8,918 research outputs found
Statistical properties of ultraluminous IRAS galaxies from an HST imaging survey
We perform photometric measurements on a large HST snapshot imaging survey
sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative
nuclei from bright clumps in all the sample targets, mainly based on a
quantitative criterion of I-band luminosity as well as the global and local
morphological information. All the sources are then classified into three
categories with multiple, double and single nucleus/nuclei. The resultant
fractions of multiple, double and single nucleus/nuclei ULIRGs are 18%, 39% and
43%, respectively. This supports the multiple merger scenario as a possible
origin of ULIRGs, in addition to the commonly-accepted pair merger model.
Further statistical studies indicate that the AGN fraction increases from
multiple (36%) to double (65%) and then to single (80%) nucleus/nuclei ULIRGs.
For the single nucleus category, there is a high luminosity tail in the
luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This
indicates that active galactic nuclei tend to appear at final merging stage.
For multiple/double nuclei galaxies, we also find a high fraction of very close
nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This
strengthens the conclusion that systems at late merging phase preferentially
host ULIRGs.Comment: 38 pages, 21 figures, to be published in A
Maximal -regularity for stochastic evolution equations
We prove maximal -regularity for the stochastic evolution equation
\{{aligned} dU(t) + A U(t)\, dt& = F(t,U(t))\,dt + B(t,U(t))\,dW_H(t),
\qquad t\in [0,T],
U(0) & = u_0, {aligned}. under the assumption that is a sectorial
operator with a bounded -calculus of angle less than on
a space . The driving process is a cylindrical
Brownian motion in an abstract Hilbert space . For and
and initial conditions in the real interpolation space
\XAp we prove existence of unique strong solution with trajectories in
L^p(0,T;\Dom(A))\cap C([0,T];\XAp), provided the non-linearities
F:[0,T]\times \Dom(A)\to L^q(\mathcal{O},\mu) and B:[0,T]\times \Dom(A) \to
\g(H,\Dom(A^{\frac12})) are of linear growth and Lipschitz continuous in their
second variables with small enough Lipschitz constants. Extensions to the case
where is an adapted operator-valued process are considered as well.
Various applications to stochastic partial differential equations are worked
out in detail. These include higher-order and time-dependent parabolic
equations and the Navier-Stokes equation on a smooth bounded domain
\OO\subseteq \R^d with . For the latter, the existence of a unique
strong local solution with values in (H^{1,q}(\OO))^d is shown.Comment: Accepted for publication in SIAM Journal on Mathematical Analysi
Strangeness at SIS energies
In this contribution we discuss the physics of strange hadrons in low energy
() heavy ion collision. In this energy range the relevant
strange particle are the kaons and anti-kaons. The most interesting aspect
concerning these particles are so called in-medium modifications. We will
attempt to review the current status of understanding of these in medium
modifications. In addition we will briefly discuss other issues related with
kaon production, such as the nuclear equation of state and chemical
equilibrium.Comment: Proceedings Strange Quark Matter 2003, Atlantic Beach, NC, USA, March
200
Neutrino, Neutron, and Cosmic Ray Production in the External Shock Model of Gamma Ray Bursts
The hypothesis that ultra-high energy (>~ 10^19 eV) cosmic rays (UHECRs) are
accelerated by gamma-ray burst (GRB) blast waves is assumed to be correct.
