1,088 research outputs found
Cosmological Implications of the Fundamental Relations of X-ray Clusters
Based on the two-parameter family nature of X-ray clusters of galaxies
obtained in a separate paper, we discuss the formation history of clusters and
cosmological parameters of the universe. Utilizing the spherical collapse model
of cluster formation, and assuming that the cluster X-ray core radius is
proportional to the virial radius at the time of the cluster collapse, the
observed relations among the density, radius, and temperature of clusters imply
that cluster formation occurs in a wide range of redshift. The observed
relations favor the low-density universe. Moreover, we find that the model of
is preferable.Comment: 7 pages, 4 figures. To be published in ApJ Letter
A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing
Spergel & Steinhardt proposed the possibility that the dark matter particles
are self-interacting, as a solution to two discrepancies between the
predictions of cold dark matter models and the observations: first, the
observed dark matter distribution in some dwarf galaxies has large,
constant-density cores, as opposed to the predicted central cusps; and second,
small satellites of normal galaxies are much less abundant than predicted. The
dark matter self-interaction would produce isothermal cores in halos, and would
also expel the dark matter particles from dwarfs orbiting within large halos.
However, another inevitable consequence of the model is that halos should
become spherical once most particles have interacted. Here, I rule out this
model by the fact that the innermost regions of dark matter halos in massive
clusters of galaxies are elliptical, as shown by gravitational lensing and
other observations. The absence of collisions in the lensing cores of massive
clusters implies that any dark matter self-interaction is too weak to have
affected the observed density profiles in the dark-matter dominated dwarf
galaxies, or to have eased the destruction of dwarf satellites in galactic
halos. If is the cross section and the mass of the dark matter
particle, then s_x/m_x < 10^{-25.5} \cm^2/\gev.Comment: to appear in ApJ, January 1 200
Confronting cold dark matter cosmologies with strong clustering of Lyman break galaxies at
We perform a detailed analysis of the statistical significance of a
concentration of Lyman break galaxies at recently discovered by
Steidel et al. (1997), using a series of N-body simulations with
particles in a (100\himpc)^3 comoving box. While the observed number density
of Lyman break galaxies at implies that they correspond to systems
with dark matter halos of \simlt 10^{12}M_\odot, the resulting clustering of
such objects on average is not strong enough to be reconciled with the
concentration if it is fairly common; we predict one similar concentration
approximately per () fields in three representative cold dark matter
models. Considering the current observational uncertainty of the frequency of
such clustering at , it would be premature to rule out the models, but
the future spectroscopic surveys in a dozen fields could definitely challenge
all the existing cosmological models a posteriori fitted to the universe.Comment: the final version which matchs that published in ApJ Letters (Feb
1998); compared with the previous versions, the predictions for the SCDM
model are slightly changed; Latex, 11 pages, including 3 ps figure
Tidal Effects in Clusters of Galaxies
High-redshift clusters of galaxies show an over-abundance of spirals by a
factor of 2-3, and the corresponding under-abundance of S0 galaxies, relative
to the nearby clusters. This morphological evolution can be explained by tidal
interactions with neighboring galaxies and with the hierarchically growing
cluster halo. The efficiency of tidal interactions depends on the size and
structure of the cluster, as well as on the epoch of its formation. I simulate
the formation and evolution of Virgo-type clusters in three cosmologies: a
critical density model Omega_0=1, an open model Omega_0=0.4, and a flat model
Omega_0=0.4 with a cosmological constant. The orbits of identified halos are
traced with a high temporal resolution (~10^7 yr). Halos with low relative
velocities merge only shortly after entering the cluster; after virialization
mergers are suppressed. The dynamical evolution of galaxies is determined by
the tidal field along their trajectories. The maxima of the tidal force do not
always correspond to closest approach to the cluster center. They are produced
to a large extent by the local density structures, such as the massive galaxies
and the unvirialized remnants of infalling groups of galaxies. Collisions of
galaxies are intensified by the substructure, with about 10 encounters within
10 kpc per galaxy in the Hubble time. These very close encounters add an
important amount (10-50%) of the total heating rate. The integrated effect of
tidal interactions is insufficient to transform a spiral galaxy into an
elliptical, but can produce an S0 galaxy. Overall, tidal heating is stronger in
the low Omega_0 clusters
Dwarf Dark Matter Halos
We study properties of dark matter halos at high redshifts z=2-10 for a vast
range of masses with the emphasis on dwarf halos with masses 10^7-10^9 Msun/h.
We find that the density profiles of relaxed dwarf halos are well fitted by the
NFW profile and do not have cores. We compute the halo mass function and the
halo spin parameter distribution and find that the former is very well
reproduced by the Sheth & Tormen model while the latter is well fitted by a
lognormal distribution with lambda_0 = 0.042 and sigma_lambda = 0.63. We
estimate the distribution of concentrations for halos in mass range that covers
six orders of magnitude from 10^7 Msun/h to 10^13} Msun/h, and find that the
data are well reproduced by the model of Bullock et al. The extrapolation of
our results to z = 0 predicts that present-day isolated dwarf halos should have
a very large median concentration of ~ 35. We measure the subhalo circular
velocity functions for halos with masses that range from 4.6 x 10^9 Msun/h to
10^13 Msun/h and find that they are similar when normalized to the circular
velocity of the parent halo. Dwarf halos studied in this paper are many orders
of magnitude smaller than well-studied cluster- and Milky Way-sized halos. Yet,
in all respects the dwarfs are just down-scaled versions of the large halos.
