Publications


Probing the complex ion structure in liquid carbon at 100 GPa

Physical Review Letters 111 (2013)

D Kraus, J Vorberger, DO Gericke, V Bagnoud, A Blažević, W Cayzac, A Frank, G Gregori, A Ortner, A Otten, F Roth, G Schaumann, D Schumacher, K Siegenthaler, F Wagner, K Wünsch, M Roth

We present the first direct experimental test of the complex ion structure in liquid carbon at pressures around 100 GPa, using spectrally resolved x-ray scattering from shock-compressed graphite samples. Our results confirm the structure predicted by ab initio quantum simulations and demonstrate the importance of chemical bonds at extreme conditions similar to those found in the interiors of giant planets. The evidence presented here thus provides a firmer ground for modeling the evolution and current structure of carbon-bearing icy giants like Neptune, Uranus, and a number of extrasolar planets. © 2013 American Physical Society.


The wobbly Galaxy: Kinematics north and south with RAVE red-clump giants

Monthly Notices of the Royal Astronomical Society 436 (2013) 101-121

MEK Williams, M Steinmetz, J Binney, A Siebert, H Enke, B Famaey, I Minchev, RS de Jong, C Boeche, KC Freeman, O Bienaymé, J Bland-Hawthorn, BK Gibson, GF Gilmore, EK Grebel, A Helmi, G Kordopatis, U Munari, JF Navarro, QA Parker, W Reid, GM Seabroke, S Sharma, A Siviero, FG Watson, RFG Wyse, T Zwitter

TheRAdialVelocity Experiment survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using 72 365 red-clump stars, we examine the mean velocity components in 3D between 6 < R < 10 kpc and -2 < Z < 2 kpc, concentrating on north-south differences. Simple parametric fits to the (R, Z) trends for Vφ and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, VR, finding that the gradient is marked below the plane (δ(VR)/δR=-8 kms-1 kpc-1 for Z<0, vanishing to zero above the plane), with a Z gradient thus also present. The vertical velocity, VZ, also shows clear, large-amplitude (|VZ| = 17 km s-1) structure, with indications of a rarefaction- compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the north-south differences in VR and VZ along the line of sight, with the VR estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disc, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Orbital-free density-functional theory simulations of the dynamic structure factor of warm dense aluminum

Physical Review Letters 111 (2013)

TG White, S Richardson, BJB Crowley, LK Pattison, JWO Harris, G Gregori

Here, we report orbital-free density-functional theory (OF DFT) molecular dynamics simulations of the dynamic ion structure factor of warm solid density aluminum at T=0.5 eV and T=5 eV. We validate the OF DFT method in the warm dense matter regime through comparison of the static and thermodynamic properties with the more complete Kohn-Sham DFT. This extension of OF DFT to dynamic properties indicates that previously used models based on classical molecular dynamics may be inadequate to capture fully the low frequency dynamics of the response function. © 2013 American Physical Society.


The ATLAS3D project - XVI. Physical parameters and spectral line energy distributions of the molecular gas in gas-rich early-type galaxies

Monthly Notices of the Royal Astronomical Society 432 (2013) 1742-1767

E Bayet, M Bureau, TA Davis, LM Young, AF Crocker, K Alatalo, L Blitz, M Bois, F Bournaud, M Cappellari, RL Davies, PT de Zeeuw, P-A Duc, E Emsellem, S Khochfar, D Krajnović, H Kuntschner, RM McDermid, R Morganti, T Naab, T Oosterloo, M Sarzi, N Scott, P Serra, A-M Weijmans

We present a detailed study of the physical properties of the molecular gas in a sample of 18 molecular gas-rich early-type galaxies (ETGs) from the ATLAS3D sample. Our goal is to better understand the star formation processes occurring in those galaxies, starting here with the dense star-forming gas. We use existing integrated 12CO (1-0, 2-1), 13CO (1-0, 2-1), HCN (1-0) and HCO+ (1-0) observations and new 12 CO (3-2) single-dish data. From these, we derive for the first time the average kinetic temperature, H2 volume density and column density of the emitting gas in a significant sample of ETGs, using a non-local thermodynamical equilibrium theoretical model. Since the CO lines trace different physical conditions than of those the HCN and HCO+ lines, the two sets of lines are treated separately. For most of the molecular gas-rich ETGs studied here, the CO transitions can be reproduced with kinetic temperatures of 10-20 K, H2 volume densities of 103-4 cm-3 and CO column densities of 1018-20 cm-2. The physical conditions corresponding to the HCN and HCO+ gas component have large uncertainties and must be considered as indicative only. We also compare for the first time the predicted CO spectral line energy distributions and gas properties of our molecular gas-rich ETGs with those of a sample of nearby well-studied disc galaxies. The gas excitation conditions in 13 of our 18 ETGs appear analogous to those in the centre of theMilky Way, hence the star formation activity driving these conditions is likely of a similar strength and nature. Such results have never been obtained before for ETGs and open a new window to explore further star-formation processes in the Universe. The conclusions drawn should nevertheless be considered carefully, as they are based on a limited number of observations and on a simple model. In the near future, with higher CO transition observations, it should be possible to better identify the various gas components present in ETGs, as well as more precisely determine their associated physical conditions. To achieve these goals, we show here from our theoretical study, that mid-J CO lines [such as the 12CO (6-5) line] are particularly useful. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Modeling HEDLA magnetic field generation experiments on laser facilities

