Publications


Towards a more realistic population of bright spiral galaxies in cosmological simulations

Monthly Notices of the Royal Astronomical Society 434 (2013) 3142-3164

M Aumer, SDM White, T Naab, C Scannapieco

We present an update to the multiphase smoothed particle hydrodynamics galaxy formation code by Scannapieco et al. We include a more elaborate treatment of the production of metals, cooling rates based on individual element abundances and a scheme for the turbulent diffusion of metals. Our supernova feedback model now transfers energy to the interstellar medium (ISM) in kinetic and thermal form, and we include a prescription for the effects of radiation pressure from massive young stars on the ISM. We calibrate our new code on the well-studied Aquarius haloes and then use it to simulate a sample of 16 galaxies with halo masses between 1 × 1011 and 3 × 1012Modot. In general, the stellar masses of the sample agree well with the stellar mass to halo mass relation inferred from abundance matching techniques for redshifts z = 0-4. There is however a tendency to overproduce stars at z > 4 and to underproduce them at z < 0.5 in the least massive haloes. Overly high star formation rates (SFRs) at z < 1 for the most massive haloes are likely connected to the lack of active galactic nuclei feedback in our model. The simulated sample also shows reasonable agreement with observed SFRs, sizes, gas fractions and gas-phase metallicities at z = 0-3. Remaining discrepancies can be connected to deviations from predictions for star formation histories from abundance matching. At z = 0, the model galaxies show realistic morphologies, stellar surface density profiles, circular velocity curves and stellar metallicities, but overly flat metallicity gradients. 15 out of 16 of our galaxies contain disc components with kinematic disc fraction ranging between 15 and 65 per cent. The disc fraction depends on the time of the last destructive merger or misaligned infall event. Considering the remaining shortcomings of our simulations we conclude that even higher kinematic disc fractions may be possible for λ cold dark matter haloes with quiet merger histories, such as the Aquarius haloes. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Kinetic effects on a tokamak pedestal ion flow, ion heat transport and bootstrap current

Plasma Physics and Controlled Fusion 55 (2013)

PJ Catto, FI Parra, G Kagan, JB Parker, I Pusztai, M Landreman

We consider the effects of a finite radial electric field on ion orbits in a subsonic pedestal. Using a procedure that makes a clear distinction between a transit average and a flux surface average we are able to solve the kinetic equation to retain the modifications due to finite E→ × B→ drift orbit departures from flux surfaces. Our approach properly determines the velocity space localized, as well as the nonlocal, portion of the ion distribution function in the banana and plateau regimes in the small aspect ratio limit. The rapid variation of the poloidal ion flow coefficient and the electrostatic potential in the total energy modify previous banana regime evaluations of the ion flow, the bootstrap current, and the radial ion heat flux in a subsonic pedestal. In the plateau regime, the rapid variation of the poloidal flow coefficient alters earlier results for the ion flow and bootstrap current, while leaving the ion heat flux unchanged since the rapid poloidal variation of the total energy was properly retained. © 2013 IOP Publishing Ltd.


Thermal instabilities in cooling galactic coronae: Fuelling star formation in galactic discs

Monthly Notices of the Royal Astronomical Society 434 (2013) 1849-1868

A Hobbs, J Read, C Power, D Cole

We investigate the means by which cold gas can accrete on to Milky Way mass galaxies from a hot corona of gas, using a new smoothed particle hydrodynamics code, 'SPHS'. We find that the 'cold clumps' seen in many classic SPH simulations in the literature are not present in our SPHS simulations. Instead, cold gas condenses from the halo along filaments that form at the intersection of supernovae-driven bubbles from previous phases of star formation. This positive feedback feeds cold gas to the galactic disc directly, fuelling further star formation. The resulting galaxies in the SPH and SPHS simulations differ greatly in their morphology, gas phase diagrams and stellar content. We show that the classic SPH cold clumps owe to a numerical thermal instability caused by an inability for cold gas to mix in the hot halo. The improved treatment of mixing in SPHS suppresses this instability leading to a dramatically different physical outcome. In our highest resolution SPHS simulation, we find that the cold filaments break up into bound, physically motivated clumps that form stars. The filaments are overdense by a factor of 10-100 compared to the surrounding gas, suggesting that the fragmentation results from a physical non-linear instability driven by the overdensity. This 'fragmenting filament' mode of disc growth has important implications for galaxy formation, in particular the role of star formation in bringing cold gas into disc galaxies. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Nonlinear gyrokinetic simulations of intrinsic rotation in up-down asymmetric tokamaks

