3,335 research outputs found
Pre-Heated Isentropic Gas in Groups of Galaxies
We confirm that the standard assumption of isothermal, shock-heated gas in
cluster potentials is unable to reproduce the observed X-ray luminosity-
temperature relation of groups of galaxies. As an alternative, we construct a
physically motivated model for the adiabatic collapse of pre-heated gas into an
isothermal potential that improves upon the original work of Kaiser (1991). The
luminosity and temperature of the gas is calculated, assuming an appropriate
distribution of halo formation times and radiation due to both bremsstrahlung
and recombination processes. This model successfully reproduces the slope and
dispersion of the luminosity-temperature relation of galaxy groups. We also
present calculations of the temperature and luminosity functions for galaxy
groups under the prescription of this model. This model makes two strong
predictions for haloes with total masses M<10^13 M_sun, which are not yet
testable with current data: (1) the gas mass fraction will increase in direct
proportion to the halo mass; (2) the gas temperature will be larger than the
virial temperature of the mass. The second effect is strong enough that group
masses determined from gas temperatures will be overestimated by about an order
of magnitude if it is assumed that the gas temperature is the virial
temperature. The entropy required to match observations can be obtained by
heating the gas at the turnaround time, for example, to about 3 X 10^6 K at
z=1, which is too high to be generated by a normal rate of supernova
explosions. This model breaks down on the scale of low mass clusters, but this
is an acceptable limitation, as we expect accretion shocks to contribute
significantly to the entropy of the gas in such objects.Comment: Final, refereed version, accepted by MNRAS. One new figure and
several clarifying statements have been added. Uses mn.a4.sty (hacked
mn.sty). Also available from
http://astrowww.phys.uvic.ca/~balogh/entropy.ps.g
Magnetic and Transport Properties of Fe-Ag granular multilayers
Results of magnetization, magnetotransport and Mossbauer spectroscopy
measurements of sequentially evaporated Fe-Ag granular composites are
presented. The strong magnetic scattering of the conduction electrons is
reflected in the sublinear temperature dependence of the resistance and in the
large negative magnetoresistance. The simultaneous analysis of the magnetic
properties and the transport behavior suggests a bimodal grain size
distribution. A detailed quantitative description of the unusual features
observed in the transport properties is given
Color bimodality: Implications for galaxy evolution
We use a sample of 69726 galaxies from the SDSS to study the variation of the
bimodal color-magnitude (CM) distribution with environment. Dividing the galaxy
population by environment (Sigma_5) and luminosity (-23<M_r<-17), the u-r color
functions are modeled using double-Gaussian functions. This enables a
deconvolution of the CM distributions into two populations: red and blue
sequences. The changes with increasing environmental density can be separated
into two effects: a large increase in the fraction of galaxies in the red
distribution, and a small color shift in the CM relations of each distribution.
The average color shifts are 0.05+-0.01 and 0.11+-0.02 for the red and blue
distributions, respectively, over a factor of 100 in projected neighbor
density. The red fraction varies between about 0% and 70% for low-luminosity
galaxies and between about 50% and 90% for high-luminosity galaxies. This
difference is also shown by the variation of the luminosity functions with
environment. We demonstrate that the effects of environment and luminosity can
be unified. A combined quantity, Sigma_mod = Sigma_5/Mpc^{-2} + L_r/L_{-20.2},
predicts the fraction of red galaxies, which may be related to the probability
of transformation events. Our results are consistent with major interactions
(mergers and/or harassment) causing galaxies to transform from the blue to the
red distribution. We discuss this and other implications for galaxy evolution
from earlier results and model the effect of slow transformations on the color
functions.Comment: 14 pages, 8 figures, in AIP Conf. Proc., The New Cosmology, eds. R.
E. Allen et al. (aka. The Mitchell Symposium), see
http://proceedings.aip.org/proceedings/confproceed/743.jsp ; v2: replaced
Figure 5 which was incomplete in original submissio
Thermal stability of the microstructure of severely deformed copper
Copper specimens were deformed by equal channel angular pressing (ECAP) up to 8 passes. The microstructure was studied by X-ray line profile analysis. The crystallite size is reduced to a few tens of nanometers even after the first ECAP pass and it does not change significantly during further deformation. At the same time, the dislocation density increases gradually up to 4 ECAP passes. The thermal stability of the microstructure is examined by differential scanning calorimetry (DSC). The temperature of the DSC peak decreases whereas the stored energy increases with increasing strain. At the beginning of the heat release a bimodal grain structure develops indicated by a special double-peak shape of the diffraction line profiles
Wavelet analysis of magnetic turbulence in the Earth's plasma sheet
Recent studies provide evidence for the multi-scale nature of magnetic
turbulence in the plasma sheet. Wavelet methods represent modern time series
analysis techniques suitable for the description of statistical characteristics
of multi-scale turbulence. Cluster FGM (fluxgate magnetometer) magnetic field
high-resolution (~67 Hz) measurements are studied during an interval in which
the spacecraft are in the plasma sheet. As Cluster passes through different
plasma regions, physical processes exhibit non-steady properties on
magnetohydrodynamic (MHD) and small, possibly kinetic scales. As a consequence,
the implementation of wavelet-based techniques becomes complicated due to the
statistically transitory properties of magnetic fluctuations and finite size
effects. Using a supervised multi-scale technique which allows existence test
of moments, the robustness of higher-order statistics is investigated. On this
basis the properties of magnetic turbulence are investigated for changing
thickness of the plasma sheet.Comment: 17 pages, 5 figure
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