112,818 research outputs found
The galaxy halo formation rate
The rate at which galaxy halos form is thought to play a key role in
explaining many observable cosmological phenomena such as the initial epoch at
which luminous matter forms and the distribution of active galaxies. Here we
show how Press-Schechter theory can be used to provide a simple, completely
analytic model of the halo formation rate. This model shows good agreement with
both Monte-Carlo and N-body simulation results.Comment: 2 pages, 1 figure, to appear in proceedings of the Xth Recontres de
Blois, "The Birth of Galaxies," LaTeX style file include
Cosmological evolution and hierarchical galaxy formation
We provide a new multi-waveband compilation of the data describing the
cosmological evolution of quasars, and discuss a model that attributes the
evolution to variation in the rate of merging between dark halos in a
hierarchical universe. We present a new Press-Schechter calculation of the
expected merger rate and show that this can reproduce the principal features of
the evolution. We also show that the evolution in the star-formation history of
the universe is well-described by this model.Comment: 4 pages, 1 figure. Presented at Xth Rencontres de Blois, "The Birth
of Galaxies", June 199
The Halo Formation Rate and its link to the Global Star Formation Rate
The star formation history of the universe shows strong evolution with
cosmological epoch. Although we know mergers between galaxies can cause
luminous bursts of star formation, the relative importance of such mergers to
the global star formation rate (SFR) is unknown. We present a simple analytic
formula for the rate at which halos merge to form higher-mass systems, derived
from Press-Schechter theory and confirmed by numerical simulations (for high
halo masses). A comparison of the evolution in halo formation rate with the
observed evolution in the global SFR indicates that the latter is largely
driven by halo mergers at z>1. Recent numerical simulations by Kolatt et al.
(1999) and Knebe & Muller (1999) show how merging systems are strongly biased
tracers of mass fluctuations, thereby explaining the strong clustering observed
for Lyman-break galaxies without any need to assume that Lyman-break galaxies
are associated only with the most massive systems at z~3.Comment: 4 pages, 2 figures. To appear in `The Hy-redshift universe: Galaxy
formation and evolution at high redshift' eds. A.J. Bunker and W.J.M. van
Breuge
Long-term energy capture and the effects of optimizing wind turbine operating strategies
Methods of increasing energy capture without affecting the turbine design were investigated. The emphasis was on optimizing the wind turbine operating strategy. The operating strategy embodies the startup and shutdown algorithm as well as the algorithm for determining when to yaw (rotate) the axis of the turbine more directly into the wind. Using data collected at a number of sites, the time-dependent simulation of a MOD-2 wind turbine using various, site-dependent operating strategies provided evidence that site-specific fine tuning can produce significant increases in long-term energy capture as well as reduce the number of start-stop cycles and yawing maneuvers, which may result in reduced fatigue and subsequent maintenance
The design and development of a solar tracking unit
The solar tracking unit was developed to support the Laser Heterodyne Spectrometer (LHS) airborne instrument, but has application to a general class of airborne solar occultation research instruments. The unit consists of a mirror mounted on two gimbals, one of which is hollow. The mirror reflects a 7.6 cm (3.0 in.) diameter beam of sunlight through the hollow gimbal into the research instrument optical axis. A portion of the reflected sunlight is directed into a tracking telescope which uses a four quadrant silicon detector to produce the servo error signals. The colinearity of the tracker output beam and the research instrument optical axis is maintained to better than + or - 1 arc-minute. The unit is microcomputer controlled and is capable of stand alone operation, including automatic Sun acquisition or operation under the control of the research instrument
An analytic model for the epoch of halo creation
In this paper we describe the Bayesian link between the cosmological mass
function and the distribution of times at which isolated halos of a given mass
exist. By assuming that clumps of dark matter undergo monotonic growth on the
time-scales of interest, this distribution of times is also the distribution of
`creation' times of the halos. This monotonic growth is an inevitable aspect of
gravitational instability. The spherical top-hat collapse model is used to
estimate the rate at which clumps of dark matter collapse. This gives the prior
for the creation time given no information about halo mass. Applying Bayes'
theorem then allows any mass function to be converted into a distribution of
times at which halos of a given mass are created. This general result covers
both Gaussian and non-Gaussian models. We also demonstrate how the mass
function and the creation time distribution can be combined to give a joint
density function, and discuss the relation between the time distribution of
major merger events and the formula calculated. Finally, we determine the
creation time of halos within three N-body simulations, and compare the link
between the mass function and creation rate with the analytic theory.Comment: 7 pages, 2 figures, submitted to MNRA
Life history and catch analysis of the Blue Rockfish (Sebastes mystinus) off central California- 1961-1965
A New S-S' Pair Creation Rate Expression Improving Upon Zener Curves for I-E Plots
To simplify phenomenology modeling used for charge density wave
(CDW)transport, we apply a wavefunctional formulation of tunneling Hamiltonians
to a physical transport problem characterized by a perturbed washboard
potential. To do so, we consider tunneing between states that are
wavefunctionals of a scalar quantum field. I-E curves that match Zener curves -
used to fit data experimentally with wavefunctionals congruent with the false
vacuum hypothesis. This has a very strong convergence with electron-positron
pair production representations.The similarities in plot behavior of the
current values after the threshold electric field values argue in favor of the
Bardeen pinning gap paradigm proposed for quasi-one-dimensional metallic
transport problems.Comment: 22 pages,6 figures, and extensive editing of certain segments.Paper
has been revised due to acceptance by World press scientific MPLB journal.
This is word version of file which has been submitted to MPLBs editor for
final proofing. Due for publication perhaps in mid spring to early summer
200
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