2,195 research outputs found
Constrained energy minimization and orbital stability for the NLS equation on a star graph
We consider a nonlinear Schr\"odinger equation with focusing nonlinearity of
power type on a star graph , written as , where is the selfadjoint operator
which defines the linear dynamics on the graph with an attractive
interaction, with strength , at the vertex. The mass and energy
functionals are conserved by the flow. We show that for the energy at
fixed mass is bounded from below and that for every mass below a critical
mass it attains its minimum value at a certain \hat \Psi_m \in H^1(\GG)
, while for there is no minimum. Moreover, the set of minimizers has
the structure {\mathcal M}={e^{i\theta}\hat \Psi_m, \theta\in \erre}.
Correspondingly, for every there exists a unique
such that the standing wave is orbitally
stable. To prove the above results we adapt the concentration-compactness
method to the case of a star graph. This is non trivial due to the lack of
translational symmetry of the set supporting the dynamics, i.e. the graph. This
affects in an essential way the proof and the statement of
concentration-compactness lemma and its application to minimization of
constrained energy. The existence of a mass threshold comes from the
instability of the system in the free (or Kirchhoff's) case, that in our
setting corresponds to \al=0.Comment: 26 pages, 1 figur
Reduction criterion for separability
We introduce a separability criterion based on the positive map Î:Ïâ(Tr Ï)-Ï, where Ï is a trace-class Hermitian operator. Any separable state is mapped by the tensor product of Î and the identity into a non-negative operator, which provides a simple necessary condition for separability. This condition is generally not sufficient because it is vulnerable to the dilution of entanglement. In the special case where one subsystem is a quantum bit, Î reduces to time reversal, so that this separability condition is equivalent to partial transposition. It is therefore also sufficient for 2Ă2 and 2Ă3 systems. Finally, a simple connection between this map for two qubits and complex conjugation in the âmagicâ basis [Phys. Rev. Lett. 78, 5022 (1997)] is displayed
Variational properties and orbital stability of standing waves for NLS equation on a star graph
We study standing waves for a nonlinear Schr\"odinger equation on a star
graph {} i.e. half-lines joined at a vertex. At the vertex an
interaction occurs described by a boundary condition of delta type with
strength . The nonlinearity is of focusing power type. The
dynamics is given by an equation of the form , where is the Hamiltonian operator which
generates the linear Schr\"odinger dynamics. We show the existence of several
families of standing waves for every sign of the coupling at the vertex for
every . Furthermore, we determine the ground
states, as minimizers of the action on the Nehari manifold, and order the
various families. Finally, we show that the ground states are orbitally stable
for every allowed if the nonlinearity is subcritical or critical, and
for otherwise.Comment: 36 pages, 2 figures, final version appeared in JD
On the nature of faint Low Surface Brightness galaxies in the Coma cluster
This project is the continuation of our study of faint Low Surface Brightness
Galaxies (fLSBs) in one of the densest nearby galaxy regions known, the Coma
cluster. Our goal is to improve our understanding of the nature of these
objects by comparing the broad band spectral energy distribution with
population synthesis models. The data were obtained with the MEGACAM and CFH12K
cameras at the CFHT. We used the resulting photometry in 5 broad band filters
(u*, B, V, R, and I), that included new u*-band data, to fit spectral models.
With these spectral fits we inferred a cluster membership criterium, as well as
the ages, dust extinctions, and photometric types of these fLSBs. We show that
about half of the Coma cluster fLSBs have a spectral energy distribution well
represented in our template library while the other half present a flux deficit
at ultraviolet wavelengths. Among the well represented, ~80% are probably part
of the Coma cluster based on their spectral energy distribution. They are
relatively young (younger than 2.3 Gyrs for 90% of the sample) non-starburst
objects. The later their type, the younger fLSBs are. A significant part of the
fLSBs are quite dusty objects. fLSBs are low stellar mass objects (the later
their type the less massive they are), with stellar masses comparable to
globular clusters for the faintest ones. Their characteristics are correlated
with infall directions, confirming the disruptive origin for part of them.Comment: Accepted for publication in A&A, 10 pages, 10 figure
Photometric redshifts as a tool to study the Coma cluster galaxy populations
We investigate the Coma cluster galaxy luminosity function (GLF) at faint
magnitudes, in particular in the u* band by applying photometric redshift
techniques applied to deep u*, B, V, R, I images covering a region of ~1deg2 (R
24). Global and local GLFs in the B, V, R and I bands obtained with photometric
redshift selection are consistent with our previous results based on a
statistical background subtraction.
In the area covered only by the u* image, the GLF was also derived after
applying a statistical background subtraction. The GLF in the u* band shows an
increase of the faint end slope towards the outer regions of the cluster (from
alpha~1 in the cluster center to alpha~2 in the cluster periphery). This could
be explained assuming a short burst of star formation in these galaxies when
entering the cluster.
The analysis of the multicolor type spatial distribution reveals that late
type galaxies are distributed in clumps in the cluster outskirts, where X-ray
substructures are also detected and where the GLF in the u* band is steeper.Comment: 14 pages, 2 figures in jpeg format, accepted in A&
Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
Over half of GeV gamma-ray sources observed by the EGRET experiment have not
yet been identified as known astronomical objects. There is an isotropic
component of such unidentified sources, whose number is about 60 in the whole
sky. Here we calculate the expected number of dynamically forming clusters of
galaxies emitting gamma-rays by high energy electrons accelerated in the shock
wave when they form, in the framework of the standard theory of structure
formation. We find that a few tens of such forming clusters should be
detectable by EGRET and hence a considerable fraction of the isotropic
unidentified sources can be accounted for, if about 5% of the shock energy is
going into electron acceleration. We argue that these clusters are very
difficult to detect in x-ray or optical surveys compared with the conventional
clusters, because of their extended angular size of about 1 degree. Hence they
define a new population of ``gamma-ray clusters''. If this hypothesis is true,
the next generation gamma-ray telescopes such as GLAST will detect more than a
few thousands of gamma-ray clusters. It would provide a new tracer of
dynamically evolving structures in the universe, in contrast to the x-ray
clusters as a tracer of hydrodynamically stabilized systems. We also derive the
strength of magnetic field required for the extragalactic gamma-ray background
by structure formation to extend up to 100 GeV as observed, that is about
10^{-5} of the shock-heated baryon energy density.Comment: Accepted by ApJ after minor revisions. Received May 9, Accepted
August 3. 8 pages including 2 figure
Coma cluster object populations down to M_R~-9.5
This study follows a recent analysis of the galaxy luminosity functions and
colour-magnitude red sequences in the Coma cluster (Adami et al. 2007). We
analyze here the distribution of very faint galaxies and globular clusters in
an east-west strip of arcmin crossing the Coma cluster
center (hereafter the CS strip) down to the unprecedented faint absolute
magnitude of M. This work is based on deep images obtained at the
CFHT with the CFH12K camera in the B, R, and I bands. The analysis shows that
the observed properties strongly depend on the environment, and thus on the
cluster history. When the CS is divided into four regions, the westernmost
region appears poorly populated, while the regions around the brightest
galaxies NGC 4874 and NGC 4889 (NGC 4874 and NGC 4889 being masked) are
dominated by faint blue galaxies. They show a faint luminosity function slope
of -2, very significantly different from the field estimates. Results are
discussed in the framework of galaxy destruction (which can explain part of the
very faint galaxy population) and of structures infalling on to Coma.Comment: To be published in A&
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