449 research outputs found
X-ray spectroscopy at TEXTOR
At TEXTOR, an X-ray spectrometer in a Johann mount is utilized to measure the X-ray spectra of He-like elements with intermediate Z. Up to now, the spectra of He-like argon have been investigated. The spectra have been modeled with the most recent atomic data using physically relevant parameters only. Good agreement has been found both in modeling the experimental spectra and in the determination of the plasma parameters, such as ion temperature and plasma motion and electron temperature. The deviations between the theoretical and experimental spectra are below 7% for all lines; the precision of the plasma parameters obtained by X-ray spectroscopy agrees with the accuracy of the standard diagnostics at TEXTOR.In addition, the abundance of Li-/He-like ions, as well as the H-/He-like ions, has been measured. For the higher densities, the abundance approaches the coronal expectation. Larger deviations to the coronal limit have been found with neutral beam injection. The system is now being upgraded for spatial resolution
The Omega Dependence of the Evolution of xi(r)
The evolution of the two-point correlation function, xi(r,z), and the
pairwise velocity dispersion, sigma(r,z), for both the matter and halo
population, in three different cosmological models:
(Omega_M,Omega_Lambda)=(1,0), (0.2,0) and (0.2,0.8) are described. If the
evolution of xi is parameterized by xi(r,z)=(1+z)^{-(3+eps)}xi(r,0), where
xi(r,0)=(r/r_0)^{-gamma}, then eps(mass) ranges from 1.04 +/- 0.09 for (1,0) to
0.18 +/- 0.12 for (0.2,0), as measured by the evolution of at 1 Mpc (from z ~ 5
to the present epoch). For halos, eps depends on their mean overdensity. Halos
with a mean overdensity of about 2000 were used to compute the halo two-point
correlation function tested with two different group finding algorithms: the
friends of friends and the spherical overdensity algorithm. It is certainly
believed that the rate of growth of this xihh will give a good estimate of the
evolution of the galaxy two-point correlation function, at least from z ~ 1 to
the present epoch. The values we get for eps(halos) range from 1.54 for (1,0)
to -0.36 for (0.2,0), as measured by the evolution of xi(halos) from z ~ 1.0 to
the present epoch. These values could be used to constrain the cosmological
scenario. The evolution of the pairwise velocity dispersion for the mass and
halo distribution is measured and compared with the evolution predicted by the
Cosmic Virial Theorem (CVT). According to the CVT, sigma(r,z)^2 ~ G Q rho(z)
r^2 xi(r,z) or sigma proportional to (1+z)^{-eps/2}. The values of eps measured
from our simulated velocities differ from those given by the evolution of xi
and the CVT, keeping gamma and Q constant: eps(CVT) = 1.78 +/- 0.13 for (1,0)
or 1.40 +/- 0.28 for (0.2,0).Comment: Accepted for publication in the ApJ. Also available at
http://manaslu.astro.utoronto.ca/~carlberg/cnoc/xiev/xi_evo.ps.g
The Expected Mass Function for Low Mass Galaxies in a CDM Cosmology: Is There a Problem?
It is well known that the mass function for_halos_ in CDM cosmology is a
relatively steep power law for low masses, possibly too steep to be consistent
with observations. But how steep is the_galaxy_ mass function? We have analyzed
the stellar and gas mass functions of the first massive luminous objects formed
in a \Lambda CDM universe, as calculated in the numerical simulation described
in Gnedin (2000ab). We found that while the dark matter mass function is steep,
the stellar and gas mass functions are flatter for low mass objects. The
stellar mass function is consistently flat at the low mass end. Moreover, while
the gas mass function follows the dark matter mass function until reionization
at z~7, between z=7 and z=4, the gas mass function also flattens considerably
at the low mass end. At z=4, the gas and stellar mass functions are fit by a
Schechter function with \alpha ~ -1.2 +/- 0.1, significantly shallower than the
dark matter halo mass function and consistent with some recent observations.
