3,605 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
Decoherence of quantum wavepackets due to interaction with conformal spacetime fluctuations
One of the biggest problems faced by those attempting to combine quantum
theory and general relativity is the experimental inaccessibility of the
unification scale. In this paper we show how incoherent conformal waves in the
gravitational field, which may be produced by quantum mechanical zero-point
fluctuations, interact with the wavepackets of massive particles. The result of
this interaction is to produce decoherence within the wavepackets which could
be accessible in experiments at the atomic scale.
Using a simple model for the coherence properties of the gravitational field
we derive an equation for the evolution of the density matrix of such a
wavepacket. Following the primary state diffusion programme, the most promising
source of spacetime fluctuations for detection are the above zero-point energy
fluctuations. According to our model, the absence of intrinsic irremoveable
decoherence in matter interferometry experiments puts bounds on some of the
parameters of quantum gravity theories. Current experiments give \lambda > 18.
, where \lambda t_{Planck} is an effective cut-off for the validity of
low-energy quantum gravity theories.Comment: REVTeX forma
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
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
Censusing manatees: a report on the feasibility of using aerial surveys and mark and recapture techniques to conduct a population survey of the West Indian Manatee
This report results from an invitation to review the needs and
prospects for capture-recapture and aerial census studies of the
manatee (Trichechus manatus) in Florida. Three aerial reconnaissance flights provided a
first hand view of manatee habitats, as follows: May 3, Suwannee
River to Kings Bay and Crystal River (Rathbun, Eberhardt), May 4,
Vero Beach to Ft. Lauderdale and Ft. Myers by way of Whitewater Bay
(Rose, Percival, Eberhardt), and May 5, Cape Canaveral to Jacksonville,
St. Johns River and Blue Spring (Rose, Kinnaird, Eberhardt). (24 page document
Conceptual model of salt marsh management on Merritt Island National Wildlife Refuge, Florida: final report
Diking and holding water on salt marshes ("impounding" the marsh) is a
management technique used on Merritt Island National Wildlife Refuge
(MINWR) and elsewhere in the Southeast to: a) prevent the reproduction of
saltmarsh mosquitos, and b) attract wintertering waterfowl and other marsh,
shore, and wading birds. Because of concern that diking and holding water
may interfere with the production of estuarine fish and shellfish,
impoundment managers are being asked to consider altering management
protocol to reduce or eliminate any such negative influence. How to change
protocol and preserve effective mosquito control and wildlife management is
a decision of great complexity because: a) the relationships between
estuarine organisms and the fringing salt marshes at the land-water interface
are complex, and b) impounded marshes are currently good habitat for a
variety of species of fish and wildlife. Most data collection by scientists
and managers in the area has not been focused on this particular
problem. Furthermore, collection of needed data may not be possible before
changes in protocol are demanded. Therefore, the purpose of this document
is two-fold: 1) to suggest management alternatives, given existing information,
and 2) to help identify research needs that have a high probability
of leading to improved simultaneous management of mosquitos, waterfowl,
other wildlife, freshwater fish, and estuarine fish and shellfish on the
marshland of the Merritt Island National Wildlife Refuge. (92 page document
Dairy, Democracy, & PR: A Political Economic Analysis of Associated Milk Producers, Inc. 1988-1989
This paper, using a political economic approach, explores the communication techniques used during a second-wave of consolidation in the dairy industry in the 1980s. After providing a historical context of the dairy industry and its connections with federal policy, this paper follows the story of a large dairy cooperative: Association Milk Producers, Inc. (AMPI) and how it influenced public policy and consumption through interconnected dairy organizations like the National Milk Producers Federation and by using a political action committee. This paper provides an example of Carey\u27s (1997) treetops propaganda by way of the powerful political action committee C-TAPE and the AMPI women. The government influence in regulating the production and consumption of dairy dispels the myth of the free market
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