5731989
The shape of the gravitational potential in cold dark matter haloes
We use a set of cosmological N-body simulations to investigate the structural shape of galaxysized cold dark matter (CDM) haloes. Unlike most previous work on the subject ‐ which dealt with shapes as measured by the inertia tensor ‐ we focus here on the shape of the gravitational potential, a quantity more directly relevant to comparison with observational probes. A further advantage is that the potential is less sensitive to the effects of substructure and, as a consequence, the isopotential surfaces are typically smooth and well approximated by concentric ellipsoids. Our main result is that the asphericity of the potential increases rapidly towards the centre of the halo. The radial trend is more pronounced than expected from constant flattening in the mass distribution, and reflects a strong tendency for dark matter haloes to become increasingly aspherical inwards. Near the centre the halo potential is approximately prolate on average [(c/a)0 = 0.72 ± 0.04, (b/a)0 = 0.78 ± 0.08], but it becomes less axisymmetric and more spherical in the outer regions. The principal axes of the isopotential surfaces remain well aligned, and in most haloes the angular momentum tends to be parallel to the minor axis and perpendicular to the major axis. This suggests that galactic discs may form in a plane where the potential is elliptical and where its ellipticity varies rapidly with radius. This can result in significant deviations from circular motion in systems such as low surface brightness (LSB) galaxies, even for relatively minor deviations from circular symmetry. Simulated long-slit rotation curves often appear similar to those of LSBs cited as evidence for constant density ‘cores’. This suggests that taking into account the 3D shape of the dark mass distribution might help to reconcile such evidence with the cuspy mass profile of CDM haloes.
14663098
Disk Galaxy Rotation Curves in Triaxial CDM Halos
We use N-body hydrodynamical simulations to study the structure of disks in triaxial potentials resembling CDM halos. Our analysis focuses on the accuracy of the dark mass distribution inferred from rotation curves derived from simulated long-slit spectra. We consider a massless disk embedded in a halo with axis ratios of 0.5:0.6:1.0 and with its rotation axis aligned with the minor axis of the halo. Closed orbits for the gaseous particles deviate from coplanar circular symmetry, resulting in a variety of long-slit rotation curve shapes, depending on the orientation of the disk relative to the line of sight. Rotation curves may thus differ significantly from the spherically-averaged circular velocity profile of the dark matter halo. "Solid-body" rotation curves--typically interpreted as a signature of a constant density core in the dark matter distribution--are obtained about 25% of the time for random orientations although the dark matter follows the cuspy density profile proposed by Navarro, Frenk & White (NFW). We conclude that the discrepancies reported between the shape of the rotation curve of low surface brightness galaxies and the structure of CDM halos may be resolved once the complex effects of halo triaxiality on the dynamics of the gas component is properly taken into account.
14631759
Hiding cusps in cores: kinematics of disc galaxies in triaxial dark matter haloes
We study the kinematics of gaseous discs in triaxial dark matter haloes using the closed-loop orbit solutions in non-axisymmetric potentials. The orbits are in general non-circular and, for a given triaxiality, their ellipticity depends on the ratio of escape to circular velocities, V 2 esc /V 2 c . This ratio increases steeply towards the centre for cold dark matter (CDM) halo density profiles, implying that even minor deviations from spherical symmetry may induce large deviations from circular orbits in the velocity field of a gaseous disc, especially near the centre. This result suggests that caution should be exercised when interpreting constraints on the presence of density cusps in the dark halo derived from the innermost velocity profile. Simulated long-slit rotation curves vary greatly in shape, depending primarily on the viewing angle of the disc and on its orientation relative to the principal axes of the potential. 'Solid-body' rotation curves - typically interpreted as a signature of a constant density core in the dark matter distribution - are often obtained when the slit samples velocities near the major axis of the closed-loop orbits. Triaxial potentials imprint specific symmetries in 2D velocity fields, generally inducing 'twists' in the isovelocity contours and antisymmetric patterns in opposite quadrants. We suggest that triaxial haloes may be responsible for the variety of shapes of long-slit rotation curves of low surface brightness (LSB) galaxies, as well as for the complex central kinematics of LSBs, which are sometimes ascribed to the presence of 'radial motions' in the gas. We argue that LSB rotation curves might be reconciled with the structure of CDM haloes once the effects of halo triaxiality on the dynamics of gaseous discs are properly taken into account.
64351367
Consumer server: A UNIX based event distributor in new CDF data acquisition system
Consumer Server is a program to handle event data and consumer trigger requests I/Os among Level 3 farm and consumer processes in CDF new data acquisition system. This program uses standard UNIX libraries and commercial network technologies to obtain higher portability. The authors describe the concept and configuration of the Consumer Server and report its performance.
16747752
Comments on “Angular Momentum Transport in Quasi-Keplerian Accretion Disks”
Subramanian, Pujari and Becker (2004) claim that the correct expression for the angular momentum transport in an accretion disc, which is proportional to dΩ/dR, can be derived on the basis of the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disc. We study their argument and show that their derivation contains several fundamental errors: 1) the biased choice of the desired formula from an infinite number of formulae; 2) the biased choice of parcel trajectories; and 3) confusion regarding the reference frames. Following 1) we could derive, for example, a (invalid) formula in which the angular momentum transport is proportional to dv Φ /dR, and from 2) we could even prove that the angular momentum transport is either inward or null. We present the correct approach to the problem of angular momentum transport in an accretion disc in terms of mean free path theory.
17908059
0 Numerical simulation of accretion discs in close binary systems and discovery of spiral shocks
The history of hydrodynamic numerical simulations for accretion disks in close binary systems is reviewed, in which emphasis is placed, in particular, on the facts that spiral shock waves were numerically found in 1986 by researchers including one of the present authors and that spiral structure was discovered in IP Pegasi in 1997 by Steeghs et al. The results of our two and three-dimensional numerical simulations in recent years are then summarized, with comparison being made with observations.
59385487
Beam position monitor for the LNLS UVX synchrotron light source
The beam orbit measuring system for the LNLS UVX ring is presented. The system consists of 24 strip-line beam position monitors and associated electronics. The beam position monitors (BPM's) are capable of detecting down to 10 mA of stored beam current or 100 ns-long LINAC injection pulses (single pulse detection mode) with 0.05 mm resolution. Acquisition electronics is divided into two parts: four fast-acquisition channels for the pulsed mode operation (used during injection) and a single PIN-switch multiplexed channel for CW operation (closed orbit measurement). Experimental results obtained on a characterization bench are presented and compared with theoretical estimates.
55404585
STATUS OF THE 476 MHz 50 kW SOLID STATE AMPLIFIER FOR THE LNLS STORAGE RING
In November 2010, LNLS plans to replace the two 50 kW UHF klystron system which currently provides power to the RF cavities installed in the storage ring. To replace this system, two 50 kW solid state amplifiers have been built and are now under commissioning. Thanks to a close collaboration with the Synchrotron SOLEIL started in 1999, LNLS adapted the characteristics of the French project to 476 MHz. A short description of the amplifiers is presented, as well as the power tests performed with one of them using a resistive load.