20,509 research outputs found
Clonal analysis of meningococci during a 26 year period prior to the introduction of meningococcal serogroup C vaccines
Meningococcal disease remains a public health burden in the UK and elsewhere. Invasive Neisseria meningitidis, isolated in Scotland between 1972 and 1998, were characterised retrospectively to examine the serogroup and clonal structure of the circulating population. 2607 isolates causing invasive disease were available for serogroup and MLST analysis whilst 2517 were available for multilocus sequence typing (MLST) analysis only. Serogroup distribution changed from year to year but serogroups B and C were dominant throughout. Serogroup B was dominant throughout the 1970s and early 1980s until serogroup C became dominant during the mid-1980s. The increase in serogroup C was not associated with one particular sequence type (ST) but was associated with a number of STs, including ST-8, ST-11, ST-206 and ST-334. This is in contrast to the increase in serogroup C disease seen in the 1990s that was due to expansion of the ST-11 clonal complex. While there was considerable diversity among the isolates (309 different STs among the 2607 isolates), a large proportion of isolates (59.9%) were associated with only 10 STs. These data highlight meningococcal diversity over time and the need for ongoing surveillance during the introduction of new meningococcal vaccines
Cosmic Variance In the Transparency of the Intergalactic Medium After Reionization
Following the completion of cosmic reionization, the mean-free-path of
ionizing photons was set by a population of Ly-limit absorbers. As the
mean-free-path steadily grew, the intensity of the ionizing background also
grew, thus lowering the residual neutral fraction of hydrogen in ionization
equilibrium throughout the diffuse intergalactic medium (IGM). Ly-alpha photons
provide a sensitive probe for tracing the distribution of this residual
hydrogen at the end of reionization. Here we calculate the cosmic variance
among different lines-of-sight in the distribution of the mean Ly-alpha optical
depths. We find fractional variations in the effective post-reionization
optical depth that are of order unity on a scale of ~100 co-moving Mpc, in
agreement with observations towards high-redshift quasars. Significant
contributions to these variations are provided by the cosmic variance in the
density contrast on the scale of the mean-free-path for ionizing photons, and
by fluctuations in the ionizing background induced by delayed or enhanced
structure formation. Cosmic variance results in a highly asymmetric
distribution of transmission through the IGM, with fractional fluctuations in
Ly-alpha transmission that ar larger than in Ly-beta transmission.Comment: 7 pages 3 figures. Replaced with version accepted for publication in
Ap
Self-Regulated Growth of Supermassive Black Holes in Galaxies as the Origin of the Optical and X-ray Luminosity Functions of Quasars
We postulate that supermassive black-holes grow in the centers of galaxies
until they unbind the galactic gas that feeds them. We show that the
corresponding self-regulation condition yields a correlation between black-hole
mass (Mbh) and galaxy velocity dispersion (sigma) as inferred in the local
universe, and recovers the observed optical and X-ray luminosity functions of
quasars at redshifts up to z~6 based on the hierarchical evolution of galaxy
halos in a Lambda-CDM cosmology. With only one free parameter and a simple
algorithm, our model yields the observed evolution in the number density of
optically bright or X-ray faint quasars between 2<z<6 across 3 orders of
magnitude in bolometric luminosity and 3 orders of magnitude in comoving
density per logarithm of luminosity. The self-regulation condition identifies
the dynamical time of galactic disks during the epoch of peak quasar activity
(z~2.5) as the origin of the inferred characteristic quasar lifetime of ~10
million years. Since the lifetime becomes comparable to the Salpeter e-folding
time at this epoch, the model also implies that the Mbh-sigma relation is a
product of feedback regulated accretion during the peak of quasar activity. The
mass-density in black-holes accreted by that time is consistent with the local
black-hole mass density of ~(0.8-6.3) times 10^5 solar masses per cubic Mpc,
which we have computed by combining the Mbh-sigma relation with the measured
velocity dispersion function of SDSS galaxies (Sheth et al.~2003). Applying a
similar self-regulation principle to supernova-driven winds from starbursts, we
find that the ratio between the black hole mass and the stellar mass of
galactic spheroids increases with redshift as (1+z)^1.5 although the Mbh-sigma
relation is redshift-independent.Comment: 10 pages, 5 figures, submitted to Ap
A Non-Cooperative Power Control Game for Multi-Carrier CDMA Systems
In this work, a non-cooperative power control game for multi-carrier CDMA
systems is proposed. In the proposed game, each user needs to decide how much
power to transmit over each carrier to maximize its overall utility. The
utility function considered here measures the number of reliable bits
transmitted per joule of energy consumed. It is shown that the user's utility
is maximized when the user transmits only on the carrier with the best
"effective channel". The existence and uniqueness of Nash equilibrium for the
proposed game are investigated and the properties of equilibrium are studied.
Also, an iterative and distributed algorithm for reaching the equilibrium (if
it exists) is presented. It is shown that the proposed approach results in a
significant improvement in the total utility achieved at equilibrium compared
to the case in which each user maximizes its utility over each carrier
independently.Comment: To appear in Proceedings of the 2005 IEEE Wireless Communications and
Networking Conference, New Orleans, LA, March 13 - 17, 200
Collapse of Uniformly Rotating Stars to Black Holes and the Formation of Disks
Simulations in general relativity show that the outcome of collapse of a
marginally unstable, uniformly rotating star spinning at the mass-shedding
limit depends critically on the equation of state. For a very stiff equation of
state, which is likely to characterize a neutron star, essentially all of the
mass and angular momentum of the progenitor are swallowed by the Kerr black
hole formed during the collapse, leaving nearly no residual gas to form a disk.
For a soft equation of state with an adiabatic index \Gamma - 4/3 << 1, which
characterizes a very massive or supermassive star supported predominantly by
thermal radiation pressure, as much as 10% of the mass of the progenitor avoids
capture and goes into a disk about the central hole. We present a semi-analytic
calculation that corroborates these numerical findings and shows how the final
outcome of such a collapse may be determined from simple physical
considerations. In particular, we employ a simple energy variational principle
with an approximate, post-Newtonian energy functional to determine the
structure of a uniformly rotating, polytropic star at the onset of collapse as
a function of polytropic index n, where \Gamma = 1+1/n. We then use this data
to calculate the mass and spin of the final black hole and ambient disk. We
show that the fraction of the total mass that remains in the disk falls off
sharply as 3-n (equivalently, \Gamma - 4/3) increases.Comment: 11 pages, 2 figures, 2 tables, AASTeX; accepted to appear in The
Astrophysical Journa
- …