1,149 research outputs found
Intracluster Comptonization of the CMB: Mean Spectral Distrortion and Cluster Number Counts
The mean sky-averaged Comptonization parameter, y, describing the scattering
of the CMB by hot gas in clusters of galaxies is calculated in an array of flat
and open cosmological and dark matter models. The models are globally
normalized to fit cluster X-ray data, and intracluster gas is assumed to have
evolved in a manner consistent with current observations. We predict values of
y lower than the COBE/FIRAS upper limit. The corresponding values of the
overall optical thickness to Compton scattering are < 10^{-4} for relevant
parameter values. Of more practical importance are number counts of clusters
across which a net flux (with respect to the CMB) higher than some limiting
value can be detected. Such number counts are specifically predicted for the
COBRAS/SAMBA and BOOMERANG missions.Comment: 23 pages, Latex, 11 PostScript figures, 5 PostScript tables, to
appear in Ap
On virialization with dark energy
We review the inclusion of dark energy into the formalism of spherical
collapse, and the virialization of a two-component system, made of matter and
dark energy. We compare two approaches in previous studies. The first assumes
that only the matter component virializes, e.g. as in the case of a classic
cosmological constant. The second approach allows the full system to virialize
as a whole. We show that the two approaches give fundamentally different
results for the final state of the system. This might be a signature
discriminating between the classic cosmological constant which cannot virialize
and a dynamical dark energy mimicking a cosmological constant. This signature
is independent of the measured value of the equation of state. An additional
issue which we address is energy non-conservation of the system, which
originates from the homogeneity assumption for the dark energy. We propose a
way to take this energy loss into account.Comment: 15 pages, 5 figures. Accepted for publication in JCA
Wiener Reconstruction of The Large Scale Structure
The formalism of Wiener filtering is developed here for the purpose of
reconstructing the large scale structure of the universe from noisy, sparse and
incomplete data. The method is based on a linear minimum variance solution,
given data and an assumed \prior model which specifies the covariance matrix of
the field to be reconstructed. While earlier applications of the Wiener filter
have focused on estimation, namely suppressing the noise in the measured
quantities, we extend the method here to perform both prediction and dynamical
reconstruction. The Wiener filter is used to predict the values of unmeasured
quantities, such as the density field in un-sampled regions of space, or to
deconvolve blurred data. The method is developed, within the context of linear
gravitational instability theory, to perform dynamical reconstruction of one
field which is dynamically related to some other observed field. This is the
case, for example, in the reconstruction of the real space galaxy distribution
from its redshift distribution When the field to be reconstructed is a Gaussian
random field, such as the primordial perturbation field predicted by the
canonical model of cosmology, the Wiener filter can be pushed to its fullest
potential. In such a case the Wiener estimator coincides with the Bayesian
estimator designed to maximize the {\it posterior} probability. The Wiener
filter can be also derived by assuming a quadratic regularization function, in
analogy with the `Maximum Entropy' method. The mean field obtained by the
minimal variance solution can be supplemented with constrained realizations of
the Gaussian field toComment: submitted to ApJ, 45 pages, 7 figures, compressed and uuencoded
Postscript file. (zhfl
Sum of the masses of the Milky Way and M31: A likelihood-free inference approach
We use density estimation likelihood-free inference,
Λ
cold dark matter simulations of
∼
2
M
galaxy pairs, and data from Gaia and the Hubble Space Telescope to infer the sum of the masses of the Milky Way and Andromeda (M31) galaxies, the two main components of the local group. This method overcomes most of the approximations of the traditional timing argument, makes the writing of a theoretical likelihood unnecessary, and allows the nonlinear modeling of observational errors that take into account correlations in the data and non-Gaussian distributions. We obtain an
M
200
mass estimate
M
MW
+
M
31
=
4.
6
+
2.3
−
1.8
×
10
12
M
⊙
(68% C.L.), in agreement with previous estimates both for the sum of the two masses and for the individual masses. This result is not only one of the most reliable estimates of the sum of the two masses to date, but is also an illustration of likelihood-free inference in a problem with only one parameter and only three data points
Protostellar Collapse with Various Metallicities
The thermal and chemical evolution of gravitationally collapsing protostellar
clouds is investigated, focusing attention on their dependence on metallicity.
