Publications


Observation of inhibited electron-ion coupling in strongly heated graphite.

Sci Rep 2 (2012) 889-

TG White, J Vorberger, CR Brown, BJ Crowley, P Davis, SH Glenzer, JW Harris, DC Hochhaus, S Le Pape, T Ma, CD Murphy, P Neumayer, LK Pattison, S Richardson, DO Gericke, G Gregori

Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures (T(ele)≠T(ion)) allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.


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, M Asplund, J Binney

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.


Characterizing counter-streaming interpenetrating plasmas relevant to astrophysical collisionless shocks

Physics of Plasmas 19 (2012)

JS Ross, SH Glenzer, P Amendt, R Berger, L Divol, NL Kugland, OL Landen, C Plechaty, B Remington, D Ryutov, W Rozmus, H-S Park, DH Froula, G Fiksel, C Sorce, Y Kuramitsu, T Morita, Y Sakawa, H Takabe, RP Drake, M Grosskopf, C Kuranz, G Gregori, J Meinecke, CD Murphy, M Koenig, A Pelka, A Ravasio, T Vinci, E Liang, R Presura, A Spitkovsky, F Miniati

A series of Omega experiments have produced and characterized high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks. Single and double CH foils have been irradiated with a laser intensity of ∼ 10 W/cm. The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2000 km/s, an electron temperature of ∼ 110 eV, an ion temperature of ∼ 30 eV, and a density of ∼ 10 cm were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured single foil plasma conditions were used to calculate the ion skin depth, c/ω ∼ 0.16 mm, the interaction length, l, of ∼ 8 mm, and the Coulomb mean free path, λ ∼ 27 mm. With c/ω ≪ l ≪λ, we are in a regime where collisionless shock formation is possible. © 2012 American Institute of Physics.


Analysing surveys of our Galaxy - I. Basic astrometric data

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 419 (2012) 2251-2263

PJ McMillan, J Binney


Molecular Dynamics Simulations for the Shear Viscosity of the One-Component Plasma

CONTRIBUTIONS TO PLASMA PHYSICS 52 (2012) 58-61

JP Mithen, J Daligault, G Gregori


A study of fast electron energy transport in relativistically intense laser-plasma interactions with large density scalelengths

Physics of Plasmas 19 (2012)

RHH Scott, CP Ridgers, SJ Rose, PA Norreys, KL Lancaster, RMGM Trines, AR Bell, M Tzoufras, F Perez, SD Baton, JJ Santos, P Nicolai, S Hulin, JR Davies

A systematic experimental and computational investigation of the effects of three well characterized density scalelengths on fast electron energy transport in ultra-intense laser-solid interactions has been performed. Experimental evidence is presented which shows that, when the density scalelength is sufficiently large, the fast electron beam entering the solid-density plasma is best described by two distinct populations: those accelerated within the coronal plasma (the fast electron pre-beam) and those accelerated near or at the critical density surface (the fast electron main-beam). The former has considerably lower divergence and higher temperature than that of the main-beam with a half-angle of ∼20°. It contains up to 30% of the total fast electron energy absorbed into the target. The number, kinetic energy, and total energy of the fast electrons in the pre-beam are increased by an increase in density scalelength. With larger density scalelengths, the fast electrons heat a smaller cross sectional area of the target, causing the thinnest targets to reach significantly higher rear surface temperatures. Modelling indicates that the enhanced fast electron pre-beam associated with the large density scalelength interaction generates a magnetic field within the target of sufficient magnitude to partially collimate the subsequent, more divergent, fast electron main-beam. © 2012 American Institute of Physics.


More dynamical models of our Galaxy

ArXiv (2012)

J Binney

Using a new algorithm for estimating the actions of orbits a parametrised distribution function is automatically fitted to observational data for the solar neighbourhood. We adopt a gravitational potential that is generated by three discs (gas and both thin and thick stellar discs), a bulge and a dark halo, and fit the thin-disc component of the distribution function to the solar-neighbourhood velocity distribution from the Geneva-Copenhagen Survey. We find that the disc's vertical density profile is in good agreement with data at z<~500 pc. The thick-disc component of the distribution function is then used to extend the fit to data from Gilmore & Reid (1983) for z<~2.5 kpc. The resulting model predicts excellent fits to the profile of the vertical velocity dispersion \sigma_z(z) from the RAVE survey and to the distribution of v_\phi velocity components at |z|~1kpc from the SDSS survey. The ability of this model to predict successfully data that was not used in the fitting process suggests that the adopted gravitational potential (which is close to a maximum-disc potential) is close to the true one. We show that if another plausible potential is used, the predicted values of \sigma_z are too large. The models imply that in contrast to the thin disc, the thick disc has to be hotter vertically than radially, a prediction that it will be possible to test in the near future. When the model parameters are adjusted in an unconstrained manner, there is a tendency to produce models that predict unexpected radial variations in quantities such as scale height. This finding suggests that to constrain these models adequately one needs data that extends significantly beyond the solar cylinder. The models presented in this paper might prove useful to the interpretation of data for external galaxies that has been taken with an integral field unit.