Massachusetts Institute of Technology (2013)

J Ball


Idealized models for galactic disc formation and evolution in 'realistic' ΛCDM haloes

Monthly Notices of the Royal Astronomical Society 428 (2013) 1055-1076

M Aumer, SDM White

We study the dynamics of galactic disc formation and evolution in 'realistic'Λcold darkmatter haloes with idealized baryonic initial conditions. We add rotating spheres of hot gas at z = 1.3 to two fully cosmological dark-matter-only halo (re)simulations. The gas cools according to an artificial and adjustable cooling function to form a rotationally supported galaxy. The simulations evolve in the full cosmological context until z=0.We vary the angular momentum and density profiles of the initial gas sphere, the cooling time and the orientation of the angular momentum vector to study the effects on the formation and evolution of the disc. The final discs show exponential radial and (double)-exponential vertical stellar density profiles, and stellar velocity dispersions that increase with age of the stars, as in real disc galaxies. The slower the cooling/accretion processes, the higher the kinematic disc-to-bulge (D/B) ratio of the resulting system. We find that the initial orientation of the baryonic angular momentum with respect to the halo has a major effect on the resulting D/B. The most stable systems result from orientations parallel to the halo minor axis. Despite the spherical and coherently rotating initial gas distribution, the orientation of the central disc and of the outer gas components, and the relative angle between the components can all change by more than 90{ring operator} over several billion years. Initial orientations perpendicular to the major axis tend to align with the minor axis during their evolution, but the sign of the spin can have a strong effect. Discs can form from initial conditions oriented parallel to the major axis, but there is often strong misalignment between inner and outer material. The more the orientation of the baryonic angular momentum changes during the evolution, the lower the final D/B. The behaviour varies strongly from halo to halo. Even our very simple initial conditions can lead to strong bars, dominant bulges, massive, misaligned rings and counter-rotating components. We discuss how our results may relate to the failure or success of fully cosmological disc formation simulations. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Corrigendum to “AstroGK: Astrophysical gyrokinetics code” [J. Comput. Phys. 229 (2010) 9347–9372]

Journal of Computational Physics 245 (2013) C

R Numata, GG Howes, T Tatsuno, M Barnes, W Dorland


Multi-channel transport experiments at Alcator C-Mod and comparison with gyrokinetic simulations

PHYSICS OF PLASMAS 20 (2013) ARTN 056106

AE White, NT Howard, M Greenwald, ML Reinke, C Sung, S Baek, M Barnes, J Candy, A Dominguez, D Ernst, C Gao, AE Hubbard, JW Hughes, Y Lin, D Mikkelsen, F Parra, M Porkolab, JE Rice, J Walk, SJ Wukitch, AC-M Team


Multi-objective methods for determining optimal ventilation rates in dwellings

BUILDING AND ENVIRONMENT 66 (2013) 72-81

P Das, Z Chalabi, B Jones, J Milner, C Shrubsole, M Davies, I Hamilton, I Ridley, P Wilkinson


IONIZED ABSORBERS AS EVIDENCE FOR SUPERNOVA-DRIVEN COOLING OF THE LOWER GALACTIC CORONA

ASTROPHYSICAL JOURNAL LETTERS 764 (2013) ARTN L21

F Fraternali, A Marasco, F Marinacci, J Binney


Analysing surveys of our Galaxy - II. Determining the potential

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 433 (2013) 1411-1424

PJ McMillan, JJ Binney


A new code for orbit analysis and Schwarzschild modelling of triaxial stellar systems

