29,280 research outputs found
Convergence of Scalar-Tensor theories toward General Relativity and Primordial Nucleosynthesis
In this paper, we analyze the conditions for convergence toward General
Relativity of scalar-tensor gravity theories defined by an arbitrary coupling
function (in the Einstein frame). We show that, in general, the
evolution of the scalar field is governed by two opposite mechanisms:
an attraction mechanism which tends to drive scalar-tensor models toward
Einstein's theory, and a repulsion mechanism which has the contrary effect. The
attraction mechanism dominates the recent epochs of the universe evolution if,
and only if, the scalar field and its derivative satisfy certain boundary
conditions. Since these conditions for convergence toward general relativity
depend on the particular scalar-tensor theory used to describe the universe
evolution, the nucleosynthesis bounds on the present value of the coupling
function, , strongly differ from some theories to others. For
example, in theories defined by analytical
estimates lead to very stringent nucleosynthesis bounds on
(). By contrast, in scalar-tensor theories defined by
much larger limits on () are
found.Comment: 20 Pages, 3 Figures, accepted for publication in Class. and Quantum
Gravit
Cationic ordering control of magnetization in Sr2FeMoO6 double perovskite
The role of the synthesis conditions on the cationic Fe/Mo ordering in
Sr2FeMoO6 double perovskite is addressed. It is shown that this ordering can be
controlled and varied systematically. The Fe/Mo ordering has a profound impact
on the saturation magnetization of the material. Using the appropriate
synthesis protocol a record value of 3.7muB/f.u. has been obtained. Mossbauer
analysis reveals the existence of two distinguishable Fe sites in agreement
with the P4/mmm symmetry and a charge density at the Fe(m+) ions significantly
larger than (+3) suggesting a Fe contribution to the spin-down conduction band.
The implications of these findings for the synthesis of Sr2FeMoO6 having
optimal magnetoresistance response are discussed.Comment: 9 pages, 4 figure
Disk Galaxy Formation in a LambdaCDM Universe
We describe hydrodynamical simulations of galaxy formation in a Lambda cold
dark matter (CDM) cosmology performed using a subresolution model for star
formation and feedback in a multiphase interstellar medium (ISM). In
particular, we demonstrate the formation of a well-resolved disk galaxy. The
surface brightness profile of the galaxy is exponential, with a B-band central
surface brightness of 21.0 mag arcsec^-2 and a scale-length of R_d = 2.0 h^-1
kpc. We find no evidence for a significant bulge component. The simulated
galaxy falls within the I-band Tully-Fisher relation, with an absolute
magnitude of I = -21.2 and a peak stellar rotation velocity of V_rot=121.3 km
s^-1. While the total specific angular momentum of the stars in the galaxy
agrees with observations, the angular momentum in the inner regions appears to
be low by a factor of ~2. The star formation rate of the galaxy peaks at ~7
M_sun yr^-1 between redshifts z=2-4, with the mean stellar age decreasing from
\~10 Gyrs in the outer regions of the disk to ~7.5 Gyrs in the center,
indicating that the disk did not simply form inside-out. The stars exhibit a
metallicity gradient from 0.7 Z_sun at the edge of the disk to 1.3 Z_sun in the
center. Using a suite of idealized galaxy formation simulations with different
models for the ISM, we show that the effective pressure support provided by
star formation and feedback in our multiphase model is instrumental in allowing
the formation of large, stable disk galaxies. If ISM gas is instead modeled
with an isothermal equation of state, or if star formation is suppressed
entirely, growing gaseous disks quickly violate the Toomre stability criterion
and undergo catastrophic fragmentation.Comment: 14 pages, 12 figures, LaTex (emulateapj.cls), submitted to ApJ, high
resolution images available at
http://www-cfa.harvard.edu/~brobertson/papers/galaxy
Dark-Halo Cusp: Asymptotic Convergence
We propose a model for how the buildup of dark halos by merging satellites
produces a characteristic inner cusp, of a density profile \rho \prop r^-a with
a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical
clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of
a<1 exerts tidal compression which prevents local deposit of satellite
material; the satellite sinks intact into the halo center thus causing a rapid
steepening to a>1. Using merger N-body simulations, we learn that this cusp is
stable under a sequence of mergers, and derive a practical tidal mass-transfer
recipe in regions where the local slope of the halo profile is a>1. According
to this recipe, the ratio of mean densities of halo and initial satellite
within the tidal radius equals a given function psi(a), which is significantly
smaller than unity (compared to being 1 according to crude resonance criteria)
and is a decreasing function of a. This decrease makes the tidal mass transfer
relatively more efficient at larger a, which means steepening when a is small
and flattening when a is large, thus causing converges to a stable solution.