Lyman-alpha emission properties of simulated galaxies: interstellar medium structure and inclination effects

ArXiv (2012)

A Verhamme, Y Dubois, J Blaizot, T Garel, R Bacon, J Devriendt, B Guiderdoni, A Slyz

[abridged] Aims. The aim of this paper is to assess the impact of the interstellar medium (ISM) physics on Lyman-alpha (Lya) radiation transfer and to quantify how galaxy orientation with respect to the line of sight alters observational signatures. Methods. We compare the results of Lya radiation transfer calculations through the ISM of a couple of idealized galaxy simulations with different ISM models. Results. First, the small-scale structuration of the ISM plays a determinant role in shaping a galaxys Lya properties.The artificially warm, and hence smooth, ISM of G1 yields an escape fraction of 50 percent at the Lya line center, and produces symmetrical double-peak profiles. On the contrary, in G2, most young stars are embedded in thick star-forming clouds, and the result is a 10 times lower escape fraction. G2 also displays a stronger outflowing velocity field, which favors the escape of red-shifted photons, resulting in an asymmetric Lya line. Second, the Lya properties of G2 strongly depend on the inclination at which it is observed: From edge-on to face-on, the line goes from a double-peak profile with an equivalent width of -5 Angstrom to a 15 times more luminous red-shifted asymmetric line with EW 90 Angstrom. Conclusions. Lya radiation transfer calculations can only lead to realistic properties in simulations where galaxies are resolved into giant molecular clouds, putting these calculations out of reach of current large scale cosmological simulations. Finally, we find inclination effects to be much stronger for Lya photons than for continuum radiation. This could potentially introduce severe biases in the selection function of narrow-band Lya emitter surveys, which could indeed miss a significant fraction of the high-z galaxy population.


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


Feeding compact bulges and supermassive black holes with low angular-momentum cosmic gas at high redshift

ArXiv (2011)

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

We use cosmological hydrodynamical simulations to show that a significant fraction of the gas in high redshift rare massive halos falls nearly radially to their very centre on extremely short timescales. This process results in the formation of very compact bulges with specific angular momentum a factor 5-30$smaller than the average angular momentum of the baryons in the whole halo. Such low angular momentum originates both from segregation and effective cancellation when the gas flows to the centre of the halo along well defined cold filamentary streams. These filaments penetrate deep inside the halo and connect to the bulge from multiple rapidly changing directions. Structures falling in along the filaments (satellite galaxies) or formed by gravitational instabilities triggered by the inflow (star clusters) further reduce the angular momentum of the gas in the bulge. Finally, the fraction of gas radially falling to the centre appears to increase with the mass of the halo; we argue that this is most likely due to an enhanced cancellation of angular momentum in rarer halos which are fed by more isotropically distributed cold streams. Such an increasingly efficient funnelling of low-angular momentum gas to the centre of very massive halos at high redshift may account for the rapid pace at which the most massive supermassive black holes grow to reach observed masses around $10^9$M$_\odot$ at an epoch when the Universe is barely 1 Gyr old.


Actions for axisymmetric potentials

ArXiv (2012)

J Binney

We give an algorithm for the economical calculation of angles and actions for stars in axisymmetric potentials. We test the algorithm by integrating orbits in a realistic model of the Galactic potential, and find that, even for orbits characteristic of thick-disc stars, the errors in the actions are typically smaller than 2 percent. We describe a scheme for obtaining actions by interpolation on tabulated values that significantly accelerates the process of calculating observables quantities, such as density and velocity moments, from a distribution function.


Measurement of radiative shock properties by X-ray Thomson scattering

Physical Review Letters 108 (2012)

AJ Visco, RP Drake, MJ Grosskopf, SH Glenzer, T Döppner, G Gregori, DH Froula

X-ray Thomson scattering has enabled us to measure the temperature of a shocked layer, produced in the laboratory, that is relevant to shocks emerging from supernovas. High energy lasers are used to create a shock in argon gas which is probed by x-ray scattering. The scattered, inelastic Compton feature allows inference of the electron temperature. It is measured to be 34 eV in the radiative precursor and ∼60eV near the shock. Comparison of energy fluxes implied by the data demonstrates that the shock wave is strongly radiative. © 2012 American Physical Society.


Generation of scaled protogalactic seed magnetic fields in laser-produced shock waves

Nature 481 (2012) 480-483

G Gregori, CD Murphy, K Schaar, A Baird, AR Bell, M Edwards, W Lau, J Mithen, B Reville, S Yang, R Bingham, APL Robinson, A Ravasio, A Benuzzi-Mounaix, M Koenig, C Constantin, ET Everson, C Niemann, RP Drake, CD Gregory, NC Woolsey, Y Kuramitsu, Y Sakawa, H-S Park, BA Remington, DD Ryutov, F Miniati

The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations. Although other mechanisms may also operate, currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10 gauss (refs 7, 8). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations, astrophysical environments can effectively be reproduced in the laboratory. Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields sufficiently to affect galaxy evolution. © 2012 Macmillan Publishers Limited. All rights reserved.


