2,114 research outputs found
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
Stability and symmetry-breaking bifurcation for the ground states of a NLS with a interaction
We determine and study the ground states of a focusing Schr\"odinger equation
in dimension one with a power nonlinearity and a strong
inhomogeneity represented by a singular point perturbation, the so-called
(attractive) interaction, located at the origin. The
time-dependent problem turns out to be globally well posed in the subcritical
regime, and locally well posed in the supercritical and critical regime in the
appropriate energy space. The set of the (nonlinear) ground states is
completely determined. For any value of the nonlinearity power, it exhibits a
symmetry breaking bifurcation structure as a function of the frequency (i.e.,
the nonlinear eigenvalue) . More precisely, there exists a critical
value \om^* of the nonlinear eigenvalue \om, such that: if \om_0 < \om <
\om^*, then there is a single ground state and it is an odd function; if \om
> \om^* then there exist two non-symmetric ground states. We prove that before
bifurcation (i.e., for \om < \om^*) and for any subcritical power, every
ground state is orbitally stable. After bifurcation (\om =\om^*+0), ground
states are stable if does not exceed a value that lies
between 2 and 2.5, and become unstable for . Finally, for and \om \gg \om^*, all ground states are unstable. The branch of odd
ground states for \om \om^*,
obtaining a family of orbitally unstable stationary states. Existence of ground
states is proved by variational techniques, and the stability properties of
stationary states are investigated by means of the Grillakis-Shatah-Strauss
framework, where some non standard techniques have to be used to establish the
needed properties of linearization operators.Comment: 46 pages, 5 figure
Evolution of Biological Complexity
In order to make a case for or against a trend in the evolution of complexity
in biological evolution, complexity needs to be both rigorously defined and
measurable. A recent information-theoretic (but intuitively evident) definition
identifies genomic complexity with the amount of information a sequence stores
about its environment. We investigate the evolution of genomic complexity in
populations of digital organisms and monitor in detail the evolutionary
transitions that increase complexity. We show that because natural selection
forces genomes to behave as a natural ``Maxwell Demon'', within a fixed
environment genomic complexity is forced to increase.Comment: LaTeX 19 pages, incl. 4 fig
Cascade of Complexity in Evolving Predator-Prey Dynamics
We simulate an individual-based model that represents both the phenotype and
genome of digital organisms with predator-prey interactions. We show how
open-ended growth of complexity arises from the invariance of genetic evolution
operators with respect to changes in the complexity, and that the dynamics
which emerges is controlled by a non-equilibrium critical point. The mechanism
is analogous to the development of the cascade in fluid turbulence.Comment: 5 pages, 3 figures; added comments on system size scaling and
turbulence analogy, added error estimates of data collapse parameters.
Slightly enhanced from the version which will appear in PR
On the current correlators in QCD at finite temperature
Current correlators in QCD at a finite temperature are considered from
the viewpoint of operator product expansion. It is stressed that at low the
heat bath must be represented by hadronic, and not quark-gluon states. A
possibility to express the results in terms of -dependent resonance masses
is discussed. It is demonstrated that in order the masses do not move and
the only phenomenon which occurs is a parity and isospin mixing.Comment: 6 pages, TPI-MINN-92/64-
On the Asymptotic Dynamics of a Quantum System Composed by Heavy and Light Particles
We consider a non relativistic quantum system consisting of heavy and
light particles in dimension three, where each heavy particle interacts with
the light ones via a two-body potential . No interaction is assumed
among particles of the same kind. Choosing an initial state in a product form
and assuming sufficiently small we characterize the asymptotic
dynamics of the system in the limit of small mass ratio, with an explicit
control of the error. In the case K=1 the result is extended to arbitrary
. The proof relies on a perturbative analysis and exploits a
generalized version of the standard dispersive estimates for the
Schr\"{o}dinger group. Exploiting the asymptotic formula, it is also outlined
an application to the problem of the decoherence effect produced on a heavy
particle by the interaction with the light ones.Comment: 38 page
The VIMOS VLT Deep Survey. The different assembly history of passive and star-forming L_B >= L*_B galaxies in the group environment at z < 1
We use the VIMOS VLT Deep Survey to study the close environment of galaxies
in groups at 0.2 = L*_B galaxies (Me_B =
M_B + 1.1z <= -20) are identified with Me_B <= -18.25 and within a relative
distance 5h^-1 kpc <= rp <= 100h^-1 kpc and relative velocity Delta v <= 500
km/s . The richness N of a group is defined as the number of Me_B <= -18.25
galaxies belonging to that group. We split our principal sample into red,
passive galaxies with NUV - r >= 4.25 and blue, star-forming galaxies with NUV
- r < 4.25. We find that blue galaxies with a close companion are primarily
located in poor groups, while the red ones are in rich groups. The number of
close neighbours per red galaxy increases with N, with n_red being proportional
to 0.11N, while that of blue galaxies does not depend on N and is roughly
constant. In addition, these trends are found to be independent of redshift,
and only the average n_blue evolves, decreasing with cosmic time. Our results
support the following assembly history of L_B >= L*_B galaxies in the group
environment: red, massive galaxies were formed in or accreted by the dark
matter halo of the group at early times (z >= 1), therefore their number of
neighbours provides a fossil record of the stellar mass assembly of groups,
traced by their richness N. On the other hand, blue, less massive galaxies have
recently been accreted by the group potential and are still in their parent
dark matter halo, having the same number of neighbours irrespective of N. As
time goes by, these blue galaxies settle in the group potential and turn red
and/or fainter, thus becoming satellite galaxies in the group. With a toy
quenching model, we estimate an infall rate of field galaxies into the group
environment of R_infall = 0.9 - 1.5 x 10^-4 Mpc^-3 Gyr^-1 at z ~ 0.7.Comment: Astronomy and Astrophysics, in press. 11 pages, 11 figures, 4 tables.
Minor changes with respect to the first versio
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