8,280 research outputs found

### Feedback and its Feedback Effect on Feedback: Photoionization Suppression and its Impact on Galactic Outflows

We show that radiative feedback due to reionization has a pronounced effect
on the extent of mechanical feedback due to galactic outflows. The
photoionization of the Intergalactic Medium (IGM) suppresses low-mass galaxy
formation by photoheating the gas and limiting atomic line cooling. The number
of low-mass galaxies is central for the enrichment of the IGM as these objects
have the capacity to enrich a significant fraction (by volume) of the Universe.
We use a modified version of our galactic outflow model, combined with a simple
criterion for suppression, to investigate the potential impact upon the IGM. We
find that this suppression strongly reduces the enrichment of the IGM and is
sensitive to the reionization history. We also investigate the contribution of
halos of different masses with varying degrees of suppression.Comment: 4 pages, 3 figures, class file included, accepted by ApJ Letters,
minor changes and expanded Figure

### Anisotropic Outflows and IGM Enrichment

We have designed an analytical model for the evolution of anisotropic
galactic outflows. These outflows follow the path of least resistance, and thus
travel preferentially into low-density regions, away from cosmological
structures where galaxies form. We show that anisotropic outflows can
significantly enrich low-density systems with metals.Comment: Proceedings of Chemodynamics 2006, Lyon, 2 pages, 1 figure, style
file include

### Relative ages of lava flows at Alba Patera, Mars

Many large lava flows on the flanks of Alba Patera are astonishing in their volume and length. As a suite, these flows suggest tremendously voluminous and sustained eruptions, and provide dimensional boundary conditions typically a factor of 100 larger than terrestrial flows. One of the most striking features associated with Alba Patera is the large, radially oriented lava flows that exhibit a variety of flow morphologies. These include sheet flows, tube fed and tube channel flows, and undifferentiated flows. Three groups of flows were studied; flows on the northwest flank, southeast flank, and the intracaldera region. The lava flows discussed probably were erupted as a group during the same major volcanic episode as suggested by the data presented. Absolute ages are poorly constrained for both the individual flows and shield, due in part to disagreement as to which absolute age curve is representative for Mars. A relative age sequence is implied but lacks precision due to the closeness of the size frequency curves

### Resummation Effects in the Search of SM Higgs Boson at Hadron Colliders

We examine the soft-gluon resummation effects, including the exact spin
correlations among the final state particles, in the search of the Standard
Model Higgs boson, via the process $gg\to H\to WW/ZZ \to 4 leptons, at the
Tevatron and the LHC. A comparison between the resummation and the
Next-to-Leading order (NLO) calculation is performed after imposing various
kinematics cuts suggested in the literature for the Higgs boson search. For the$H\to ZZ$mode, the resummation effects increase the acceptance of the signal
events by about 25%, as compared to the NLO prediction, and dramatically alter
various kinematics distributions of the final state leptons. For the$H\to WW$
mode, the acceptance rates of the signal events predicted by the resummation
and NLO calculations are almost the same, but some of the predicted kinematical
distributions are quite different. Thus, to precisely determine the properties
of the Higgs boson at hadron colliders, the soft-gluon resummation effects have
to be taken into account.Comment: The version to appear in PR

### Anisotropy probe of galactic and extra-galactic Dark Matter annihilations

We study the flux and the angular power spectrum of gamma-rays produced by
Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic
halos. The annihilation signal receives contributions from: a) the smooth MW
halo, b) resolved and unresolved substructures in the MW, c) external DM halos
at all redshifts, including d) their substructures. Adopting a self-consistent
description of local and extra-galactic substructures, we show that the
annihilation flux from substructures in the MW dominates over all the other
components for angles larger than O(1) degrees from the Galactic Center, unless
an extreme prescription is adopted for the substructures concentration. We also
compute the angular power spectrum of gamma-ray anisotropies and find that, for
an optimistic choice of the particle physics parameters, an interesting
signature of DM annihilations could soon be discovered by the Fermi LAT
satellite at low multipoles, l<100, where the dominant contribution comes from
MW substructures with mass M>10^4 solar masses. For the substructures models we
have adopted, we find that the contribution of extra-galactic annihilations is
instead negligible at all scales.Comment: 14 pages, 7 figure