Monthly Notices of the Royal Astronomical Society 434 (2013) 3174-3195

E Vasiliev

We review the methods used to study the orbital structure and chaotic properties of various galactic models and to construct self-consistent equilibrium solutions by Schwarzschild's orbit superposition technique. These methods are implemented in a new publicly available software tool, SMILE, which is intended to be a convenient and interactive instrument for studying a variety of 2D and 3D models, including arbitrary potentials represented by a basis-set expansion, a spherical-harmonic expansion with coefficients being smooth functions of radius (splines) or a set of fixed point masses. We also propose two new variants of Schwarzschild modelling, in which the density of each orbit is represented by the coefficients of the basis-set or spline spherical-harmonic expansion, and the orbit weights are assigned in such a way as to reproduce the coefficients of the underlying density model. We explore the accuracy of these general-purpose potential expansions and show that they may be efficiently used to approximate a wide range of analytic density models and serve as smooth representations of discrete particle sets (e.g. snapshots from an N-body simulation), for instance, for the purpose of orbit analysis of the snapshot. For the variants of Schwarzschild modelling, we use two test cases - a triaxial Dehnen model containing a central black hole and a model re-created from an N-body snapshot obtained by a cold collapse. These tests demonstrate that all modelling approaches are capable of creating equilibrium models. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Freely decaying turbulence in two-dimensional electrostatic gyrokinetics

PHYSICS OF PLASMAS 19 (2012) ARTN 122305

T Tatsuno, GG Plunk, M Barnes, W Dorland, GG Howes, R Numata


The detection and treatment of distance errors in kinematic analyses of stars

Monthly Notices of the Royal Astronomical Society 420 (2012) 1281-1293

R Schönrich, J Binney, M Asplund

We present a new method for detecting and correcting systematic errors in the distances to stars when both proper motions and line-of-sight velocities are available. The method, which is applicable for samples of 200 or more stars that have a significant extension on the sky, exploits correlations between the measured U, V and W velocity components that are introduced by distance errors. We deliver a formalism to describe and interpret the specific imprints of distance errors including spurious velocity correlations and shifts of mean motion in a sample. We take into account correlations introduced by measurement errors, Galactic rotation and changes in the orientation of the velocity ellipsoid with position in the Galaxy. Tests on pseudo-data show that the method is more robust and sensitive than traditional approaches to this problem. We investigate approaches to characterizing the probability distribution of distance errors, in addition to the mean distance error, which is the main theme of the paper. Stars with the most overestimated distances bias our estimate of the overall distance scale, leading to the corrected distances being slightly too small. We give a formula that can be used to correct for this effect. We apply the method to samples of stars from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey, exploring optimal gravity cuts, sample contamination, and correcting the used distance relations. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the beam emission spectroscopy system on the mega amp spherical tokamak

Plasma Physics and Controlled Fusion 54 (2012)

YC Ghim, AR Field, D Dunai, S Zoletnik, L Bardóczi, AA Schekochihin

The mean motion of turbulent patterns detected by a two-dimensional beam emission spectroscopy (BES) diagnostic on the mega amp spherical tokamak (MAST) is determined using a cross-correlation time delay method. Statistical reliability of the method is studied by means of synthetic data analysis. The experimental measurements on MAST indicate that the apparent mean poloidal motion of the turbulent density patterns in the lab frame arises because the longest correlation direction of the patterns (parallel to the local background magnetic fields) is not parallel to the direction of the fastest mean plasma flows (usually toroidal when strong neutral-beam injection is present). This effect is particularly pronounced in a spherical tokamak because of the relatively large mean rotation and large magnetic pitch angle. The experimental measurements are consistent with the mean motion of plasma being toroidal. The sum of all other contributions (mean poloidal plasma flow, phase velocity of the density patterns in the plasma frame, non-linear effects, etc) to the apparent mean poloidal velocity of the density patterns is found to be negligible. These results hold in all investigated L-mode, H-mode and internal transport barrier discharges. The one exception is a high-poloidal-beta (the ratio of the plasma pressure to the poloidal magnetic field energy density) discharge, where a large magnetic island exists. In this case BES detects very little motion. This effect is currently theoretically unexplained. © 2012 IOP Publishing Ltd.