Given this mass-transfer recipe, linear perturbation analysis, supported by toy
simulations, shows that a sequence of cosmological mergers with homologous
satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1.
The slope depends only weakly on the fluctuation power spectrum, in agreement
with cosmological simulations. During a long interim period the profile has an
NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough
compact satellite remnants make it intact into the inner halo. In order to
maintain a flat core, satellites must be disrupted outside the core, possibly
as a result of a modest puffing up due to baryonic feedback.Comment: 37 pages, Latex, aastex.cls, revised, ApJ, 588, in pres
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
Improved Approximate String Matching and Regular Expression Matching on Ziv-Lempel Compressed Texts
We study the approximate string matching and regular expression matching
problem for the case when the text to be searched is compressed with the
Ziv-Lempel adaptive dictionary compression schemes. We present a time-space
trade-off that leads to algorithms improving the previously known complexities
for both problems. In particular, we significantly improve the space bounds,
which in practical applications are likely to be a bottleneck
On The Reduced Canonical Quantization Of The Induced 2D-Gravity
The quantization of the induced 2d-gravity on a compact spatial section is
carried out in three different ways. In the three approaches the supermomentum
constraint is solved at the classical level but they differ in the way the
hamiltonian constraint is imposed. We compare these approaches establishing an
isomorphism between the resulting Hilbert spaces.Comment: 17 pages, plain LaTeX. FTUV/93-15, IFIC/93-10, Imperial-TP/93-94/1
Tidal Torques and the Orientation of Nearby Disk Galaxies
We use numerical simulations to investigate the orientation of the angular
momentum axis of disk galaxies relative to their surrounding large scale
structure. We find that this is closely related to the spatial configuration at
turnaround of the material destined to form the galaxy, which is often part of
a coherent two-dimensional slab criss-crossed by filaments. The rotation axis
is found to align very well with the intermediate principal axis of the inertia
momentum tensor at this time. This orientation is approximately preserved
during the ensuing collapse, so that the rotation axis of the resulting disk
ends up lying on the plane traced by the protogalactic material at turnaround.
This suggests a tendency for disks to align themselves so that their rotation
axis is perpendicular to the minor axis of the structure defined by surrounding
matter. One example of this trend is provided by our own Galaxy, where the
Galactic plane is almost at right angles with the supergalactic plane (SGP)
drawn by nearby galaxies; indeed, the SGP latitude of the North Galactic Pole
is just 6 degrees. We have searched for a similar signature in catalogs of
nearby disk galaxies, and find a significant excess of edge-on spirals (for
which the orientation of the disk rotation axis may be determined
unambiguously) highly inclined relative to the SGP. This result supports the
view that disk galaxies acquire their angular momentum as a consequence of
early tidal torques acting during the expansion phase of the protogalactic
material.Comment: 5 pages, 2 figures, accepted for publication in ApJ
Star Formation and Feedback in Dwarf Galaxies
We examine the star formation history and stellar feedback effects of dwarf
galaxies under the influence of extragalactic ultraviolet radiation. We
consider the dynamical evolution of gas in dwarf galaxies using a
one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute
the effects of radiative transfer and photoionization. We include a
physically-motivated star formation recipe and consider the effects of
feedback. Our results indicate that star formation in the severe environment of
dwarf galaxies is a difficult and inefficient process. For intermediate mass
systems, such as the dSphs around the Galaxy, star formation can proceed with
in early cosmic epochs despite the intense background UV flux. Triggering
processes such as merger events, collisions, and tidal disturbance can lead to
density enhancements, reducing the recombination timescale, allowing gas to
cool and star formation to proceed. However, the star formation and gas
retention efficiency may vary widely in galaxies with similar dark matter
potentials, because they depend on many factors, such as the baryonic fraction,
external perturbation, IMF, and background UV intensity. We suggest that the
presence of very old stars in these dwarf galaxies indicates that their initial
baryonic to dark matter content was comparable to the cosmic value. This
constraint suggests that the initial density fluctuation of baryonic matter may
be correlated with that of the dark matter. For the more massive dwarf
elliptical galaxies, the star formation efficiency and gas retention rate is
much higher. Their mass to light ratio is regulated by star formation feedback,
and is expected to be nearly independent of their absolute luminosity. The
results of our theoretical models reproduce the observed correlation.Comment: 35 pages, 13 figure
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