Publications


Velocity and abundance precisions for future high-resolution spectroscopic surveys: A study for 4MOST

Astronomische Nachrichten 334 (2012) 197-216

E Caffau, A Koch, L Sbordone, P Sartoretti, CJ Hansen, F Royer, N Leclerc, P Bonifacio, N Christlieb, H-G Ludwig, EK Grebel, RS de Jong, C Chiappini, J Walcher, S Mignot, S Feltzing, M Cohen, I Minchev, A Helmi, T Piffl, E Depagne, O Schnurr

In preparation for future, large-scale, multi-object, high-resolution spectroscopic surveys of the Galaxy, we present a series of tests of the precision in radial velocity and chemical abundances that any such project can achieve at a 4 m class telescope. We briefly discuss a number of science cases that aim at studying the chemo-dynamical history of the major Galactic components (bulge, thin and thick disks, and halo) - either as a follow-up to the Gaia mission or on their own merits. Based on a large grid of synthetic spectra that cover the full range in stellar parameters of typical survey targets, we devise an optimal wavelength range and argue for a moderately high-resolution spectrograph. As a result, the kinematic precision is not limited by any of these factors, but will practically only suffer from systematic effects, easily reaching uncertainties <1 km s-1. Under realistic survey conditions (namely, considering stars brighter than r=16 mag with reasonable exposure times) we prefer an ideal resolving power of R˜20 000 on average, for an overall wavelength range (with a common two-arm spectrograph design) of [395;456.5] nm and [587;673] nm. We show for the first time on a general basis that it is possible to measure chemical abundance ratios to better than 0.1 dex for many species (Fe, Mg, Si, Ca, Ti, Na, Al, V, Cr, Mn, Co, Ni, Y, Ba, Nd, Eu) and to an accuracy of about 0.2 dex for other species such as Zr, La, and Sr. While our feasibility study was explicitly carried out for the 4MOST facility, the results can be readily applied to and used for any other conceptual design study for high-resolution spectrographs.


Global gyrokinetic turbulence simulations of MAST plasmas

Plasma Physics and Controlled Fusion 54 (2012)

S Saarelma, G Colyer, AR Field, CM Roach, A Bottino, P Hill, B McMillan, A Peeters

Electrostatic gyrokinetic analyses are presented for an L-mode discharge with an internal transport barrier, from the spherical tokamak, MAST. Local and global microstability analysis finds similar linear growth rates for ion temperature gradient (ITG) driven modes. When the electron response is assumed to be adiabatic, growth rates are found to be lower than the experimental E×B flow shearing rate. Including kinetic electrons, without collisions, increases the ITG growth rates above the flow shearing rate, and these modes are found to be linearly unstable in the outer part of the plasma only. In global simulations the flow shear stabilization is found to be asymmetric with respect to the direction of the flow: there is a small destabilizing effect at low flow shear when the flow is in the co-direction. Global non-linear simulations with kinetic electrons and including the flow shear effects predict turbulent ion heat transport that is well above the neoclassical level in the region outside the internal transport barrier in this MAST plasma. In non-linear simulations we also find turbulence extending from the outer part of the plasma into the linearly stable core region. © 2012 IOP Publishing Ltd.


Scaling of spontaneous rotation with temperature and plasma current in tokamaks

ArXiv (2011)

FI Parra, MFF Nave, AA Schekochihin, C Giroud, JSD Grassie, JHF Severo, PD Vries, KD Zastrow, JETEFDA Contributors

Using theoretical arguments, a simple scaling law for the size of the intrinsic rotation observed in tokamaks in the absence of momentum injection is found: the velocity generated in the core of a tokamak must be proportional to the ion temperature difference in the core divided by the plasma current, independent of the size of the device. The constant of proportionality is of the order of $10\,\mathrm{km \cdot s^{-1} \cdot MA \cdot keV^{-1}}$. When the intrinsic rotation profile is hollow, i.e. it is counter-current in the core of the tokamak and co-current in the edge, the scaling law presented in this Letter fits the data remarkably well for several tokamaks of vastly different size and heated by different mechanisms.


Angular momentum transfer to a Milky Way disk at high redshift

ArXiv (2012)

H Tillson, J Devriendt, A Slyz, L Miller, C Pichon

An Adaptive Mesh Refinement cosmological resimulation is analyzed in order to test whether filamentary flows of cold gas are responsible for the build-up of angular momentum within a Milky Way like disk at z>=3. A set of algorithms is presented that takes advantage of the high spatial resolution of the simulation (12 pc) to identify: (i) the central gas disk and its plane of orientation; (ii) the complex individual filament trajectories that connect to the disk, and; (iii) the infalling satellites. The results show that two filaments at z>5.5, which later merge to form a single filament at z<4, drive the angular momentum and mass budget of the disk throughout its evolution, whereas luminous satellite mergers make negligible fractional contributions. Combined with the ubiquitous presence of such filaments in all large-scale cosmological simulations that include hydrodynamics, these findings provide strong quantitative evidence that the growth of thin disks in haloes with masses below 10^{12} M_{sun}, which host the vast majority of galaxies, is supported via inflowing streams of cold gas at intermediate and high redshifts.


