4,300 research outputs found
Non-circular Gas Kinematics and Star Formation in the Ringed Galaxy NGC 4736
We analyze the gas kinematics and star formation properties of the nearby
RSab galaxy NGC 4736 using interferometric and single-dish CO(1-0) data and
previously published Halpha and HI data. The CO morphology is dominated by a
central molecular bar and tightly wound spiral arms associated with a bright
ring of star formation. Strong HI emission is also found in the ring, but HI is
absent from the central regions. Comparison of the HI and Halpha distributions
suggests that HI in the ring is primarily dissociated H. Modeling of the CO
kinematics reveals gas motion in elliptical orbits around the central bar, and
we argue that the ring represents both the OLR of the bar and the ILR of a
larger oval distortion. The HI kinematics show evidence for axisymmetric inflow
towards the ring and are inconsistent with streaming in aligned elliptical
orbits, but the highly supersonic (~40 km/s) inflow velocities required,
corresponding to mass inflow rates of ~2 Msol/yr, suggest that more
sophisticated models (e.g., gas orbiting in precessed elliptical orbits) should
be considered. The radial CO and Halpha profiles are poorly correlated in the
vicinity of the nuclear bar, but show a better correlation (in rough agreement
with the Schmidt law) at the ring. Even along the ring, however, the azimuthal
correspondence between CO and Halpha is poor, suggesting that massive stars
form more efficiently at some (perhaps resonant) locations than at others.
These results indicate that the star formation rate per unit gas mass exhibits
strong spatial variations and is not solely a function of the available gas
supply. The localization of star formation to the ring is broadly consistent
with gravitational instability theory, although the instability parameter on average in the ring, only falling below 1 in localized regions.Comment: Revised version accepted by ApJ, with new section on p-v diagrams. 24
pages with 24 figures (emulateapj5). Full resolution and color versions are
available at http://astro.berkeley.edu/~twong/preprint
Magnetic-film atom chip with 10 m period lattices of microtraps for quantum information science with Rydberg atoms
We describe the fabrication and construction of a setup for creating lattices
of magnetic microtraps for ultracold atoms on an atom chip. The lattice is
defined by lithographic patterning of a permanent magnetic film. Patterned
magnetic-film atom chips enable a large variety of trapping geometries over a
wide range of length scales. We demonstrate an atom chip with a lattice
constant of 10 m, suitable for experiments in quantum information science
employing the interaction between atoms in highly-excited Rydberg energy
levels. The active trapping region contains lattice regions with square and
hexagonal symmetry, with the two regions joined at an interface. A structure of
macroscopic wires, cut out of a silver foil, was mounted under the atom chip in
order to load ultracold Rb atoms into the microtraps. We demonstrate
loading of atoms into the square and hexagonal lattice sections simultaneously
and show resolved imaging of individual lattice sites. Magnetic-film lattices
on atom chips provide a versatile platform for experiments with ultracold
atoms, in particular for quantum information science and quantum simulation.Comment: 7 pages, 7 figure
Density Functional for Anisotropic Fluids
We propose a density functional for anisotropic fluids of hard body
particles. It interpolates between the well-established geometrically based
Rosenfeld functional for hard spheres and the Onsager functional for elongated
rods. We test the new approach by calculating the location of the the
nematic-isotropic transition in systems of hard spherocylinders and hard
ellipsoids. The results are compared with existing simulation data. Our
functional predicts the location of the transition much more accurately than
the Onsager functional, and almost as good as the theory by Parsons and Lee. We
argue that it might be suited to study inhomogeneous systems.Comment: To appear in J. Physics: Condensed Matte
An Improved BKW Algorithm for LWE with Applications to Cryptography and Lattices
In this paper, we study the Learning With Errors problem and its binary
variant, where secrets and errors are binary or taken in a small interval. We
introduce a new variant of the Blum, Kalai and Wasserman algorithm, relying on
a quantization step that generalizes and fine-tunes modulus switching. In
general this new technique yields a significant gain in the constant in front
of the exponent in the overall complexity. We illustrate this by solving p
within half a day a LWE instance with dimension n = 128, modulus ,
Gaussian noise and binary secret, using
samples, while the previous best result based on BKW claims a time
complexity of with samples for the same parameters. We then
introduce variants of BDD, GapSVP and UniqueSVP, where the target point is
required to lie in the fundamental parallelepiped, and show how the previous
algorithm is able to solve these variants in subexponential time. Moreover, we
also show how the previous algorithm can be used to solve the BinaryLWE problem
with n samples in subexponential time . This
analysis does not require any heuristic assumption, contrary to other algebraic
approaches; instead, it uses a variant of an idea by Lyubashevsky to generate
many samples from a small number of samples. This makes it possible to
asymptotically and heuristically break the NTRU cryptosystem in subexponential
time (without contradicting its security assumption). We are also able to solve
subset sum problems in subexponential time for density , which is of
