30,609 research outputs found
Footballs, Conical Singularities and the Liouville Equation
We generalize the football shaped extra dimensions scenario to an arbitrary
number of branes. The problem is related to the solution of the Liouville
equation with singularities and explicit solutions are presented for the case
of three branes. The tensions of the branes do not need to be tuned with each
other but only satisfy mild global constraints.Comment: 15 pages, Refs. added, minor changes. Typo in eq. 4.3 corrected.
Version to be published in PR
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
Simulations of galaxy formation in a Λ cold dark matter universe : I : dynamical and photometric properties of a simulated disk galaxy.
We present a detailed analysis of the dynamical and photometric properties of a disk galaxy simulated in the cold dark matter (CDM) cosmogony. The galaxy is assembled through a number of high-redshift mergers followed by a period of quiescent accretion after z1 that lead to the formation of two distinct dynamical components: a spheroid of mostly old stars and a rotationally supported disk of younger stars. The surface brightness profile is very well approximated by the superposition of an R1/4 spheroid and an exponential disk. Each photometric component contributes a similar fraction of the total luminosity of the system, although less than a quarter of the stars form after the last merger episode at z1. In the optical bands the surface brightness profile is remarkably similar to that of Sab galaxy UGC 615, but the simulated galaxy rotates significantly faster and has a declining rotation curve dominated by the spheroid near the center. The decline in circular velocity is at odds with observation and results from the high concentration of the dark matter and baryonic components, as well as from the relatively high mass-to-light ratio of the stars in the simulation. The simulated galaxy lies 1 mag off the I-band Tully-Fisher relation of late-type spirals but seems to be in reasonable agreement with Tully-Fisher data on S0 galaxies. In agreement with previous simulation work, the angular momentum of the luminous component is an order of magnitude lower than that of late-type spirals of similar rotation speed. This again reflects the dominance of the slowly rotating, dense spheroidal component, to which most discrepancies with observation may be traced. On its own, the disk component has properties rather similar to those of late-type spirals: its luminosity, its exponential scale length, and its colors are all comparable to those of galaxy disks of similar rotation speed. This suggests that a different form of feedback than adopted here is required to inhibit the efficient collapse and cooling of gas at high redshift that leads to the formation of the spheroid. Reconciling, without fine-tuning, the properties of disk galaxies with the early collapse and high merging rates characteristic of hierarchical scenarios such as CDM remains a challenging, yet so far elusive, proposition
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
More examples of structure formation in the Lemaitre-Tolman model
In continuing our earlier research, we find the formulae needed to determine
the arbitrary functions in the Lemaitre-Tolman model when the evolution
proceeds from a given initial velocity distribution to a final state that is
determined either by a density distribution or by a velocity distribution. In
each case the initial and final distributions uniquely determine the L-T model
that evolves between them, and the sign of the energy-function is determined by
a simple inequality. We also show how the final density profile can be more
accurately fitted to observational data than was done in our previous paper. We
work out new numerical examples of the evolution: the creation of a galaxy
cluster out of different velocity distributions, reflecting the current data on
temperature anisotropies of CMB, the creation of the same out of different
density distributions, and the creation of a void. The void in its present
state is surrounded by a nonsingular wall of high density.Comment: LaTeX 2e with eps figures. 30 pages, 11 figures, 30 figure files.
Revision matches published versio
Codimension Two Branes and Distributional Curvature
In general relativity, there is a well-developed formalism for working with
the approximation that a gravitational source is concentrated on a shell, or
codimension one surface. By contrast, there are obstacles to concentrating
sources on surfaces that have a higher codimension, for example, a string in a
spacetime with dimension greater than or equal to four. Here it is shown that,
by giving up some of the generality of the codimension one case, curvature can
be concentrated on submanifolds that have codimension two. A class of metrics
is identified such that (1) the scalar curvature and Ricci densities exist as
distributions with support on a co-dimension two submanifold, and (2) using the
Einstein equation, the distributional curvature corresponds to a concentrated
stress-energy with equation of state p equals minus the energy density, where p
is the isotropic pressure tangent to the submanifold. This is the appropriate
stress-energy to describe a self-gravitating brane that is governed by an area
action, or a brane world deSitter cosmology. The possibility of having a
different equation of state arise from a wider class of metrics is discussed.Comment: 18 pages; v2 references added; typos corrected, references added;
additional references adde
The Effects of a Photoionizing UV Background on the Formation of Disk Galaxies
We use high resolution N-body/gasdynamical simulations to investigate the
effects of a photoionizing UV background on the assembly of disk galaxies in
hierarchically clustering universes. We focus on the mass and rotational
properties of gas that can cool to form centrifugally supported disks in dark
matter halos of different mass. Photoheating can significantly reduce the
amount of gas that can cool in galactic halos. Depending on the strength of the
UV background field, the amount of cooled gas can be reduced by up to in
systems with circular speeds in the range - \kms. The magnitude of the
effect, however, is not enough to solve the ``overcooling'' problem that
plagues hierarchical models of galaxy formation if the UV background is chosen
to be consistent with estimates based on recent observations of QSO absorption
systems. Photoionization has little effect on the collapse of gas at high
redshift and affects preferentially gas that is accreted at late times. Since
disks form inside-out, accreting higher angular momentum gas at later times,
disks formed in the presence of a UV background have spins that are even
smaller than those formed in simulations that do not include the effects of
photoionization. This exacerbates the angular momentum problem that afflicts
hierarchical models of disk formation. We conclude that photoionization cannot
provide the heating mechanism required to reconcile hierarchically clustering
models with observations. Energy feedback and enrichment processes from the
formation and evolution of stars must therefore be indispensable ingredients
for any successful model of the formation of disk galaxies.Comment: 36 pages, w/ embedded figures, submitted to ApJ. Also available at
http://penedes.as.arizona.edu/~jfn/preprints/dskform.ps.g
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