The SAURON project - XXI. The spatially resolved UV-line strength relations of early-type galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 423 (2012) 1921-1939

H Jeong, SK Yi, M Bureau, RL Davies, R Bacon, M Cappellari, PT de Zeeuw, E Emsellem, J Falcon-Barroso, D Krajnovic, H Kuntschner, RM McDermid, RF Peletier, M Sarzi, RCE van den Bosch, G van de Ven


Subcritical fluctuations and suppression of turbulence in differentially rotating gyrokinetic plasmas

PLASMA PHYSICS AND CONTROLLED FUSION 54 (2012) ARTN 055011

AA Schekochihin, EG Highcock, SC Cowley


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.


Laser absorption in relativistically underdense plasmas by synchrotron radiation

Physical Review Letters 109 (2012) 245006

CS Brady, CP Ridgers, TD Arber, AR Bell, JG Kirk


Laboratory investigations on the origins of cosmic rays

Plasma Physics and Controlled Fusion 54 (2012)

Y Kuramitsu, Y Sakawa, T Morita, K Mima, H Azechi, T Moritaka, T Sano, H Takabe, T Ide, H Tanji, S Dono, N Nakanii, R Kodama, KA Tanaka, K Nishio, H Aoki, 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, K Kondo, Y Mori, Y Kitagawa, E Miura, Y Matsumoto, A Mizuta, N Ohnishi, M Hoshino

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.


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

Nature 481 (2012) 480-483

G Gregori, A Ravasio, CD Murphy, K Schaar, A Baird, AR Bell, A Benuzzi-Mounaix, R Bingham, C Constantin, RP Drake, M Edwards, ET Everson, CD Gregory, Y Kuramitsu, W Lau, J Mithen, C Niemann, HS Park, BA Remington, B Reville, AP Robinson, DD Ryutov, Y Sakawa, S Yang, NC Woolsey, M Koenig, 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(-21) 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.


Inverse Compton X-ray halos around high-z radio galaxies: A feedback mechanism powered by far-infrared starbursts or the CMB?

ArXiv (2012)

I Smail, KM Blundell, BD Lehmer, DM Alexander

We report the detection of extended X-ray emission around two powerful high-z radio galaxies (HzRGs) at z~3.6 (4C03.24 & 4C19.71) and use these to investigate the origin of extended, Inverse Compton (IC) powered X-ray halos at high z. The halos have X-ray luminosities of Lx~3e44 erg/s and sizes of ~60kpc. Their morphologies are broadly similar to the ~60-kpc long radio lobes around these galaxies suggesting they are formed from IC scattering by relativistic electrons in the radio lobes, of either CMB or FIR photons from the dust-obscured starbursts in these galaxies. These observations double the number of z>3 HzRGs with X-ray detected IC halos. We compare the IC X-ray to radio luminosity ratios for these new detections to the two previously detected z~3.8 HzRGs. Given the similar redshifts, we would expect comparable X-ray IC luminosities if CMB mm photons are the seed field for the IC emission. Instead the two z~3.6 HzRGs, which are ~4x fainter in the FIR, also have ~4x fainter X-ray IC emission. Including a further six z>2 radio sources with IC X-ray halos from the literature, we suggest that in the more compact (lobe sizes <100-200kpc), majority of radio sources, the bulk of the IC emission may be driven by scattering of locally produced FIR photons from luminous, dust-obscured starbursts within these galaxies, rather than CMB photons. The resulting X-ray emission can ionise the gas on ~100-200-kpc scales around these systems and thus form their extended Ly-alpha emission line halos. The starburst and AGN activity in these galaxies are thus combining to produce an effective and wide-spread "feedback" process, acting on the long-term gas reservoir for the galaxy. If episodic radio activity and co-eval starbursts are common in massive, high-z galaxies, then this IC-feedback mechanism may affect the star-formation histories of massive galaxies. [Abridged]


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.


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

Monthly Notices of the Royal Astronomical Society 421 (2012) 1298-1314

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

Surveying 18 CO-bright galaxies from the ATLAS early-type galaxy sample with the Institut de Radio Astronomie Millimétrique (IRAM) 30-m telescope, we detect CO(1-0) and CO(2-1) in all 18 galaxies, HCN(1-0) in 12/18 and HCO (1-0) in 10/18. We find that the line ratios CO(1-0)/ CO(1-0) and CO(1-0)/HCN(1-0) are clearly correlated with several galaxy properties: total stellar mass, luminosity-weighted mean stellar age, molecular-to-atomic gas ratio, dust temperature and dust morphology. We suggest that these correlations are primarily governed by the optical depth in the CO lines; interacting, accreting and/or starbursting early-type galaxies have more optically thin molecular gas while those with settled dust and gas discs host optically thick molecular gas. The ranges of the integrated line intensity ratios generally overlap with those of spirals, although we note some outliers in the CO(1-0)/ CO(1-0), CO(2-1)/ CO(2-1) and HCN/HCO (1-0) ratios. In particular, three galaxies are found to have very low CO(1-0)/ CO(1-0) and CO(2-1)/ CO(2-1) ratios. Such low ratios may signal particularly stable molecular gas which creates stars less efficiently than 'normal' (i.e. below Schmidt-Kennicutt prediction), consistent with the low dust temperatures seen in these galaxies. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