Implications of this assumption are then derived for the external shock model
of gamma-ray bursts. The evolving synchrotron radiation spectrum in GRB blast
waves provides target photons for the photomeson production of neutrinos and
neutrons. Decay characteristics and radiative efficiencies of the neutral
particles that escape from the blast wave are calculated. The diffuse
high-energy GRB neutrino background and the distribution of high-energy GRB
neutrino events are calculated for specific parameter sets, and a scaling
relation for the photomeson production efficiency in surroundings with
different densities is derived. GRBs provide an intense flux of high-energy
neutrons, with neutron-production efficiencies exceeding ~ 1% of the total
energy release. The radiative characteristics of the neutron beta-decay
electrons from the GRB "neutron bomb" are solved in a special case. Galaxies
with GRB activity should be surrounded by radiation halos of ~ 100 kpc extent
from the outflowing neutrons, consisting of a nonthermal optical/X-ray
synchrotron component and a high-energy gamma-ray component from
Compton-scattered microwave background radiation. The luminosity of sources of
GRBs and relativistic outflows in L* galaxies such as the Milky Way is at the
level of ~10^40+-1 ergs/s. This is sufficient to account for UHECR generation
by GRBs. We briefly speculate on the possibility that hadronic cosmic rays
originate from the subset of supernovae that collapse to form relativistic
outflows and GRBs. (abridged)Comment: 53 pages, 8 figures, ApJ, in press, 574, July 20, 2002. Substantial
revision, previous Appendix expanded to ApJ, 556, 479; cosmic ray origin
speculations to Heidelberg (astro-ph/001054) and Hamburg ICRC
(astro-ph/0202254) proceeding
The faint counterparts of MAMBO mm sources near the NTT Deep Field
We discuss identifications for 18 sources from our MAMBO 1.2mm survey of the
region surrounding the NTT Deep Field. We have obtained accurate positions from
Very Large Array 1.4GHz interferometry and in a few cases IRAM mm
interferometry, and have also made deep BVRIzJK imaging at ESO. We find
thirteen 1.2mm sources associated with optical/near-infrared objects in the
magnitude range K=19.0 to 22.5, while five are blank fields at K>22. The median
redshift of the radio-identified mm sources is ~2.6 from the radio/mm
estimator, and the median optical/near-infrared photometric redshifts for the
objects with counterparts ~2.1. This suggests that those radio-identified mm
sources without optical/near-infrared counterparts tend to lie at higher
redshifts than those with optical/near-infrared counterparts. Compared to
published identifications of objects from 850micron surveys of similar depth,
the median K and I magnitudes of our counterparts are roughly two magnitudes
fainter and the dispersion of I-K colors is less. Real differences in the
median redshifts, residual mis-identifications with bright objects, cosmic
variance, and small number statistics are likely to contribute to this
significant difference, which also affects redshift measurement strategies. We
discuss basic properties of the near-infrared/(sub)mm/radio spectral energy
distributions of our galaxies and of interferometrically identified submm
sources from the literature. From a comparison with submm objects with
CO-confirmed spectroscopic redshifts we argue that roughly two thirds of the
(sub)mm galaxies are at z>~2.5. This fraction is probably larger when including
sources without radio counterparts. (abridged)Comment: 45 pages, 9 figures. Accepted by ApJ. The resolution of figures 2 and
3 has been degraded. A higher quality pdf version of this paper is available
at http://www.mpe.mpg.de/~dannerb
A quark model framework for the study of nuclear medium effects
A quark-model framework for studying nuclear medium effects on nucleon
resonances is described and applied here to pion photoproduction on the
deuteron, which is the simplest composite nucleon system and serves as a first
test case. Pion photoproduction on nuclei is discussed within a chiral
constituent quark model in which the quark degrees of freedom are explicitly
introduced through an effective chiral Lagrangian for the
quark-pseudoscalar-meson coupling. The advantage of this model is that a
complete set of nucleon resonances can be systematically included with a
limited number of parameters. Also, the systematic description of the nucleon
and its resonances at quark level allows us to self-consistently relate the
nuclear medium's influence on the baryon properties to the intrinsic dynamic
aspects of the baryons. As the simplest composite nucleus, the deuteron
represents the first application of this effective theory for meson
photoproduction on light nuclei. The influence of the medium on the transition
operators for a free nucleon is investigated in the Delta resonance region. No
evidence is found for a change of the Delta properties in the pion
photoproduction reaction on the deuteron since the nuclear medium here involves
just one other nucleon and the low binding energy implies low nuclear density.