High Energy Density Physics 9 (2013) 172-177

M Fatenejad, AR Bell, A Benuzzi-Mounaix, R Crowston, RP Drake, N Flocke, G Gregori, M Koenig, C Krauland, D Lamb, D Lee, JR Marques, J Meinecke, F Miniati, CD Murphy, H-S Park, A Pelka, A Ravasio, B Remington, B Reville, A Scopatz, P Tzeferacos, K Weide, N Woolsey, R Young, R Yurchak

The Flash Center is engaged in a collaboration to simulate laser driven experiments aimed at understanding the generation and amplification of cosmological magnetic fields using the FLASH code. In these experiments a laser illuminates a solid plastic or graphite target launching an asymmetric blast wave into a chamber which contains either Helium or Argon at millibar pressures. Induction coils placed several centimeters away from the target detect large scale magnetic fields on the order of tens to hundreds of Gauss. The time dependence of the magnetic field is consistent with generation via the Biermann battery mechanism near the blast wave. Attempts to perform simulations of these experiments using the FLASH code have uncovered previously unreported numerical difficulties in modeling the Biermann battery mechanism near shock waves which can lead to the production of large non-physical magnetic fields. We report on these difficulties and offer a potential solution. © 2012 Elsevier B.V.


Fast electron beam measurements from relativistically intense, frequency-doubled laser-solid interactions

New Journal of Physics 15 (2013)

RHH Scott, F Pérez, MJV Streeter, EL Clark, JR Davies, H-P Schlenvoigt, JJ Santos, S Hulin, KL Lancaster, F Dorchies, C Fourment, B Vauzour, AA Soloviev, SD Baton, SJ Rose, PA Norreys

Experimental measurements of the fast electron beam created by the interaction of relativistically intense, frequency-doubled laser light with planar solid targets and its subsequent transport within the target are presented and compared with those of a similar experiment using the laser fundamental frequency. Using frequency-doubled laser light, the fast electron source size is significantly reduced, while evidence suggests the divergence angle may be reduced. Pyrometric measurements of the target rear surface temperature and the Cu Kα imager data indicate the laser to fast electron absorption fraction is reduced using frequency doubled laser light. Bremsstrahlung measurements indicate the fast electron temperature is 125 keV, while the laser energy absorbed into forward-going fast electrons was found to be 16 ± 4% for frequency doubled light at a mean laser intensity of 5 ± 3 × 1018 W cm-2. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Laboratory experiments on plasma jets in a magnetic field using high-power lasers

EPJ Web of Conferences 59 (2013)

K Nishio, Y Sakawa, Y Kuramitsu, T Morita, T Ide, M Kuwada, M Koga, T Kato, T Norimatsu, C Gregory, N Woolsey, C Murphy, G Gregori, K Schaar, A Diziere, M Koenig, A Pelka, S Wang, Q Dong, Y Li, H Takabe

The experiments to simulate astrophysical jet generation are performed using Gekko XII (GXII) HIPER laser system at the Institute of Laser Engineering. In the experiments a fast plasma flow generated by shooting a CH plane (10 μm thickness) is observed at the rear side of the plane. By separating the focal spot of the main beams, a non-uniform plasma is generated. The non-uniform plasma flow in an external magnetic field (0.2∼0.3 T) perpendicular to the plasma is more collimated than that without the external magnetic field. The plasma β, the ratio between the plasma and magnetic pressure, is ≠1, and the magnetic Reynolds number is ∼150 in the collimated plasma. It is considered that the magnetic field is distorted by the plasma flow and enhances the jet collimation. © Owned by the authors, published by EDP Sciences, 2013.


