1,297 research outputs found
Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
Over half of GeV gamma-ray sources observed by the EGRET experiment have not
yet been identified as known astronomical objects. There is an isotropic
component of such unidentified sources, whose number is about 60 in the whole
sky. Here we calculate the expected number of dynamically forming clusters of
galaxies emitting gamma-rays by high energy electrons accelerated in the shock
wave when they form, in the framework of the standard theory of structure
formation. We find that a few tens of such forming clusters should be
detectable by EGRET and hence a considerable fraction of the isotropic
unidentified sources can be accounted for, if about 5% of the shock energy is
going into electron acceleration. We argue that these clusters are very
difficult to detect in x-ray or optical surveys compared with the conventional
clusters, because of their extended angular size of about 1 degree. Hence they
define a new population of ``gamma-ray clusters''. If this hypothesis is true,
the next generation gamma-ray telescopes such as GLAST will detect more than a
few thousands of gamma-ray clusters. It would provide a new tracer of
dynamically evolving structures in the universe, in contrast to the x-ray
clusters as a tracer of hydrodynamically stabilized systems. We also derive the
strength of magnetic field required for the extragalactic gamma-ray background
by structure formation to extend up to 100 GeV as observed, that is about
10^{-5} of the shock-heated baryon energy density.Comment: Accepted by ApJ after minor revisions. Received May 9, Accepted
August 3. 8 pages including 2 figure
An Economic Assessment of the Impact of ICT on Performance Indicators of Water Resource Management in Africa: A Suggested Strategy for avoiding the eminent International Water Wars
Using data from 16 West African countries, this paper examines the links between Per Capita Income, Trade and Financial indicators, Education and Freedom indicators. Others are Internet users, Broadband and Mobile Cell phone Subscribers. Mean while Fresh Water Supply (which is assumed as a bench mark public sector-led water resource management performance indicators) and Access to Safe Drinking Water (a bench mark private sector-led water resource management performance indicators) represents indicators of water resources management. The results show that income, ICT and government trade policies influence the efficient management of cross-country water resource. Freedom indicators strongly affect water resource management performance indicators (WRMPI). Moreso, Internet Users, Broadband Subscribers, and Mobile cell phones Subscribers have a positive association with WRMPI. Contrary to wide spread expectations, education does not influence WRMPI.Key words: ICT, Water economics, water Resource Management, Nigeria, International Water War
Diffuse gamma-ray background and cosmic-ray positrons from annihilating dark matter
We study the annihilating dark matter contribution to the extra-galactic
diffuse gamma-ray background spectrum, motivated by the recent observations of
cosmic-ray positron/electron anomalies. The observed diffuse gamma-ray flux
provides stringent constraint on dark matter models and we present upper bounds
on the annihilation cross section of the dark matter. It is found that for the
case of cored dark matter halo profile, the diffuse gamma-rays give more
stringent bound compared with gamma-rays from the Galactic center. The Fermi
satellite will make the bound stronger.Comment: 20 pages, 11 figures; references added; to appear in PR
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
Responsiveness to Knowledge and Organisational Performance of Listed-Companies in the Construction Sector
In order to improve performance, the place of tangible and intangible resources deployed in operations has become critical. However, reliance on tangible resources as the bastion of better organisational performance seems to be waning; partly due to the ease with which these are copied. This implies that reliance on intangible resources, of which knowledge is a prime component, becomes inevitable. Rather than take on the titanic knowledge management construct holistically, the interest of this study is in the sub-construct of responsiveness to knowledge (RTK) largely because of its relevance in the construction sector in South Africa where client expectations, work methods and indeed, project employees are in a constant state of flux. Empirical in nature, the study uses a census of construction companies listed on the Johannesburg stock exchange (JSE) and focussed on the issue of responsiveness to knowledge and its association with the organisational performance (OP). Quantitative data collected from employees in a cross-sectional manner, were analysed. In terms of results, the study points to a positive association between RTK and OP in construction companies. Despite this finding, it would appear that companies undertake knowledge management on an ad-hoc and informal manner rather than by following a systematic process. Consequently, the study contends that attention, investment and institutionalisation of a mechanism for responding to knowledge as an integral part of the knowledge management bouquet, can enhance organisational performance
Strong Evolution in the Luminosity-Velocity Relation at z>1?
We present a method for constraining the evolution of the galaxy
luminosity-velocity (LV) relation in hierarchical scenarios of structure
formation. The comoving number density of dark-matter halos with circular
velocity of 200 km/s is predicted in favored CDM cosmologies to be nearly
constant over the redshift range 0<z<5. Any observed evolution in the density
of bright galaxies implies in turn a corresponding evolution in the LV
relation. We consider several possible forms of evolution for the zero-point of
the LV relation and predict the corresponding evolution in galaxy number
density. The Hubble Deep Field suggests a large deficit of bright (M_V < -19)
galaxies at 1.4 < z < 2. If taken at face value, this implies a dimming of the
LV zero-point by roughly 2 magnitudes. Deep, wide-field, near-IR selected
surveys will provide more secure measurements to compare with our predictions.Comment: 4 pages, 2 figures. Submitted to ApJ Letter
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
Physical Bias of Galaxies From Large-Scale Hydrodynamic Simulations
We analyze a new large-scale (Mpc) numerical hydrodynamic
simulation of the popular CDM cosmological model, including in our
treatment dark matter, gas and star-formation, on the basis of standard
physical processes. The method, applied with a numerical resolution of
kpc (which is still quite coarse for following individual galaxies,
especially in dense regions), attempts to estimate where and when galaxies
form. We then compare the smoothed galaxy distribution with the smoothed mass
distribution to determine the "bias" defined as on scales large compared with the code
numerical resolution (on the basis of resolution tests given in the appendix of
this paper). We find that (holding all variables constant except the quoted
one) bias increases with decreasing scale, with increasing galactic age or
metallicity and with increasing redshift of observations. At the Mpc
fiducial comoving scale bias (for bright regions) is 1.35 at reaching to
3.6 at , both numbers being consistent with extant observations. We also
find that Mpc voids in the distribution of luminous objects are
as observed (i.e., observed voids are not an argument against CDM-like models)
and finally that the younger systems should show a colder Hubble flow than do
the early type galaxies (a testable proposition). Surprisingly, little
evolution is found in the amplitude of the smoothed galaxy-galaxy correlation
function (as a function of {\it comoving} separation). Testing this prediction
vs observations will allow a comparison between this work and that of Kauffmann
et al which is based on a different physical modelingmethod.Comment: in press, ApJ, 26 latex pages plus 7 fig
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
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
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