However, we show that the reaction mechanism is in principle sensitive to
changes of Delta properties that would be produced by the denser nuclear medium
of heavier nuclei through the modification of the quark model parameters.Comment: Revtex, 8 pages, 4 figure
Dynamics and freeze-out of hadron resonances at RHIC
Yields, rapidity and transverse momentum spectra of ,
, and the meson resonances ,
, and are predicted. Hadronic rescattering leads to a
suppression of reconstructable resonances, especially at low . A mass
shift of the of 10 MeV is obtained from the microscopic simulation, due
to late stage formation in the cooling pion gas.Comment: Proceedings of the Strange Quark Matter 2003, eprint version differs
from published versio
Kaon differential flow in relativistic heavy-ion collisions
Using a relativistic transport model, we study the azimuthal momentum
asymmetry of kaons with fixed transverse momentum, i.e., the differential flow,
in heavy-ion collisions at beam momentum of 6 GeV/c per nucleon, available from
the Alternating Gradient Synchrotron (AGS) at the Brookhaven National
Laboratory (BNL). We find that in the absence of kaon potential the kaon
differential flow is positive and increases with transverse momentum as that of
nucleons. The repulsive kaon potential as predicted by theoretical models,
however, reduces the kaon differetnial flow, changing it to negative for kaons
with low momenta. Cancellation between the negative differential flow at low
mementa and the positive one at high momenta is then responsible for the
experimentally observed nearly vanishing in-plane transverse flow of kaons in
heavy ion experiments.Comment: Phys. Rev. C in pres
Global stabilization of feedforward systems under perturbations in sampling schedule
For nonlinear systems that are known to be globally asymptotically
stabilizable, control over networks introduces a major challenge because of the
asynchrony in the transmission schedule. Maintaining global asymptotic
stabilization in sampled-data implementations with zero-order hold and with
perturbations in the sampling schedule is not achievable in general but we show
in this paper that it is achievable for the class of feedforward systems. We
develop sampled-data feedback stabilizers which are not approximations of
continuous-time designs but are discontinuous feedback laws that are
specifically developed for maintaining global asymptotic stabilizability under
any sequence of sampling periods that is uniformly bounded by a certain
"maximum allowable sampling period".Comment: 27 pages, 5 figures, submitted for possible publication to SIAM
Journal Control and Optimization. Second version with added remark
Low Temperature Precursor Route for Highly Efficient Spherically Shaped LED-Phosphors M2Si5N8:Eu2+ (M = Eu, Sr, Ba)
The highly efficient nitridosilicate phosphors M2Si5N8 (M = Sr, Ba, Eu) for phosphor-converted pc-LEDs were synthesized at low temperatures using a novel precursor route involving metal amides M(NH2)2. These precursors have been synthesized by dissolution of the respective metals in supercritical ammonia at 150°C and 300 bar. The thermal behavior and decomposition process of the amides were investigated with temperature programmed powder X-ray diffractometry and thermoanalytical measurements (DTA/TG). These investigations rendered the amides as suitable intermediates for reaction with silicon diimide (Si(NH)2). Thus, the desired nitridosilicate phosphors were obtained at relatively low temperatures around 1150â1400°C which is approximately 300°C lower compared to common synthetic approaches starting from metals or oxides. The influence of the thermal treatment on the phosphor morphology has been studied extensively. The accessibility of spherical phosphor particles represents another striking feature of this route since it improves light extraction from the crystallites due to decreasing light guiding and decreasing re-absorption inside the phosphor particle. The synthesized luminescent materials M2Si5N8:Eu2+ (M = Sr, Ba) exhibit quantum efficiencies and emission band widths (FWHM 70â90 nm) comparable to standard phosphor powders. Employment of Eu(NH2)2 as dopant reagent for synthesis of Ba2Si5N8:Eu2+ proved favorable for the formation of spherical crystallites compared to doping with Eu metal, halides, or oxide
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