272 research outputs found
Clustering of the Diffuse Infrared Light from the COBE DIRBE maps. I. and limits on the near-IR background
This paper is devoted to studying the CIB through its correlation properties.
We studied the limits on CIB anisotropy in the near IR (1.25, 2.2, and 3.5 \um,
or ) bands at a scale of 0.7\deg\ using the COBE\footnote{ The
National Aeronautics and Space Administration/Goddard Space Flight Center
(NASA/GSFC) is responsible for the design, development, and operation of the
{\it COBE}. Scientific guidance is provided by the {\it COBE} Science Working
Group. GSFC is also responsible for the development of the analysis software
and for the production of the mission data sets.} Diffuse Infrared Background
Experiment (DIRBE) data. In single bands we obtain the upper limits on the
zero-lag correlation signal \w2m4sr2 for the
bands respectively. The DIRBE data exhibit a clear color between the
various bands with a small dispersion. On the other hand most of the CIB is
expected to come from redshifted galaxies and thus should have different color
properties. We use this observation to develop a `color subtraction' method of
linear combinations of maps at two different bands. This method is expected to
suppress the dominant fluctuations from foreground stars and nearby galaxies,
while not reducing (or perhaps even amplifying) the extragalactic contribution
to . Applying this technique gives significantly lower and more isotropic
limits.Comment: 44 pages postcript; includes 5 tables, 14 figures. Astrophysical
Journal, in pres
Clustering of DIRBE Light and IR Background
We outline a new method for estimating the cosmic infrared background using
the spatial and spectral correlation properties of infrared maps. The cosmic
infrared background from galaxies should have a minimum fluctuation of the
order of 10\% on angular scales of the order of 1\deg. We show that a linear
combination of maps at different wavelengths can greatly reduce the
fluctuations produced by foreground stars, while not eliminating the
fluctuations of the background from high redshift galaxies. The method is
potentially very powerful, especially at wavelengths where the foreground is
bright but smooth.Comment: 7 pages postcript, talk at "Unveiling the cosmic infrared background"
workshop, College Park, M
Low-Velocity Impacts on a Polymeric Foam for the Passive Safety Improvement of Sports Fields: Meshless Approach and Experimental Validation
Over the past few years, foam materials have been increasingly used in the passive safety of sport fields, to mitigate the risk of crash injury. Currently, the passive safety certification process of these materials represents an expensive and time-consuming task, since a considerable number of impact tests on material samples have to be carried out by an ad hoc testing apparatus. To overcome this difficulty and speed up the design process of new protective devices, a virtual model for the low-velocity impact behaviour of foam protective mats is needed. In this study a modelling approach based on the mesh-free Element Galerkin method was developed to investigate the impact behaviour of ethylene-vinyl acetate (EVA) foam protective mats. The main advantage of this novel technique is that the difficulties related to the computational mesh distortion and caused by the large deformation of the foam material are avoided and a good accuracy is achieved at a relatively low computational cost. The numerical model was validated statistically by comparing numerical and experimental acceleration data acquired during a series of impact events on EVA foam mats of various thicknesses. The findings of this study are useful for the design and improvement of foam protective devices and allow for optimizing sports fieldsâ facilities by reducing head injury risk by a reliable computational method
A transient network of telechelic polymers and microspheres : structure and rheology
We study the structure and dynamics of a transient network composed of
droplets of microemulsion connected by telechelic polymers. The polymer induces
a bridging attraction between droplets without changing their shape. A
viscoelastic behaviour is induced in the initially liquid solution,
characterised in the linear regime by a stretched exponential stress
relaxation. We analyse this relaxation in the light of classical theories of
transient networks. The role of the elastic reorganisations in the deformed
network is emphasized. In the non linear regime, a fast relaxation dynamics is
followed by a second one having the same rate as in the linear regime. This
behaviour, under step strain experiments, should induce a non monotonic
behaviour in the elastic component of the stress under constant shear rate.
However, we obtain in this case a singularity in the flow curve very different
from the one observed in other systems, that we interpret in terms of fracture
behaviour.Comment: 9 pages, 4 figure
Experimental investigations of critical hydraulic gradients for a soil prone to suffusion
The presence of soils, which are at the limit state of internal stability, is a potential risk to earthworks under seepage flow. Therefore, it is necessary to identify unstable soils and to estimate hydraulic gradients at which the suffusion can be initiated respectively progressed. An experimental study has been carried out to quantify critical hydraulic gradients for a widely graded soil. For the tested soil, in downwards vertical percolation experiments, the global critical hydraulic gradients lie in the different ranges between icrit = 0:1 to 5:5 with dependency on the particle arrangement. The critical hydraulic gradient was investigated using various types of sample preparation technique. Moreover, suffusion tests using several types of samples with the same particle size distribution have been carried out. It states that for such a widely graded soil, the main problem is the particle arrangement. In other words, the suffusion might be not problematic if there is no segregation. Sometimes a specific amount of segregation also stabilizes the sample against suffusion. The common way of sample preparation delivers comparable results to the results of other researchers
Detection of small scale fluctuations in the near-IR cosmic infrared background from long exposure 2MASS fields
We report first results for the cosmic infrared background (CIB) fluctuations
at 1.25, 1.65 and 2.17 micron obtained from long exposures constructed from
2MASS standard star fields. We have co-added and analyzed scans from one such
field with a total exposure time > 1 hour, and removed sources and other
artifacts. The stars and galaxies were clipped out to K_s~19^m leaving only
high-z galaxies (or possibly local low-surface-brightness systems). The
residual component of the diffuse emission on scales from a few arc-sec to a
few arc-min has a power-law slope consistent with emission produced by
clustered galaxies. The noise (and residual artifacts) contribution to the
signal is small and the colors of the signal are very different from Galactic
stars or air-glow. We therefore identify the signal as CIB fluctuations from
the faint unresolved galaxies. We show that the present-day galaxies with no
evolution would produce a significant deficit in the observed CIB fluctuations.