DIFFUSE INTERSTELLAR BAND AT 8620 angstrom IN RAVE: A NEW METHOD FOR DETECTING THE DIFFUSE INTERSTELLAR BAND IN SPECTRA OF COOL STAR

ASTROPHYSICAL JOURNAL 778 (2013) ARTN 86

J Kos, T Zwitter, EK Grebel, O Bienayme, J Binney, J Bland-Hawthorn, KC Freeman, BK Gibson, G Gilmore, G Kordopatis, JF Navarro, Q Parker, WA Reid, G Seabroke, A Siebert, A Siviero, M Steinmetz, F Watson, RFG Wyse


Blowing cold flows away: the impact of early AGN activity on the formation of a brightest cluster galaxy progenitor

ArXiv (2012)

Y Dubois, C Pichon, J Devriendt, J Silk, M Haehnelt, T Kimm, A Slyz

Supermassive black holes (BH) are powerful sources of energy that are already in place at very early epochs of the Universe (by z=6). Using hydrodynamical simulations of the formation of a massive M_vir=5 10^11 M_sun halo by z=6 (the most massive progenitor of a cluster of M_vir=2 10^15 M_sun at z=0), we evaluate the impact of Active Galactic Nuclei (AGN) on galaxy mass content, BH self-regulation, and gas distribution inside this massive halo. We find that SN feedback has a marginal influence on the stellar structure, and no influence on the mass distribution on large scales. In contrast, AGN feedback alone is able to significantly alter the stellar-bulge mass content by quenching star formation when the BH is self-regulating, and by depleting the cold gas reservoir in the centre of the galaxy. The growth of the BH proceeds first by a rapid Eddington-limited period fed by direct cold filamentary infall. When the energy delivered by the AGN is sufficiently large to unbind the cold gas of the bulge, the accretion of gas onto the BH is maintained both by smooth gas inflow and clump migration through the galactic disc triggered by merger-induced torques. The feedback from the AGN has also a severe consequence on the baryon mass content within the halo, producing large-scale hot superwinds, able to blow away some of the cold filamentary material from the centre and reduce the baryon fraction by more than 30 per cent within the halo's virial radius. Thus in the very young universe, AGN feedback is likely to be a key process, shaping the properties of the most massive galaxies.


Powering of cool filaments in cluster cores by buoyant bubbles - I. Qualitative model

Monthly Notices of the Royal Astronomical Society 436 (2013) 526-530

E Churazov, M Ruszkowski, A Schekochihin

Cool-core clusters (e.g. Perseus or M87) often possess a network of bright gaseous filaments, observed in radio, infrared, optical and X-ray bands. We propose that these filaments are powered by the reconnection of the magnetic field in the wakes of buoyant bubbles. Active galactic nucleus (AGN)-inflated bubbles of relativistic plasma rise buoyantly in the cluster atmosphere, stretching and amplifying the field in the wake to values of β = 8πP/B ~ 1. The field lines in thewake have opposite directions and are forced together as the bubble motion stretches the filament. This setup bears strong similarity to the coronal loops on the Sun or to the Earth's magnetotail. The reconnection process naturally explains both the required level of local dissipation rate in filaments and the overall luminosity of filaments. The original source of power for the filaments is the potential energy of buoyant bubbles, inflated by the central AGN. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Dynamics for Galactic Archaeology

ArXiv (2013)

J Binney

Our Galaxy is a complex machine in which several processes operate simultaneously: metal-poor gas is accreted, is chemically enriched by dying stars, and then drifts inwards, surrendering its angular momentum to stars; new stars are formed on nearly circular orbits in the equatorial plane and then diffuse through orbit space to eccentric and inclined orbits; the central stellar bar surrenders angular momentum to the surrounding disc and dark halo while acquiring angular momentum from inspiralling gas; the outer parts of the disc are constantly disturbed by satellite objects, both luminous and dark, as they sweep through pericentre. We review the conceptual tools required to bring these complex happenings into focus. Our first concern must be the construction of equilibrium models of the Galaxy, for upon these hang our hopes of determining the Galaxy's mean gravitational field, which is required for every subsequent step. Ideally our equilibrium model should be formulated so that the secular evolution of the system can be modelled with perturbation theory. Such theory can be used to understand how stars diffuse through orbit space from either the thin gas disc in which we presume disc stars formed, or the debris of an accreted object, the presumed origin of many halo stars. Coupling this understanding to the still very uncertain predictions of the theory of stellar evolution and nucleosynthesis, we can finally extract a complete model of the chemodynamic evolution of our reasonably generic Galaxy. We discuss the relation of such a model to cosmological simulations of galaxy formation, which provide general guidance but cannot be relied on for quantitative detail.


