27,593 research outputs found
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
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
Remnants of Sagittarius Dwarf Spheroidal Galaxy around the young globular cluster Palomar 12
Photometry of a large field around the young globular cluster Palomar 12 has
revealed the main-sequence of a low surface-brightness stellar system. This
main-sequence is indicative of a stellar population that varies significantly
in metallicity and/or age, but in the mean is more metal poor than Pal 12.
Under different assumptions for the properties of this population, we find
distances from the Sun in the range 17-24 kpc, which encompasses the distance
to Pal 12, kpc. The stellar system is also detected in a field
2\arcdeg North of Pal 12, which indicates it has a minimum diameter of
kpc. The orbit of Pal 12 (Dinescu et al. 2000), the color-magnitude
diagram of the stellar system, their positions on the sky, and their distances
suggest that they are debris from the tidal disruption of the Sgr dSph galaxy.
We discuss briefly the implications for the evolution of Sgr and the Galactic
halo.Comment: 16 pages, 2 figures, accepted for ApJ Letters. Some importante
changes after revision, including a new figur
Ca impurity in small mixed He-He clusters
The structure of small mixed helium clusters doped with one calcium atom has
been determined within the diffusion Monte Carlo framework. The results show
that the calcium atom sits at the He-He interface. This is in agreement
with previous studies, both experimental and theoretical, performed for large
clusters. A comparison between the results obtained for the largest cluster we
have considered for each isotope shows a clear tendency of the Ca atom to
reside in a deep dimple at the surface of the cluster for He clusters, and
to become fully solvated for He clusters. We have calculated the absorption
spectrum of Ca around the transition and have found that
it is blue-shifted from that of the free-atom transition by an amount that
depends on the size and composition of the cluster.Comment: 24 pages, 11 figures. Accepted on Journal of Chemical Physic
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
Universality in the partially anisotropic three-dimensional Ising lattice
Using transfer-matrix extended phenomenological renormalization-group methods
the critical properties of spin-1/2 Ising model on a simple-cubic lattice with
partly anisotropic coupling strengths are studied.
Universality of both fundamental critical exponents and is
confirmed. It is shown that the critical finite-size scaling amplitude ratios
, , and
are independent of the lattice anisotropy
parameter . By this for the last above invariant of the
three-dimensional Ising universality class we give the first quantitative
estimate: (shape , periodic boundary
conditions in both transverse directions).Comment: 11 pages in latex; no figure
Scaling Relations of Dwarf Galaxies without Supernova-Driven Winds
Nearby dwarf galaxies exhibit tight correlations between their global stellar
and dynamical properties, such as circular velocity, mass-to-light ratio,
stellar mass, surface brightness, and metallicity. Such correlations have often
been attributed to gas or metal-rich outflows driven by supernova energy
feedback to the interstellar medium. We use high-resolution cosmological
simulations of high-redshift galaxies with and without energy feedback, as well
as analytic modeling, to investigate whether the observed correlations can
arise without supernova-driven outflows. We find that the simulated dwarf
galaxies exhibit correlations similar to those observed as early as z~10,
regardless of whether supernova feedback is included. We also show that the
correlations can be well reproduced by our analytic model that accounts for
realistic gas inflow but assumes no outflows, and star formation rate obeying
the Kennicutt-Schmidt law with a critical density threshold. We argue that
correlations in simulated galaxies arise due to the increasingly inefficient
conversion of gas into stars in low-mass dwarf galaxies rather than
supernova-driven outflows. We also show that the decrease of the observed
effective yield in low-mass objects, often used as an indicator of gas and
metal outflows, can be reasonably reproduced in our simulations without
outflows. We show that this trend can arise if a significant fraction of metals
in small galaxies is spread to the outer regions of the halo outside the
stellar extent via mixing. In this case the effective yield can be
significantly underestimated if only metals within the stellar radius are taken
into account. Measurements of gas metallicity in the outskirts of gaseous disks
of dwarfs would thus provide a key test of such explanation.Comment: accepted for publication in ApJ, 19 pages, 12 figures, uses
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