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

Monthly Notices of the Royal Astronomical Society 422 (2012) 590-599

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

We present a 100ks 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 ∼15arcsec (∼120kpc) radius and there is a drop in the calculated hardness ratio inside the central 5arcsec 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 south-west hotspot and a second hotspot visible on the counter-jet side. Although the wings of the quasar point spread function (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 the PSF emission using Chandra Ray Tracer and marx simulations. The resulting steep cluster surface brightness profile for PKS1229-021 appears similar to the profile for the FR II (Fanaroff-Riley class II) radio galaxy 3C444, which has a similarly rapid surface brightness drop caused by a powerful shock surrounding the radio lobes. Using a model surface brightness profile based on 3C444, we estimated the total cluster luminosity for PKS1229-021 to be. We discuss the difficulty of detecting cool-core clusters, which host bright X-ray sources, in high redshift surveys. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


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.


Diffusive shock acceleration and magnetic field amplification

Space Science Reviews 173 (2012) 491-519

KM Schure, AR Bell, AM Bykov, L O'C Drury

Diffusive shock acceleration is the theory of particle acceleration through multiple shock crossings. In order for this process to proceed at a rate that can be reconciled with observations of high-energy electrons in the vicinity of the shock, and for cosmic rays protons to be accelerated to energies up to observed galactic values, significant magnetic field amplification is required. In this review we will discuss various theories on how magnetic field amplification can proceed in the presence of a cosmic ray population. On both short and long length scales, cosmic ray streaming can induce instabilities that act to amplify the magnetic field. Developments in this area that have occurred over the past decade are the main focus of this paper. © 2012 Springer Science+Business Media B.V.


Employing laser-accelerated proton beams to diagnose high intensity laser-plasma interactions

AIP Conference Proceedings 1462 (2012) 149-154

G Sarri, K Quinn, M Borghesi, CA Cecchetti, PA Norreys, R Trines, O Willi, J Fuchs, P McKenna, M Quinn, F Pegoraro, SV Bulanov

A review of the proton radiography technique will be presented. This technique employs laser-accelerated laminar bunches of protons to diagnose the temporal and spatial characteristic of the electric and magnetic fields generated during high-intensity laser-plasma interactions. The remarkable temporal and spatial resolution that this technique can achieve (of the order of a picosecond and a few microns respectively) candidates this technique as the preferrable one, if compared to other techniques, to probe high intensity laser-matterinteractions. © 2012 American Institute of Physics.


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

High Energy Density Physics 8 (2012) 76-80

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

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.


X-ray Thomson scattering on shocked graphite

High Energy Density Physics 8 (2012) 46-49

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

We present measurements of the changes in the microscopic structure of graphite in a laser-driven shock experiment with X-ray scattering. Laser radiation with intensities of ∼2 × 10 W/cm compressed the carbon samples by a factor of two reaching pressures of ∼90 GPa. Due to the change of the crystalline structure the scattered signals of the probe radiation were modified significantly in intensity and spectral composition compared to the scattering on cold samples. It is shown that the elastic scattering on tightly bound electrons increases strongly due to the phase transition whereas the inelastic scattering on weakly bound electrons remains nearly unchanged for the chosen geometry. © 2011 Elsevier B.V.


The Radius of Baryonic Collapse in Disc Galaxy Formation

ArXiv (2012)

SA Kassin, J Devriendt, SM Fall, RSD Jong, B Allgood, JR Primack

In the standard picture of disc galaxy formation, baryons and dark matter receive the same tidal torques, and therefore approximately the same initial specific angular momentum. However, observations indicate that disc galaxies typically have only about half as much specific angular momentum as their dark matter haloes. We argue this does not necessarily imply that baryons lose this much specific angular momentum as they form galaxies. It may instead indicate that galaxies are most directly related to the inner regions of their host haloes, as may be expected in a scenario where baryons in the inner parts of haloes collapse first. A limiting case is examined under the idealised assumption of perfect angular momentum conservation. Namely, we determine the density contrast Delta, with respect to the critical density of the Universe, by which dark matter haloes need to be defined in order to have the same average specific angular momentum as the galaxies they host. Under the assumption that galaxies are related to haloes via their characteristic rotation velocities, the necessary Delta is ~600. This Delta corresponds to an average halo radius and mass which are ~60% and ~75%, respectively, of the virial values (i.e., for Delta = 200). We refer to this radius as the radius of baryonic collapse R_BC, since if specific angular momentum is conserved perfectly, baryons would come from within it. It is not likely a simple step function due to the complex gastrophysics involved, therefore we regard it as an effective radius. In summary, the difference between the predicted initial and the observed final specific angular momentum of galaxies, which is conventionally attributed solely to angular momentum loss, can more naturally be explained by a preference for collapse of baryons within R_BC, with possibly some later angular momentum transfer.