Chaotic mixing and the secular evolution of triaxial cuspy galaxy models built with Schwarzschild's method

Monthly Notices of the Royal Astronomical Society 419 (2012) 3268-3279

E Vasiliev, E Athanassoula

We use both N-body simulations and integration in fixed potentials to explore the stability and the long-term secular evolution of self-consistent, equilibrium, non-rotating, triaxial spheroidal galactic models. More specifically, we consider Dehnen models built with the Schwarzschild method. We show that short-term stability depends on the degree of velocity anisotropy (radially anisotropic models are subject to rapid development of radial-orbit instability). Long-term stability, on the other hand, depends mainly on the properties of the potential and, in particular, on whether it admits a substantial fraction of strongly chaotic orbits. We show that in the case of a weak density cusp (γ= 1 Dehnen model) the N-body model is remarkably stable, while the strong-cusp (γ= 2) model exhibits substantial evolution of shape away from triaxiality, which we attribute to the effect of chaotic diffusion of orbits. The different behaviour of these two cases originates from the different phase space structure of the potential; in the weak-cusp case there exist numerous resonant orbit families that impede chaotic diffusion. We also find that it is hardly possible to affect the rate of this evolution by altering the fraction of chaotic orbits in the Schwarzschild model, which is explained by the fact that the chaotic properties of an orbit are not preserved by the N-body evolution. There are, however, parameters in Schwarzschild modelling that do affect the stability of an N-body model, so we discuss the recipes of building a 'good' Schwarzschild model. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


The mass distribution of the Fornax dSph: Constraints from its globular cluster distribution

Monthly Notices of the Royal Astronomical Society 426 (2012) 601-613

DR Cole, W Dehnen, JI Read, MI Wilkinson

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 tidally undisrupted dSph). We set up a suite of 2800 N-body simulations that sample the full range of globular cluster 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 dark matter 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 shell-like globular cluster distribution near the core radius, independent of the initial conditions. By contrast, cold dark matter-type cusped mass distributions lead to the rapid infall of at least one cluster within Δt = 1-2Gyr, except when picking unlikely initial conditions for the cluster orbits (∼2 per cent probability), and almost all clusters within Δt = 10Gyr. Alternatively, if Fornax has only a weakly cusped mass distribution, then dynamical friction is much reduced. While over Δt = 10Gyr this still leads to the infall of one to four clusters from their present orbits, the infall of any cluster within Δt = 1-2Gyr is much less likely (with probability 0-70 per cent, depending on Δt and the strength of the cusp). Such a solution to the timing problem requires (in addition to a shallow dark matter cusp) 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. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


A new formula for disc kinematics

Monthly Notices of the Royal Astronomical Society 419 (2012) 1546-1556

R Schönrich, J Binney

In a disc galaxy, the distribution of azimuthal components of velocity is very skew. In the past, this skewness has been modelled by superposed Gaussians. We use dynamical arguments to derive an analytic formula that can be fitted to observed velocity distributions, and validate it by fits to the velocities derived from a dynamically rigorous model, and to a sample of local stars with accurate space velocities. Our formula is much easier to use than a full distribution function. It has fewer parameters than a multi-Gaussian fit, and the best-fitting model parameters give insight into the underlying disc dynamics. In particular, once the azimuthal velocities of a sample have been successfully fitted, the apparatus provides a prediction for the corresponding distribution of radial velocitiesvR. An effective formula like ours is invaluable when fitting to data for stars at some distance from the Sun because it enables one to make proper allowance for the errors in distance and proper motion when determining the underlying disc kinematics. The derivation of our formula elucidates the way the horizontal and vertical motions are closely intertwined, and makes it evident that no stellar population can have a scaleheight and vertical velocity dispersions that are simultaneously independent of radius. We show that the oscillation of a star perpendicular to the Galactic plane modifies the effective potential in which the star moves radially in such a way that the more vertical energy a star has, the larger is the mean radius of its orbit. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Intrinsic rotation with gyrokinetic models