Thus the dominant contribution to the observed signal must come from high z and
may indicate high rates of star formation at those epochs.Comment: Ap. J. Letters, in pres
The structure of the cometary globule CG 12: a high latitude star forming region
The structure of the high galactic latitude Cometary Globule 12 (CG 12) has
been investigated by means of radio molecular line observations. Detailed, high
signal to noise ratio maps in C18O (1-0), C18O (2-1) and molecules tracing high
density gas, CS (3-2), DCO+ (2-1) and H13CO+ (1-0), are presented. The C18O
line emission is distributed in a 10' long North-South elongated lane with two
strong maxima, CG12 N(orth) and CG12 S(outh). In CG12 S the high density
tracers delineate a compact core, DCO+ core, which is offset by 15" from the
C18O maximum. The observed strong C18O emission traces the surface of the DCO+
core or a separate, adjacent cloud component. The emission in high density
tracers is weak in CG12 N and especially the H13CO+, DCO+ and N2H+ lines are
+0.5 km/s offset in velocity with respect to the C18O lines. Evidence is
presented that the molecular gas is highly depleted. The observed strong C18O
emission towards CG12 N originates in the envelope of this depleted cloud
component or in a separate entity seen in the same line of sight. The C18O
lines in CG 12 were analyzed using Positive Matrix Factorization, PMF. The
shape and the spatial distribution of the individual PMF factors fitted
separately to the C18O (1-0) and (2-1) transitions were consistent with each
other. The results indicate a complex velocity and line excitation structure in
the cloud. Besides separate cloud velocity components the C18O line shapes and
intensities are influenced by excitation temperature variations caused by e.g,
the molecular outflow or by molecular depletion. Assuming a distance of 630 pc
the size of the CG 12 compact head, 1.1 pc by 1.8 pc, and the C18O mass larger
than 100 Msun are comparable to those of other nearby low/intermediate mass
star formation regions.Comment: 18 pages, 17 figures Accepted A&A Sep. 22 200
Dust properties in M31.I.Basic properties and a discussion on age-dependent dust heating
Context. Spitzer Space Telescope observations and dust emission models are
used to discuss the distribution of dust and its characteristics in M31.
Together with GALEX FUV, NUV, and SDSS images we studied the age dependence of
the dust heating process. Methods.Spitzer IRAC/MIPS maps of M31 were matched
together and compared to dust emission models allowing to constrain the dust
mass, the intensity of the mean radiation field, the abundance of Polycyclic
Aromatic Hydrocarbons (PAH) particles. The total infrared emission (TIR) was
analyzed in function of UV and Optical colors and compared to predictions of
models which consider the age-dependent dust heating. Results. We demonstrate
that cold-dust component emission dominates the infrared spectral energy
distribution of M31. The mean intensity of the radiation field heating the dust
is low (typically U<2, where U=1 is the value in the solar surrounding). Due to
the lack of submillimetric measurements the dust mass (M_{dust}) is only weakly
constrained by the infrared spectrum. We show that across the spiral-ring
structure of M31 a fraction >3% of the total dust mass is in PAHs. UV and
optical colors are correlated to (TIR/FUV) ratios in \sim 670 pc-sized regions
overall the disk of M31, although deviating from the IRX-beta relationship for
starburst galaxies. We derived that in 83% of the regions analyzed across the
10kpc ring more than 50% of the energy absorbed by the dust is rediated at
\lambda > 4000 \AA and that dust in M31 appears mainly heated by populations a
few Gyr old even across the star-forming ring. The attenuation is varying
radially peaking near 10kpc and decreasing faster in the inner regions of M31
than in the outer regions. We finally derived the attenuation map of M31 at
6"/px resolution (\sim 100 pc/px along the plane of M31).[abridged]Comment: 21 pages, 18 figures, accepted for publication in A&A. Only low
resolution images included, full resolution images will be avaiable in the
journal electronic version. Fig.14 and Fig.17 will be avaiable via CD
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