independent interest: for such density, the previous best algorithm requires
exponential time. As a direct application, we can solve in subexponential time
the parameters of a cryptosystem based on this problem proposed at TCC 2010.Comment: CRYPTO 201
Bar Diagnostics in Edge-On Spiral Galaxies. III. N-Body Simulations of Disks
Present in over 45% of local spirals, boxy and peanut-shaped bulges are
generally interpreted as edge-on bars and may represent a key phase in the
evolution of bulges. Aiming to test such claims, the kinematic properties of
self-consistent 3D N-body simulations of bar-unstable disks are studied. Using
Gauss-Hermite polynomials to describe the stellar kinematics, a number of
characteristic bar signatures are identified in edge-on disks: 1) a major-axis
light profile with a quasi-exponential central peak and a plateau at moderate
radii (Freeman Type II profile); 2) a ``double-hump'' rotation curve; 3) a
sometime flat central velocity dispersion peak with a plateau at moderate radii
and occasional local central minimum and secondary peak; 4) an h3-V correlation
over the projected bar length. All those kinematic features are spatially
correlated and can easily be understood from the orbital structure of barred
disks. They thus provide a reliable and easy-to-use tool to identify edge-on
bars. Interestingly, they are all produced without dissipation and are
increasingly realized to be common in spirals, lending support to bar-driven
evolution scenarios for bulge formation. So called ``figure-of-eight''
position-velocity diagrams are never observed, as expected for realistic
orbital configurations. Although not uniquely related to triaxiality,
line-of-sight velocity distributions with a high velocity tail (i.e. an h3-V
correlation) appear as particularly promising tracers of bars. The stellar
kinematic features identified grow in strength as the bar evolves and vary
little for small inclination variations. Many can be used to trace the bar
length. Comparisons with observations are encouraging and support the view that
boxy and peanut-shaped bulges are simply thick bars viewed edge-on.Comment: 32 pages, 4 figures, AASTeX preprint. Revised following referees'
comments. Now accepted for publication in The Astrophysical Journal. We
strongly suggest you download the version with full resolution figures at
http://www.astro.columbia.edu/~bureau/Publications/Nbody_ApJ04.ps.g
The merger of vertically offset quasi-geostrophic vortices
We examine the critical merging distance between two equal-volume, equal-potential-vorticity quasi-geostrophic vortices. We focus on how this distance depends on the vertical offset between the two vortices, each having a unit mean height-to-width aspect ratio. The vertical direction is special in the quasi-geostrophic model (used to capture the leading-order dynamical features of stably stratified and rapidly rotating geophysical flows) since vertical advection is absent. Nevertheless vortex merger may still occur by horizontal advection. In this paper, we first investigate the equilibrium states for the two vortices as a function of their vertical and horizontal separation. We examine their basic properties together with their linear stability. These findings are next compared to numerical simulations of the nonlinear evolution of two spheres of potential vorticity. Three different regimes of interaction are identified, depending on the vertical offset. For a small offset, the interaction differs little from the case when the two vortices are horizontally aligned. On the other hand, when the vertical offset is comparable to the mean vortex radius, strong interaction occurs for greater horizontal gaps than in the horizontally aligned case, and therefore at significantly greater full separation distances. This perhaps surprising result is consistent with the linear stability analysis and appears to be a consequence of the anisotropy of the quasi-geostrophic equations. Finally, for large vertical offsets, vortex merger results in the formation of a metastable tilted dumbbell vortex.Publisher PDFPeer reviewe
Harm avoidance is related to mismatch negativity (MMN) amplitude in healthy subjects
peer reviewedEvent-related potential (ERP) studies evidenced that some personality dimensions induced different controlled cognitive attitudes towards the processing of information. However, few data are available on the possible relationships between personality and automatic attention or early sensory processing. In the present study the relationships between the mismatch negativity (MMN) and personality described by the Cloninger model of personality were investigated. Subjects were 32 healthy volunteers. The MMN was recorded with frequent stimuli tones of 1470 Hz, 70 dB and 40 ms duration, and target (20%) tones of 1470 Hz, 70 dB, 80 ms duration. The subjects completed a French version of the 226-item self-questionnaire TCI within the day following psychophysiological recording. The results showed that the HA dimension was negatively correlated with the MMN amplitude. The association was more present among women than men. No significant relationship existed between the other dimensions of personality and either the MMN amplitude or latency. These findings suggest that the MMN is related to the behavioral inhibition system (BIS), a fact which is consistent with clinical studies conducted on schizophrenia and anxiety disorders. In conclusion, this study suggests that personality dimensions induce different automatic attitudes towards the processing of information. (C) 2002 Published by Elsevier Science Ltd
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