The baryonic mass functions are shallower because (a) the dark matter halo mass
function is consistent with the Press-Schechter formulation at low masses n(M)
M^-2 and (b) heating/cooling and ionization processes appear to cause baryons
to collect in halos with the relationship M_b M_d^4 at low masses. Combining
(a) and (b) gives n(M_b) M_b^-5/4, comparable to the simulation results. Thus,
the well known observational fact that low mass galaxies are underabundant as
compared to expectations from numerical dark matter simulations or
Press-Schechter modeling of CDM universes emerges naturally from these results,
implying that perhaps no ``new physics'' beyond the standard model is needed.Comment: Submitted to ApJ, 17 pages including 6 figure
Calibration of the galaxy cluster M_500-Y_X relation with XMM-Newton
The quantity Y_ X, the product of the X-ray temperature T_ X and gas mass M_
g, has recently been proposed as a robust low-scatter mass indicator for galaxy
clusters. Using precise measurements from XMM-Newton data of a sample of 10
relaxed nearby clusters, spanning a Y_ X range of 10^13 -10^15 M_sun keV, we
investigate the M_500-Y_ X relation. The M_500 - Y_ X data exhibit a power law
relation with slope alpha=0.548 \pm 0.027, close to the self-similar value
(3/5) and independent of the mass range considered. However, the normalisation
is \sim 20% below the prediction from numerical simulations including cooling
and galaxy feedback. We discuss two effects that could contribute to the
normalisation offset: an underestimate of the true mass due to the HE
assumption used in X-ray mass estimates, and an underestimate of the hot gas
mass fraction in the simulations. A comparison of the functional form and
scatter of the relations between various observables and the mass suggest that
Y_ X may indeed be a better mass proxy than T_ X or M_g,500.Comment: 4 pages, 2 figures, accepted for publication in A&
POTENT Reconstruction from Mark III Velocities
We present an improved POTENT method for reconstructing the velocity and mass
density fields from radial peculiar velocities, test it with mock catalogs, and
apply it to the Mark III Catalog. Method improvments: (a) inhomogeneous
Malmquist bias is reduced by grouping and corrected in forward or inverse
analyses of inferred distances, (b) the smoothing into a radial velocity field
is optimized to reduce window and sampling biases, (c) the density is derived
from the velocity using an improved nonlinear approximation, and (d) the
computational errors are made negligible. The method is tested and optimized
using mock catalogs based on an N-body simulation that mimics our cosmological
neighborhood, and the remaining errors are evaluated quantitatively. The Mark
III catalog, with ~3300 grouped galaxies, allows a reliable reconstruction with
fixed Gaussian smoothing of 10-12 Mpc/h out to ~60 Mpc/h. We present maps of
the 3D velocity and mass-density fields and the corresponding errors. The
typical systematic and random errors in the density fluctuations inside 40
Mpc/h are \pm 0.13 and \pm 0.18. The recovered mass distribution resembles in
its gross features the galaxy distribution in redshift surveys and the mass
distribution in a similar POTENT analysis of a complementary velocity catalog
(SFI), including the Great Attractor, Perseus-Pisces, and the void in between.
The reconstruction inside ~40 Mpc/h is not affected much by a revised
calibration of the distance indicators (VM2, tailored to match the velocities
from the IRAS 1.2Jy redshift survey). The bulk velocity within the sphere of
radius 50 Mpc/h about the Local Group is V_50=370 \pm 110 km/s (including
systematic errors), and is shown to be mostly generated by external mass
fluctuations. With the VM2 calibration, V_50 is reduced to 305 \pm 110 km/s.Comment: 60 pages, LaTeX, 3 tables and 27 figures incorporated (may print the
most crucial figures only, by commenting out one line in the LaTex source
Quantifying structure in networks
We investigate exponential families of random graph distributions as a
framework for systematic quantification of structure in networks. In this paper
we restrict ourselves to undirected unlabeled graphs. For these graphs, the
counts of subgraphs with no more than k links are a sufficient statistics for
the exponential families of graphs with interactions between at most k links.
In this framework we investigate the dependencies between several observables
commonly used to quantify structure in networks, such as the degree
distribution, cluster and assortativity coefficients.Comment: 17 pages, 3 figure
Evolution of magnetic fields through cosmological perturbation theory
The origin of galactic and extra-galactic magnetic fields is an unsolved
problem in modern cosmology. A possible scenario comes from the idea of these
fields emerged from a small field, a seed, which was produced in the early
universe (phase transitions, inflation, ...) and it evolves in time.
Cosmological perturbation theory offers a natural way to study the evolution of
primordial magnetic fields. The dynamics for this field in the cosmological
context is described by a cosmic dynamo like equation, through the dynamo term.
In this paper we get the perturbed Maxwell's equations and compute the energy
momentum tensor to second order in perturbation theory in terms of gauge
invariant quantities. Two possible scenarios are discussed, first we consider a
FLRW background without magnetic field and we study the perturbation theory
introducing the magnetic field as a perturbation. The second scenario, we
consider a magnetized FLRW and build up the perturbation theory from this
background. We compare the cosmological dynamo like equation in both scenarios
The Halo Mass Function: High-Redshift Evolution and Universality
We study the formation of dark matter halos in the concordance LCDM model
over a wide range of redshifts, from z=20 to the present. Our primary focus is
the halo mass function, a key probe of cosmology. By performing a large suite
of nested-box N-body simulations with careful convergence and error controls
(60 simulations with box sizes from 4 to 256 Mpc/h, we determine the mass
function and its evolution with excellent statistical and systematic errors,
reaching a few percent over most of the considered redshift and mass range.
Across the studied redshifts, the halo mass is probed over 6 orders of
magnitude (10^7 - 10^13.5 M_sun/h). Historically, there has been considerable
variation in the high redshift mass function as obtained by different groups.
We have made a concerted effort to identify and correct possible systematic
errors in computing the mass function at high redshift and to explain the
discrepancies between some of the previous results. We discuss convergence
criteria for the required force resolution, simulation box size, halo mass
range, initial and final redshift, and time stepping. Because of conservative
cuts on the mass range probed by individual boxes, our results are relatively
insensitive to simulation volume, the remaining sensitivity being consistent
with extended Press-Schechter theory. Previously obtained mass function fits
near z=0, when scaled by linear theory, are in good agreement with our results
at all redshifts, although a mild redshift dependence consistent with that
found by Reed and collaborators exists at low redshifts.Comment: 20 pages, 15 figures. Minor changes to the text and figures; results
and conclusions unchange
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