ArXiv (0)

FI Parra, M Barnes, I Calvo, PJ Catto

The generation of intrinsic rotation by turbulence and neoclassical effects in tokamaks is considered. To obtain the complex dependences observed in experiments, it is necessary to have a model of the radial flux of momentum that redistributes the momentum within the tokamak in the absence of a preexisting velocity. When the lowest order gyrokinetic formulation is used, a symmetry of the model precludes this possibility, making small effects in the gyroradius over scale length expansion necessary. These effects that are usually small become important for momentum transport because the symmetry of the lowest order gyrokinetic formulation leads to the cancellation of the lowest order momentum flux. The accuracy to which the gyrokinetic equation needs to be obtained to retain all the physically relevant effects is discussed.


Weak Alfvén-wave turbulence revisited.

Phys Rev E Stat Nonlin Soft Matter Phys 85 (2012) 036406-

AA Schekochihin, SV Nazarenko, TA Yousef

Weak Alfvénic turbulence in a periodic domain is considered as a mixed state of Alfvén waves interacting with the two-dimensional (2D) condensate. Unlike in standard treatments, no spectral continuity between the two is assumed, and, indeed, none is found. If the 2D modes are not directly forced, k(-2) and k(-1) spectra are found for the Alfvén waves and the 2D modes, respectively, with the latter less energetic than the former. The wave number at which their energies become comparable marks the transition to strong turbulence. For imbalanced energy injection, the spectra are similar, and the Elsasser ratio scales as the ratio of the energy fluxes in the counterpropagating Alfvén waves. If the 2D modes are forced, a 2D inverse cascade dominates the dynamics at the largest scales, but at small enough scales, the same weak and then strong regimes as described above are achieved.


Spin evolution of supermassive black holes and galactic nuclei

Physical Review D - Particles, Fields, Gravitation and Cosmology 86 (2012)

D Merritt, E Vasiliev

The spin angular momentum S of a supermassive black hole (SBH) precesses due to torques from orbiting stars, and the stellar orbits precess due to dragging of inertial frames by the spinning hole. We solve the coupled post-Newtonian equations describing the joint evolution of S and the stellar angular momenta L j, j=1...N in spherical, rotating nuclear star clusters. In the absence of gravitational interactions between the stars, two evolutionary modes are found: (1)nearly uniform precession of S about the total angular momentum vector of the system and (2)damped precession, leading, in less than one precessional period, to alignment of S with the angular momentum of the rotating cluster. Beyond a certain distance from the SBH, the time scale for angular momentum changes due to gravitational encounters between the stars is shorter than spin-orbit precession times. We present a model, based on the Ornstein-Uhlenbeck equation, for the stochastic evolution of star clusters due to gravitational encounters and use it to evaluate the evolution of S in nuclei where changes in the L j are due to frame dragging close to the SBH and to encounters farther out. Long-term evolution in this case is well described as uniform precession of the SBH about the cluster's rotational axis, with an increasingly important stochastic contribution when SBH masses are small. Spin precessional periods are predicted to be strongly dependent on nuclear properties, but typical values are ∼107-108yr for low-mass SBHs in dense nuclei, ∼108-1010yr for SBH masses ∼108M ™, and ∼1010-1011yr for the most massive SBHs. We compare the evolution of SBH spins in stellar nuclei to the case of torquing by an inclined, gaseous accretion disk. © 2012 American Physical Society.