They are cuspy and, as expected, more concentrated. They have the same spin
parameter distribution and follow the same mass function that was measured for
large halos.Comment: Accepted to be pusblished by ApJ, 12 pages, 8 figures, LaTeX
(documentclass preprint2). Differences with respect to the previous
submission are: (i) abstract was modified slightly to make it more
transparent to the reader, (ii) an extra figure has been added, and (3) some
minor modifications to the main text were also don
Global Financial Crisis and its Ramifications on Capital Markets, Contributions to Economics
Many advancing economies employ life insurance mechanism to drive capital formation and accumulation. This paper presents observable facts on Nigeria relative to peers, which suggest abysmal life insurance density and penetration; perhaps are responsible for the extremely low capital formation gap. The paper is motivated by the economyâs low savings ratio, hence it hypothesis that capital formation through life insurance mechanism may not develop. Two stage least square (TSLS) method in a recursive system was employed from 1980 to 2013. The result finds that capital formation ratio (Cfr) is positively sensitive to life insurance penetration (LIP), and to a period lag of the problem variable (Cfr(1)).The outcomes of instrumental variables are in line with a priori, except for interest rate. The dynamic and static ex-post simulations, and ex-ante forecasts evaluation meet the standards. The forecast results fit the expectations of the underlining assumptions, suggesting that on average Cfr can grow by 8% per annum ceteris paribus. This finding is therefore consistent with financial development theory. It recommends that Pension regulators should step-up compliance of existing statute on compulsory life cover by employers; Insurance regulators should engage public education on life insurance; the federal government should adopt fiscal incentives to attract foreign direct investments in the industry for competitiveness and make policies for mandatory life policy for all adult working citizens
Normalizing the Temperature Function of Clusters of Galaxies
We re-examine the constraints which can be robustly obtained from the
observed temperature function of X-ray cluster of galaxies. The cluster mass
function has been thoroughly studied in simulations and analytically, but a
direct simulation of the temperature function is presented here for the first
time. Adaptive hydrodynamic simulations using the cosmological Moving Mesh
Hydro code of Pen (1997a) are used to calibrate the temperature function for
different popular cosmologies. Applying the new normalizations to the
present-day cluster abundances, we find for a hyperbolic universe, and for a spatially flat universe with a cosmological constant.
The simulations followed the gravitational shock heating of the gas and dark
matter, and used a crude model for potential energy injection by supernova
heating. The error bars are dominated by uncertainties in the heating/cooling
models. We present fitting formulae for the mass-temperature conversions and
cluster abundances based on these simulations.Comment: 20 pages incl 5 figures, final version for ApJ, corrected open
universe \gamma relation, results unchange
Weak Lensing as a Calibrator of the Cluster Mass-Temperature Relation
The abundance of clusters at the present epoch and weak gravitational lensing
shear both constrain roughly the same combination of the power spectrum
normalization sigma_8 and matter energy density Omega_M. The cluster constraint
further depends on the normalization of the mass-temperature relation.
Therefore, combining the weak lensing and cluster abundance data can be used to
accurately calibrate the mass-temperature relation. We discuss this approach
and illustrate it using data from recent surveys.Comment: Matches the version in ApJL. Equation 4 corrected. Improvements in
the analysis move the cluster contours in Fig1 slightly upwards. No changes
in the conclusion
Mass-Temperature Relation of Galaxy Clusters: A Theoretical Study
Combining conservation of energy throughout nearly-spherical collapse of
galaxy clusters with the virial theorem, we derive the mass-temperature
relation for X-ray clusters of galaxies . The normalization factor
and the scatter of the relation are determined from first principles with
the additional assumption of initial Gaussian random field. We are also able to
reproduce the recently observed break in the M-T relation at T \sim 3 \keV,
based on the scatter in the underlying density field for a low density
CDM cosmology. Finally, by combining observational data of high
redshift clusters with our theoretical formalism, we find a semi-empirical
temperature-mass relation which is expected to hold at redshifts up to unity
with less than 20% error.Comment: 43 pages, 13 figures, One figure is added and minor changes are made.
Accepted for Publication in Ap
Public-Sector Project Abandonment Decision: A Test of the Ricardian Equivalence Theory on the Failed Lagos Metroline in Nigeria
This chapter examines the implications of projects abandonment with test of the Ricardian Equivalence
on the failed Lagos metro line project in Nigeria as case study. The main variables used are Rail and
Pipeline Output, Budget Deficit, Interest Rate, Corruption Index, Savings and some others. The study
results on the Ricardian Equivalence hypothesis on deficit financing of projects using Vector autoregression
model from 1980-2012 indicate that no causal influence holds in Nigeria. Results show that
poor planning, corruption, political factors, poor support infrastructures, poor quality of local resources,
etc. were attributable. The results of the Impulse Response tests reveal that Rail and Pipeline output and
a few others responded positively to shocks in the short run (years 1-2), and negatively to others. The
result affirms that Government should privatize the railway system, legislate against project abandonment
and ensure that projects are adequately planned, funded, insured and insulated against corruptio
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