11,891 research outputs found
Dimensionality of Local Minimizers of the Interaction Energy
In this work we consider local minimizers (in the topology of transport
distances) of the interaction energy associated to a repulsive-attractive
potential. We show how the imensionality of the support of local minimizers is
related to the repulsive strength of the potential at the origin.Comment: 27 page
Nonlocal interactions by repulsive-attractive potentials: radial ins/stability
In this paper, we investigate nonlocal interaction equations with
repulsive-attractive radial potentials. Such equations describe the evolution
of a continuum density of particles in which they repulse each other in the
short range and attract each other in the long range. We prove that under some
conditions on the potential, radially symmetric solutions converge
exponentially fast in some transport distance toward a spherical shell
stationary state. Otherwise we prove that it is not possible for a radially
symmetric solution to converge weakly toward the spherical shell stationary
state. We also investigate under which condition it is possible for a
non-radially symmetric solution to converge toward a singular stationary state
supported on a general hypersurface. Finally we provide a detailed analysis of
the specific case of the repulsive-attractive power law potential as well as
numerical results. We point out the the conditions of radial ins/stability are
sharp.Comment: 42 pages, 7 figure
Linearized dynamics from the 4-simplex Regge action
We study the relation between the hessian matrix of the riemannian Reggae
action on a 4-simplex and linearized quantum gravity. We give an explicit
formula for the hessian as a function of the geometry, and show that it has a
single zero mode. We then use a 3d lattice model to show that (i) the zero mode
is a remnant of the continuum diffeomorphism invariance, and (ii) we recover
the complete free graviton propagator in the continuum limit. The results help
clarify the structure of the boundary state needed in the recent calculations
of the graviton propagator in loop quantum gravity, and in particular its role
in fixing the gauge.Comment: 16 (+9 Appendix) pages, 1 figur
Spectral decomposition of starbursts and AGNs in 5-8 micron Spitzer IRS spectra of local ULIRGs
We present an analysis of the 5-8 micron Spitzer-IRS spectra of a sample of
68 local Ultraluminous Infrared Galaxies (ULIRGs). Our diagnostic technique
allows a clear separation of the active galactic nucleus (AGN) and starburst
(SB) components in the observed mid-IR emission, and a simple analytic model
provides a quantitative estimate of the AGN/starburst contribution to the
bolometric luminosity. We show that AGNs are ~30 times brighter at 6 micron
than starbursts with the same bolometric luminosity, so that even faint AGNs
can be detected. Star formation events are confirmed as the dominant power
source for extreme infrared activity, since ~85% of ULIRG luminosity arises
from the SB component. Nonetheless an AGN is present in the majority (46/68) of
our sources.Comment: 5 Pages, 3 figures. MNRAS Letters, Accepte
The Bolocam 1.1 mm Lockman Hole Galaxy Survey: SHARC II 350 micron Photometry and Implications for Spectral Models, Dust Temperatures, and Redshift Estimation
We present 350 micron photometry of all 17 galaxy candidates in the Lockman
Hole detected in a 1.1 mm Bolocam survey. Several of the galaxies were
previously detected at 850 microns, at 1.2 mm, in the infrared by Spitzer, and
in the radio. Nine of the Bolocam galaxy candidates were detected at 350
microns and two new candidates were serendipitously detected at 350 microns
(bringing the total in the literature detected in this way to three). Five of
the galaxies have published spectroscopic redshifts, enabling investigation of
the implied temperature ranges and a comparison of photometric redshift
techniques.
Lambda = 350 microns lies near the spectral energy distribution peak for z =
2.5 thermally emitting galaxies. Thus, luminosities can be measured without
extrapolating to the peak from detection wavelengths of lambda > 850 microns.
Characteristically, the galaxy luminosities lie in the range 1.0 - 1.2 x 10^13
L_solar, with dust temperatures in the range of 40 K to 70 K, depending on the
choice of spectral index and wavelength of unit optical depth. The implied dust
masses are 3 - 5 x 10^8 M_solar. We find that the far-infrared to radio
relation for star-forming ULIRGs systematically overpredicts the radio
luminosities and overestimates redshifts on the order of Delta z ~ 1, whereas
redshifts based on either on submillimeter data alone or the 1.6 micron stellar
bump and PAH features are more accurate.Comment: In Press (to appear in Astrophysical Journal, ApJ 20 May 2006 v643 1)
47 pages, 10 figures, 4 table
Deformation of geometry and bifurcation of vortex rings
We construct a smooth family of Hamiltonian systems, together with a family
of group symmetries and momentum maps, for the dynamics of point vortices on
surfaces parametrized by the curvature of the surface. Equivariant bifurcations
in this family are characterized, whence the stability of the Thomson heptagon
is deduced without recourse to the Birkhoff normal form, which has hitherto
been a necessary tool.Comment: 26 page
Spin Networks for Non-Compact Groups
Spin networks are natural generalization of Wilson loops functionals. They
have been extensively studied in the case where the gauge group is compact and
it has been shown that they naturally form a basis of gauge invariant
observables. Physically the restriction to compact gauge group is enough for
the study of Yang-mills theories, however it is well known that non-compact
groups naturally arise as internal gauge groups for Lorentzian gravity models.