Calculations are carried out for a range of metallicities spanning the local
interstellar value to zero. During the time when clouds are transparent to
continuous radiation, the temperatures are higher for those with lower
metallicity, reflecting lower radiative ability. However, once the clouds
become opaque, in the course of the adiabatic contraction of the transient
cores, their evolutionary trajectories in the density-temperature plane
converge to a unique curve that is determined by only physical constants. The
trajectories coincide with each other thereafter. Consequently, the size of the
stellar core at the formation is the same regardless of the gas composition of
the parent cloud.Comment: 30 pages. The Astrophysical Journal, 533, in pres
Dynamical Evolution of Globular Clusters in Hierarchical Cosmology
We probe the evolution of globular clusters that could form in giant
molecular clouds within high-redshift galaxies. Numerical simulations
demonstrate that the large and dense enough gas clouds assemble naturally in
current hierarchical models of galaxy formation. These clouds are enriched with
heavy elements from earlier stars and could produce star clusters in a similar
way to nearby molecular clouds. The masses and sizes of the model clusters are
in excellent agreement with the observations of young massive clusters. Do
these model clusters evolve into globular clusters that we see in our and
external galaxies? In order to study their dynamical evolution, we calculate
the orbits of model clusters using the outputs of the cosmological simulation
of a Milky Way-sized galaxy. We find that at present the orbits are isotropic
in the inner 50 kpc of the Galaxy and preferentially radial at larger
distances. All clusters located outside 10 kpc from the center formed in the
now-disrupted satellite galaxies. The spatial distribution of model clusters is
spheroidal, with a power-law density profile consistent with observations. The
combination of two-body scattering, tidal shocks, and stellar evolution results
in the evolution of the cluster mass function from an initial power law to the
observed log-normal distribution.Comment: 5 pages, proceedings of IAU 246 "Dynamical Evolution of Dense Stellar
Systems", eds. Vesperini, Giersz, Sill
Constraints on Type Ia Supernova Models from X-ray Spectra of Galaxy Clusters
We present constraints on theoretical models of Type Ia supernovae using
spatially resolved ASCA X-ray spectroscopy of three galaxy clusters: Abell 496,
Abell 2199 and Abell 3571. All three clusters have central iron abundance
enhancements; an ensemble of abundance ratios are used to show that most of the
iron in the central regions of the clusters comes from SN Ia. These
observations are consistent with the suppressed galactic wind scenario proposed
by Dupke and White (1999). At the center of each cluster, simultaneous analysis
of spectra from all ASCA instruments shows that the nickel to iron abundance
ratio (normalized by the solar ratio) is Ni/Fe ~ 4. We use the nickel to iron
ratio as a discriminator between SN Ia explosion models: the Ni/Fe ratio of
ejecta from the "Convective Deflagration" model W7 is consistent with the
observations, while those of "delayed detonation" models are not consistent at
the 90% confidence level.Comment: 20 pages, 2 figures, accepted by The Astrophysical Journa
Constraining the Lifetime of Quasars from their Spatial Clustering
The lifetime t_Q of the luminous phase of quasars is constrained by current
observations to be between 10^6 and 10^8 years, but is otherwise unkown. We
model the quasar luminosity function in detail in the optical and X-ray bands
using the Press-Schechter formalism, and show that the expected clustering of
quasars depends strongly on their assumed lifetime. We quantify this
dependence, and find that existing measurements of the correlation length of
quasars are consistent with the range 10^6 < t_Q < 10^8 years. We then show
that future measurements of the power spectrum of quasars out to z=3, from the
2dF or Sloan Digital Sky Survey, can significantly improve this constraint, and
in principle allow a precise determination of t_Q. We estimate the systematic
errors introduced by uncertainties in the modeling of the quasar-halo
relationship, as well as by the possible existence of obscured quasars.Comment: ApJ, in press (emulateapj
Primal infon logic with conjunctions as sets
Primal infon logic was proposed by Gurevich and Neeman as an efficient yet expressive logic for policy and trust management. It is a propositional multimodal subintuitionistic logic decidable in linear time. However in that logic the principle of the replacement of equivalents fails. For example, (x ∧ y) → z does not entail (y ∧ x) → z, and similarly w → ((x ∧ y) ∧ z) does not entail w → (x ∧ (y ∧ z)). Imposing the full principle of the replacement of equivalents leads to an NP-hard logic according to a recent result of Beklemishev and Prokhorov. In this paper we suggest a way to regain the part of this principle restricted to conjunction: We introduce a version of propositional primal logic that treats conjunctions as sets, and show that the derivation problem for this logic can be decided in linear expected time and quadratic worst-case time. © 2014 IFIP International Federation for Information Processing
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