocks. We subject the new scheme to numerous tests and find that the method works at least as well as any previous technique in the strongshock regime, but becomes virtually inviscid away from shocks, while still maintaining particle order. In particular sound waves or oscillations of gas spheres are hardly damped over many periods. 
Article: Isolated dwarf galaxies: From cuspy to flat dark matter density profiles and metalicity gradients
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ABSTRACT: The chemodynamical evolution of spherical multicomponent selfgravitating models for isolated dwarf galaxies is studied. We compare their evolution with and without feedback effects from star formation processes. We find that initially cuspy dark matter profiles flatten with time as a result of star formation, without any special tuning conditions. Thus the seemingly flattened profiles found in many dwarfs do not contradict the cuspy profiles predicted by cosmological models. We also calculate the chemical evolution of stars and gas, to permit comparisons with observational data.  [Show abstract] [Hide abstract]
ABSTRACT: We reexamine the stellar kinematics of the Solar neighbourhood in terms of the velocity of the Sun with respect to the local standard of rest. We show that the classical determination of its component V_sun in the direction of Galactic rotation via Stroemberg's relation is undermined by the metallicity gradient in the disc, which introduces a correlation between the colour of a group of stars and the radial gradients of its properties. Comparing the local stellar kinematics to a chemodynamical model which accounts for these effects, we obtain (U,V,W)_sun = (11.1 +/ 0.74, 12.24 +/ 0.47, 7.25 +/0.37) km/s, with additional systematic uncertainties of ~ (1,2,0.5) km/s. In particular, V_sun is 7 km/s larger than previously estimated. The new values of solar motion are extremely insensitive to the metallicity gradient within the disc. 
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ABSTRACT: The madetomeasure Nbody method (Syer & Tremaine 1996) slowly adapts the particle weights of an Nbody model, whilst integrating the trajectories in an assumed static potential, until some constraints are satisfied, such as optimal fits to observational data. I propose a novel technique for this adaption procedure, which overcomes several limitations and shortcomings of the original method. The capability of the new technique is demonstrated by generating realistic Nbody equilibrium models for darkmatter haloes with prescribed density profile, triaxial shape, and slowly outwardly growing radial velocity anisotropy  [Show abstract] [Hide abstract]
ABSTRACT: We report the discovery of two welldefined tidal tails emerging from the sparse remote globular cluster Palomar 5. These tails stretch out symmetrically to both sides of the cluster in the direction of constant Galactic latitude and subtend an angle of 26 on the sky. The tails have been detected in commissioning data of the Sloan Digital Sky Survey, providing deep fivecolor photometry in a 25wide band along the equator. The stars in the tails make up a substantial part (~) of the current total population of cluster stars in the magnitude interval 19.5 ≤ i* ≤ 22.0. This reveals that the cluster is subject to heavy mass loss. The orientation of the tails provides an important key for the determination of the cluster's Galactic orbit.  [Show abstract] [Hide abstract]
ABSTRACT: We present the second data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities (RVs) and stellar atmosphere parameters of up to one million stars using the 6dF multiobject spectrograph on the 1.2m UK Schmidt Telescope of the AngloAustralian Observatory (AAO). It is obtaining medium resolution spectra (median R=7,500) in the Catriplet region (8,4108,795 \AA) for southern hemisphere stars in the magnitude range 9<I<12. Following the first data release (Steinmetz et al. 2006) the current release doubles the sample of published RVs, now containing 51,829 RVs for 49,327 individual stars observed on 141 nights between April 11 2003 and March 31 2005. Comparison with external data sets shows that the new data collected since April 3 2004 show a standard deviation of 1.3 km/s, about twice better than for the first data release. For the first time this data release contains values of stellar parameters from 22,407 spectra of 21,121 individual stars. They were derived by a penalized \chi^2 method using an extensive grid of synthetic spectra calculated from the latest version of Kurucz models. From comparison with external data sets, our conservative estimates of errors of the stellar parameters (for a spectrum with S/N=40) are 400 K in temperature, 0.5 dex in gravity, and 0.2 dex in metallicity. We note however that the internal errors estimated from repeat RAVE observations of 822 stars are at least a factor 2 smaller. We demonstrate that the results show no systematic offsets if compared to values derived from photometry or complementary spectroscopic analyses. The data release includes proper motion and photometric measurements. It can be accessed via the RAVE webpage: http://www.ravesurvey.org and through CDS. Comment: 85 pages, 23 figures, 14 tables, accepted for publication in the Astronomical Journal
Publication Stats
7k  Citations  
227.98  Total Impact Points  
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Institutions

20032015

University of Leicester
 Department of Physics and Astronomy
Leiscester, England, United Kingdom


19992008

Max Planck Institute for Astronomy
Heidelburg, BadenWürttemberg, Germany


19981999

University of Oxford
 Rudolf Peierls Centre for Theoretical Physics
Oxford, England, United Kingdom