Effect of collisions on amplification of laser beams by Brillouin scattering in plasmas

Physics of Plasmas 20 (2013)

KA Humphrey, DC Speirs, R Bingham, RMGM Trines, P Norreys, F Fiuza, RA Cairns, LO Silva

We report on particle in cell simulations of energy transfer between a laser pump beam and a counter-propagating seed beam using the Brillouin scattering process in uniform plasma including collisions. The results presented show that the ion acoustic waves excited through naturally occurring Brillouin scattering of the pump field are preferentially damped without affecting the driven Brillouin scattering process resulting from the beating of the pump and seed fields together. We find that collisions, including the effects of Landau damping, allow for a more efficient transfer of energy between the laser beams, and a significant reduction in the amount of seed pre-pulse produced. © 2013 AIP Publishing LLC.


Self-consistent measurement of the equation of state of liquid deuterium

High Energy Density Physics 8 (2012) 76-80

K Falk, SP Regan, J Vorberger, MA Barrios, TR Boehly, DE Fratanduono, SH Glenzer, DG Hicks, SX Hu, CD Murphy, PB Radha, S Rothman, AP Jephcoat, JS Wark, DO Gericke, G Gregori

We combine experiments and theoretical models to characterize warm dense deuterium. A shockwave was driven in a planar target by the OMEGA laser without a standard pusher making the analysis independent of a quartz or aluminium pressure standard. The conditions of the shocked material were diagnosed with VISAR and optical pyrometry which yields the shock velocity (16.9 ± 0.9 km/s) and the temperature (0.57 ± 0.05 eV). We find a self-consistent description of the data when using ab initio simulations (DFT-MD), but not for other equation of state (EOS) models tested. © 2011 Elsevier B.V.


Dynamics of secular evolution

ArXiv (2012)

J Binney

The text of lectures to the 2011 Tenerife Winter School. The School's theme was "Secular Evolution of Galaxies" and my task was to present the underlying stellar-dynamical theory. Other lecturers were speaking on the role of bars and chemical evolution, so these topics are avoided here. We start with an account of the connections between isolating integrals, quasiperiodicity and angle-action variables - these variables played a unifying role throughout the lectures. This leads on to the phenomenon of resonant trapping and how this can lead to chaos in cuspy potentials and phase-space mixing in slowly evolving potentials. Surfaces of section and frequency analysis are introduced as diagnostics of phase-space structure. Real galactic potentials include a fluctuating part that drives the system towards unattainable thermal equilibrium. Two-body encounters are only one source of fluctuations, and all fluctuations will drive similar evolution. We derive the orbit-averaged Fokker-Planck equation and relations that hold between the second-order diffusion coefficients and both the power spectrum of the fluctuations and the first-order diffusion coefficients. From the observed heating of the solar neighbourhood we show that the second-order diffusion coefficients must scale as J^{1/2}. We show that periodic spiral structure shifts angular momentum outwards, heating at the Lindblad resonances and mixing at corotation. The equation that would yield the normal modes of a stellar disc is first derived and then used to discuss the propagation of tightly-wound spiral waves. The winding up of such waves is explains why cool stellar discs are responsive systems that amplify ambient noise. An explanation is offered of why the Lin-Shu-Kalnajs dispersion relation and even global normal-mode calculations provide a very incomplete understanding of the dynamics of stellar discs.


The ATLAS 3D project - XI. Dense molecular gas properties of CO-luminous early-type galaxies

Monthly Notices of the Royal Astronomical Society (2012)

A Crocker, M Krips, M Bureau, LM Young, TA Davis, E Bayet, K Alatalo, L Blitz, M Bois, F Bournaud, M Cappellari, RL Davies, PT de Zeeuw, P-A Duc, E Emsellem, S Khochfar, D Krajnović, H Kuntschner, P-Y Lablanche, RM Mcdermid, R Morganti, T Naab, T Oosterloo, M Sarzi, N Scott, P Serra, A-M Weijmans


Constraining stellar assembly and active galactic nucleus feedback at the peak epoch of star formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 425 (2012) L96-L100

T Kimm, S Kaviraj, JEG Devriendt, SH Cohen, RA Windhorst, Y Dubois, A Slyz, NP Hathi, RRE Jr, RW O'Connell, MA Dopita, J Silk


Laboratory investigations on the origins of cosmic rays

Plasma Physics and Controlled Fusion 54 (2012)

Y Kuramitsu, Y Sakawa, T Morita, T Ide, K Nishio, H Tanji, H Aoki, S Dono, CD Gregory, JN Waugh, N Woolsey, A Dizière, A Pelka, A Ravasio, B Loupias, M Koenig, SA Pikuz, YT Li, Y Zhang, X Liu, JY Zhong, J Zhang, G Gregori, N Nakanii, K Kondo, Y Mori, E Miura, R Kodama, Y Kitagawa, K Mima, KA Tanaka, H Azechi, T Moritaka, Y Matsumoto, T Sano, A Mizuta, N Ohnishi, M Hoshino, H Takabe