In this context a proper construction of gauge invariant observables is needed.
The purpose of this work is to define the notion of spin network states for
non-compact groups. We first built, by a careful gauge fixing procedure, a
natural measure and a Hilbert space structure on the space of gauge invariant
graph connection. Spin networks are then defined as generalized eigenvectors of
a complete set of hermitic commuting operators. We show how the delicate issue
of taking the quotient of a space by non compact groups can be address in term
of algebraic geometry. We finally construct the full Hilbert space containing
all spin network states. Having in mind application to gravity we illustrate
our results for the groups SL(2,R), SL(2,C).Comment: 43pages, many figures, some comments adde
The fast transient sky with Gaia
The ESA Gaia satellite scans the whole sky with a temporal sampling ranging
from seconds and hours to months. Each time a source passes within the Gaia
field of view, it moves over 10 CCDs in 45 s and a lightcurve with 4.5 s
sampling (the crossing time per CCD) is registered. Given that the 4.5 s
sampling represents a virtually unexplored parameter space in optical time
domain astronomy, this data set potentially provides a unique opportunity to
open up the fast transient sky. We present a method to start mining the wealth
of information in the per CCD Gaia data. We perform extensive data filtering to
eliminate known on-board and data processing artefacts, and present a
statistical method to identify sources that show transient brightness
variations on ~2 hours timescales. We illustrate that by using the Gaia
photometric CCD measurements, we can detect transient brightness variations
down to an amplitude of 0.3 mag on timescales ranging from 15 seconds to
several hours. We search an area of ~23.5 square degrees on the sky, and find
four strong candidate fast transients. Two candidates are tentatively
classified as flares on M-dwarf stars, while one is probably a flare on a giant
star and one potentially a flare on a solar type star. These classifications
are based on archival data and the timescales involved. We argue that the
method presented here can be added to the existing Gaia Science Alerts
infrastructure for the near real-time public dissemination of fast transient
events.Comment: 10 pages, 5 figures and 5 tables; MNRAS in pres
The Opacity of Nearby Galaxies from Counts of Background Galaxies: II. Limits of the Synthetic Field Method
Recently, we have developed and calibrated the Synthetic Field Method (SFM)
to derive the total extinction through disk galaxies. The method is based on
the number counts and colors of distant background field galaxies that can be
seen through the foreground object, and has been successfully applied to NGC
4536 and NGC 3664, two late-type galaxies located, respectively, at 16 and 11
Mpc. Here, we study the applicability of the SFM to HST images of galaxies in
the Local Group, and show that background galaxies cannot be easily identified
through these nearby objects, even with the best resolution available today. In
the case of M 31, each pixel in the HST images contains 50 to 100 stars, and
the background galaxies cannot be seen because of the intrinsic granularity due
to strong surface brightness fluctuations. In the LMC, on the other hand, there
is only about one star every six linear pixels, and the lack of detectable
background galaxies results from a ``secondary'' granularity, introduced by
structure in the wings of the point spread function. The success of the SFM in
NGC 4536 and NGC 3664 is a natural consequence of the reduction of the
intensity of surface brightness fluctuations with distance. When the dominant
confusion factor is structure in the PSF wings, as is the case of HST images of
the LMC, and would happen in M 31 images obtained with a 10-m diffraction-
limited optical telescope, it becomes in principle possible to improve the
detectability of background galaxies by subtracting the stars in the foreground
object. However, a much better characterization of optical PSFs than is
currently available would be required for an adequate subtraction of the wings.
Given the importance of determining the dust content of Local Group galaxies,
efforts should be made in that direction.Comment: 45 pages, 10 Postscript figure
Genomics of the new species Kingella negevensis: diagnostic issues and identification of a locus encoding a RTX toxin.
Kingella kingae, producing the cytotoxic RTX protein, is a causative agent of serious infections in humans such as bacteremia, endocarditis and osteoarticular infection, especially in young children. Recently, Kingella negevensis, a related species, has been isolated from the oral cavity of healthy children. In this study, we report the isolation of K. negevensis strain eburonensis, initially misidentified as K. kingae with MALDI-TOF MS, from a vaginal specimen of a patient suffering of vaginosis. The genome sequencing and analysis of this strain together with comparative genomics of the Kingella genus revealed that K. negevensis possesses a full homolog of the rtx operon of K. kingae involved in the synthesis of the RTX toxin. We report that a K. kingae specific diagnostic PCR, based on the rtxA gene, was positive when tested on K. negevensis strain eburonensis DNA. This cross-amplification, and risk of misidentification, was confirmed by in silico analysis of the target gene sequence. To overcome this major diagnostic issue we developed a duplex real-time PCR to detect and distinguish K. kingae and K. negevensis. In addition to this, the identification of K. negevensis raises a clinical issue in term of pathogenic potential given the production of a RTX hemolysin
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