### A Survey of Metal Lines at High-redshift (I) : SDSS Absorption Line Studies - The Methodology and First Search Results for OVI

We report the results of a systematic search for signatures of metal lines in
quasar spectra of the Sloan Digital Sky Survey (SDSS) Data Release 3(DR3),
focusing on finding intervening absorbers via detection of their OVI doublet.
Here we present the search algorithm, and criteria for distinguishing
candidates from spurious Lyman $\alpha${} forest lines. In addition, we compare
our findings with simulations of the Lyman $\alpha${} forest in order to
estimate the detectability of OVI doublets over various redshift intervals. We
have obtained a sample of 1756 OVI doublet candidates with rest-frame
equivalent width > 0.05 \AA{} in 855 AGN spectra (out of 3702 objects with
redshifts in the accessible range for OVI detection). This sample is further
subdivided into 3 groups according to the likelihood of being real and the
potential for follow-up observation of the candidate. The group with the
cleanest and most secure candidates is comprised of 145 candidates. 69 of these
reside at a velocity separation > 5000 km/s from the QSO, and can therefore be
classified tentatively as intervening absorbers. Most of these absorbers have
not been picked up by earlier, automated QSO absorption line detection
algorithms. This sample increases the number of known OVI absorbers at
redshifts beyond z$_{abs} > 2.7 substantially.Comment: 41 pages, 10 figures, 2 tables, accepted by AJ. This is a
substantially altered version, including an appendix with details on the
validity of the search algorithm on one pixel rather than binning. Also note
that M. Pieri was added as autho

### Low density ferromagnetism in the Hubbard model

A single-band Hubbard model with nearest and next-nearest neighbour hopping
is studied for $d=1$, 2, 3, using both analytical and numerical techniques. In
one dimension, saturated ferromagnetism is found above a critical value of $U$
for a band structure with two minima and for small and intermediate densities.
This is an extension of a scenario recently proposed by M\"uller--Hartmann. For
three dimensions and non-pathological band structures, it is proven that such a
scenario does not work.Comment: 4 pages, 3 postscript figure

### Spin Chains in an External Magnetic Field. Closure of the Haldane Gap and Effective Field Theories

We investigate both numerically and analytically the behaviour of a spin-1
antiferromagnetic (AFM) isotropic Heisenberg chain in an external magnetic
field. Extensive DMRG studies of chains up to N=80 sites extend previous
analyses and exhibit the well known phenomenon of the closure of the Haldane
gap at a lower critical field H_c1. We obtain an estimate of the gap below
H_c1. Above the lower critical field, when the correlation functions exhibit
algebraic decay, we obtain the critical exponent as a function of the net
magnetization as well as the magnetization curve up to the saturation (upper
critical) field H_c2. We argue that, despite the fact that the SO(3) symmetry
of the model is explicitly broken by the field, the Haldane phase of the model
is still well described by an SO(3) nonlinear sigma-model. A mean-field theory
is developed for the latter and its predictions are compared with those of the
numerical analysis and with the existing literature.Comment: 11 pages, 4 eps figure

### Time-dependent Gross-Pitaevskii equation for composite bosons as the strong-coupling limit of the fermionic BCS-RPA approximation

The linear response to a space- and time-dependent external disturbance of a
system of dilute condensed composite bosons at zero temperature, as obtained
from the linearized version of the time-dependent Gross-Pitaevskii equation, is
shown to result also from the strong-coupling limit of the time-dependent BCS
(or broken-symmetry RPA) approximation for the constituent fermions subject to
the same external disturbance. In this way, it is possible to connect
excited-state properties of the bosonic and fermionic systems by placing the
Gross-Pitaevskii equation in perspective with the corresponding fermionic
approximationsComment: 4 pages, 1 figur

- â€¦