We report our recent efforts on the experimental investigations related to the origins of cosmic rays. The origins of cosmic rays are long standing open issues in astrophysics. The galactic and extragalactic cosmic rays are considered to be accelerated in non-relativistic and relativistic collisionless shocks in the universe, respectively. However, the acceleration and transport processes of the cosmic rays are not well understood, and how the collisionless shocks are created is still under investigation. Recent high-power and high-intensity laser technologies allow us to simulate astrophysical phenomena in laboratories. We present our experimental results of collisionless shock formations in laser-produced plasmas. © 2012 IOP Publishing Ltd.


The X-ray luminous cluster underlying the z = 1.04 quasar PKS1229-021

ArXiv (2012)

HR Russell, AC Fabian, GB Taylor, JS Sanders, KM Blundell, CS Crawford, RM Johnstone, E Belsole

We present a 100 ks Chandra observation studying the extended X-ray emission around the powerful z=1.04 quasar PKS1229-021. The diffuse cluster X-ray emission can be traced out to ~15 arcsec (~120 kpc) radius and there is a drop in the calculated hardness ratio inside the central 5 arcsec consistent with the presence of a cool core. Radio observations of the quasar show a strong core and a bright, one-sided jet leading to the SW hot spot and a second hot spot visible on the counter-jet side. Although the wings of the quasar PSF provided a significant contribution to the total X-ray flux at all radii where the extended cluster emission was detected, we were able to accurately subtract off the PSF emission using ChaRT and marx simulations. The resulting steep cluster surface brightness profile for PKS1229-021 appears similar to the profile for the FRII radio galaxy 3C444, which has a similarly rapid surface brightness drop caused by a powerful shock surrounding the radio lobes (Croston et al.). Using a model surface brightness profile based on 3C444, we estimated the total cluster luminosity for PKS1229-021 to be L_X ~ 2 x 10^{44} erg/s. We discuss the difficulty of detecting cool core clusters, which host bright X-ray sources, in high redshift surveys.


AGN feedback driven molecular outflow in NGC 1266

Proceedings of the International Astronomical Union 8 (2012) 175-176

K Alatalo, KE Nyland, G Graves, S Deustua, J Wrobel, LM Young, TA Davis, M Bureau, E Bayet, L Blitz, M Bois, F Bournaud, M Cappellari, RL Davies, PT De Zeeuw, E Emsellem, S Khochfar, D Krajnovic, H Kuntschner, S Martín, RM Mcdermid, R Morganti, T Naab, M Sarzi, N Scott, P Serra, A Weijmans

NGC 1266 is a nearby field galaxy observed as part of the ATLAS 3D survey (Cappellari et al. 2011). NGC 1266 has been shown to host a compact (< 200 pc) molecular disk and a mass-loaded molecular outflow driven by the AGN (Alatalo et al. 2011). Very Long Basline Array (VLBA) observations at 1.65 GHz revealed a compact (diameter < 1.2 pc), high brightness temperature continuum source most consistent with a low-level AGN origin. The VLBA continuum source is positioned at the center of the molecular disk and may be responsible for the expulsion of molecular gas in NGC 1266. Thus, the candidate AGN-driven molecular outflow in NGC 1266 supports the picture in which AGNs do play a significant role in the quenching of star formation and ultimately the evolution of the red sequence of galaxies. © International Astronomical Union 2013.


A review of Vlasov-Fokker-Planck numerical modeling of inertial confinement fusion plasma

Journal of Computational Physics 231 (2012) 1051-1079

AGR Thomas, M Tzoufras, APL Robinson, RJ Kingham, CP Ridgers, M Sherlock, AR Bell

The interaction of intense lasers with solid matter generates a hot plasma state that is well described by the Vlasov-Fokker-Planck equation. Accurate and efficient modeling of the physics in these scenarios is highly pertinent, because it relates to experimental campaigns to produce energy by inertial confinement fusion on facilities such as the National Ignition Facility. Calculations involving the Vlasov-Fokker-Planck equation are computationally intensive, but are crucial to proper understanding of a wide variety of physical effects and instabilities in inertial fusion plasmas. In this topical review, we will introduce the background physics related to Vlasov-Fokker-Planck simulation, and then proceed to describe results from numerical simulation of inertial fusion plasma in a pedagogical manner by discussing some key numerical algorithm developments that enabled the research to take place. A qualitative comparison of the techniques is also given. © 2011 Elsevier Inc.