3,017 research outputs found
Stellar Over-densities in the Outer Halo of the Milky Way
This study presents a tomographic survey of a subset of the outer halo (10-40
kpc) drawn from the Sloan Digital Sky Survey Data Release 6. Halo substructure
on spatial scales of degrees is revealed as an excess in the local density
of sub-giant stars. With an appropriate assumption of a model stellar isochrone
it is possible for us to then derive distances to the sub-giant population. We
describe three new candidate halo substructures; the 160- and 180-degree
over-densities (at distances of 17 and 19 kpc respectively and radii of 1.3 and
1.5 kpc respectively) and an extended feature at 28 kpc that covers at least
162 square degrees, the Virgo Equatorial Stream. In addition, we recover the
Sagittarius dwarf galaxy (Sgr) leading arm material and the Virgo Over-density.
The derived distances, together with the number of sub-giant stars associated
with each substructure, enables us to derive the integrated luminosity for the
features. The tenuous, low surface brightness of the features strongly suggests
an origin from the tidal disruption of an accreted galaxy or galaxies. Given
the dominance of the tidal debris of Sgr in this region of the sky we
investigate if our observations can be accommodated by tidal disruption models
for Sgr. The clear discordance between observations and model predictions for
known Sgr features means it is difficult to tell unambiguously if the new
substructures are related to Sgr or not. Radial velocities in the stellar
over-densities will be critical in establishing their origins.Comment: 14 pages, 7 figures, PASA accepte
Dynamic effects induced by renormalization in anisotropic pattern forming systems
The dynamics of patterns in large two-dimensional domains remains a challenge
in non-equilibrium phenomena. Often it is addressed through mild extensions of
one-dimensional equations. We show that full 2D generalizations of the latter
can lead to unexpected dynamical behavior. As an example we consider the
anisotropic Kuramoto-Sivashinsky equation, that is a generic model of
anisotropic pattern forming systems and has been derived in different instances
of thin film dynamics. A rotation of a ripple pattern by occurs in
the system evolution when nonlinearities are strongly suppressed along one
direction. This effect originates in non-linear parameter renormalization at
different rates in the two system dimensions, showing a dynamical interplay
between scale invariance and wavelength selection. Potential experimental
realizations of this phenomenon are identified.Comment: 5 pages, 3 figures; supplemental material available at journal web
page and/or on reques
Second-Order Self-Consistent-Field Density-Matrix Renormalization Group
We present a matrix-product state (MPS)-based quadratically convergent
density-matrix renormalization group self-consistent-field (DMRG-SCF) approach.
Following a proposal by Werner and Knowles (JCP 82, 5053, (1985)), our DMRG-SCF
algorithm is based on a direct minimization of an energy expression which is
correct to second-order with respect to changes in the molecular orbital basis.
We exploit a simultaneous optimization of the MPS wave function and molecular
orbitals in order to achieve quadratic convergence. In contrast to previously
reported (augmented Hessian) Newton-Raphson and super-configuration-interaction
algorithms for DMRG-SCF, energy convergence beyond a quadratic scaling is
possible in our ansatz. Discarding the set of redundant active-active orbital
rotations, the DMRG-SCF energy converges typically within two to four cycles of
the self-consistent procedureComment: 40 pages, 5 figures, 3 table
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