THE EPOCH OF DISK SETTLING: z similar to 1 TO NOW

ASTROPHYSICAL JOURNAL 758 (2012) ARTN 106

SA Kassin, BJ Weiner, SM Faber, JP Gardner, CNA Willmer, AL Coil, MC Cooper, J Devriendt, AA Dutton, P Guhathakurta, DC Koo, AJ Metevier, KG Noeske, JR Primack


Determining the velocity dispersion of the thick disc

ArXiv (2012)

J Sanders

We attempt to recover the mean vertical velocity and vertical velocity dispersion as a function of the Galactic height for a sample drawn from a realistic Galaxy distribution function by following the method presented in Moni Bidin et al. (2012). We find that, for the sample size used, the observational error in the velocities is much smaller than the Poisson noise which has not been accounted for by Moni Bidin et al. We repeat the analysis on a large number of samples to estimate the contribution of the Poisson noise and to uncover any systematics. We find that the dispersion is systematically overestimated at low Galactic heights and slightly underestimated at high Galactic heights leading to an underestimate of the gradient of the dispersion with Galactic height. The causes of the systematics are revealed by repeating the calculation using a method inspired by Girard et al. (2006). This method recovers the expected dispersion much more successfully and in particular yields a gradient of the dispersion with Galactic height which is approximately three times that found using the method presented by Moni Bidin et al.


An oxford swift integral field spectroscopy study of 14 early-type galaxies in the coma cluster

Monthly Notices of the Royal Astronomical Society 425 (2012) 1521-1526

N Scott, R Houghton, RL Davies, M Cappellari, N Thatte, F Clarke, M Tecza

As a demonstration of the capabilities of the new Oxford SWIFT integral field spectrograph, we present first observations for a set of 14 early-type galaxies in the core of the Coma cluster. Our data consist of I- and z-band spatially resolved spectroscopy obtained with the Oxford SWIFT spectrograph, combined with r-band photometry from the Sloan Digital Sky Survey archive for 14 early-type galaxies. We derive spatially resolved kinematics for all objects from observations of the calcium triplet absorption features at ∼8500Å. Using this kinematic information we classify galaxies as either fast rotators or slow rotators. We compare the fraction of fast and slow rotators in our sample, representing the densest environment in the nearby Universe, to results from the ATLAS survey, finding that the slow rotator fraction is ∼50per cent larger in the core of the Coma cluster than in the volume-limited ATLAS sample, a 1.2σ increase given our selection criteria. Comparing our sample to the Virgo cluster core only (which is 24 times less dense than the Coma core) we find no evidence of an increase in the slow rotator fraction. Combining measurements of the effective velocity dispersion σ with the photometric data we determine the Fundamental Plane for our sample of galaxies. We find that the use of the average velocity dispersion within 1 effective radius, σ , reduces the residuals by 13per cent with respect to comparable studies using central velocity dispersions, consistent with other recent integral field Fundamental Plane determinations. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Enhancing and inhibiting star formation: High-resolution simulation studies of the impact of cold accretion, mergers and feedback on individual massive galaxies

Proceedings of the International Astronomical Union 8 (2012) 13-16

LC Powell, S Khochfar, F Bournaud, D Chapon, R Teyssier, J Devriendt, A Slyz, V Gaibler

The quest for a better understanding of the evolution of massive galaxies can be broadly summarised with 2 questions: how did they build up their large (stellar) masses and what eventually quenched their star formation (SF)? To tackle these questions, we use high-resolution ramses simulations (Teyssier 2002) to study several aspects of the detailed interplay between accretion (mergers and cold flows), SF and feedback in individual galaxies. We examine SF in major mergers; a process crucial to stellar mass assembly. We explore whether the merger-induced, clustered SF is as important a mechanism in average mergers, as it is in extreme systems like the Antennae. We find that interaction-induced turbulence drives up the velocity dispersion, and that there is a correlated rise in SFR in all our simulated mergers as the density pdf evolves to have an excess of very dense gas. Next, we introduce a new study into whether mechanical jet feedback can impact upon the ability of hot gas haloes to provide a supply of fuel for SF during mergers and in their remnants. Finally, we briefly review our recent study, in which we examine the effect of supernova (SN) feedback on galaxies accreting via the previously overlooked cold-mode, by resimulating a stream-fed galaxy at z ~ 9. A far-reaching galactic wind results yet it cannot suppress the cold, filamentary accretion or eject significant mass in order to reduce the SFR, suggesting that SN feedback may not be as effective as is often assumed. © 2013 International Astronomical Union.


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.