4,431 research outputs found
The Merging History of Massive Black Holes
We investigate a hierarchical structure formation scenario describing the
evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the
local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first
POPIII stars. As these pregalactic holes become incorporated through a series
of mergers into larger and larger halos, they sink to the center owing to
dynamical friction, accrete a fraction of the gas in the merger remnant to
become supermassive, form a binary system, and eventually coalesce. A simple
model in which the damage done to a stellar cusps by decaying BH pairs is
cumulative is able to reproduce the observed scaling relation between galaxy
luminosity and core size. An accretion model connecting quasar activity with
major mergers and the observed BH mass-velocity dispersion correlation
reproduces remarkably well the observed luminosity function of
optically-selected quasars in the redshift range 1<z<5. We finally asses the
potential observability of the gravitational wave background generated by the
cosmic evolution of SMBH binaries by the planned space-born interferometer
LISA.Comment: 4 pages, 2 figures, Contribute to "Multiwavelength Cosmology",
Mykonos, Greece, June 17-20, 200
A Non-Sequential Representation of Sequential Data for Churn Prediction
We investigate the length of event sequence giving best predictions
when using a continuous HMM approach to churn prediction from sequential
data. Motivated by observations that predictions based on only the few most recent
events seem to be the most accurate, a non-sequential dataset is constructed
from customer event histories by averaging features of the last few events. A simple
K-nearest neighbor algorithm on this dataset is found to give significantly
improved performance. It is quite intuitive to think that most people will react
only to events in the fairly recent past. Events related to telecommunications occurring
months or years ago are unlikely to have a large impact on a customer’s
future behaviour, and these results bear this out. Methods that deal with sequential
data also tend to be much more complex than those dealing with simple nontemporal
data, giving an added benefit to expressing the recent information in a
non-sequential manner
Real Lives II: findings from the All-Ireland Gay Men’s Sex Surveys, 2005 and 2006
All Ireland Gay Men's Sex Survey (Vital Statistics)
Duration: March 2000 - September 2010
Sigma Research has been working with Ireland's Gay Health Network (GHN) since 2000. GHN is an umbrella organisation working towards gay men's health and HIV prevention. GHN instigated a community-based, self-completion survey to take place across The Republic of Ireland and Northern Ireland during the summer of 2000 and commissioned Sigma Research to work with them. This large-scale community research project was the third such survey among gay men in Ireland, and built on previous findings.
After the development and piloting of the survey, recruitment commenced at Dublin Pride in June 2000 and continued throughout the summer at similar events in Belfast, Derry, Galway, Limerick and Waterford. Recruitment in bars and clubs took place in Dublin and Cork, and social groups in more rural area were sent copies of the questionnaire and a request to distribute them to their members. 1,290 questionnaires were returned by gay men (81%), bisexual men (11%) and other homosexually active men living in Ireland. 19% of all respondents lived in Northern Ireland. A full survey report, including implications for HIV prevention planning is available to download.
Since 2003 Gay Health Network members - particularly The Gay Men's Health Service (Health Services Executive) and the Rainbow Project, Northern Ireland - have collaborated with our online UK version of the Gay Men’s Sex Survey (Vital Statistics) by promoting it to men in Ireland via community websites and postcards distributed on the gay scene
A Hybrid N-body--Coagulation Code for Planet Formation
We describe a hybrid algorithm to calculate the formation of planets from an
initial ensemble of planetesimals. The algorithm uses a coagulation code to
treat the growth of planetesimals into oligarchs and explicit N-body
calculations to follow the evolution of oligarchs into planets. To validate the
N-body portion of the algorithm, we use a battery of tests in planetary
dynamics. Several complete calculations of terrestrial planet formation with
the hybrid code yield good agreement with previously published calculations.
These results demonstrate that the hybrid code provides an accurate treatment
of the evolution of planetesimals into planets.Comment: Astronomical Journal, accepted; 33 pages + 11 figure
A New Family of Multistep Methods with Improved Phase Lag Characteristics for the Integration of Orbital Problems
In this work we introduce a new family of ten-step linear multistep methods
for the integration of orbital problems. The new methods are constructed by
adopting a new methodology which improves the phase lag characteristics by
vanishing both the phase lag function and its first derivatives at a specific
frequency. The efficiency of the new family of methods is proved via error
analysis and numerical applications.Comment: 21 pages, 3 figures, 1 tabl
An intelligent assistant for exploratory data analysis
In this paper we present an account of the main features of SNOUT, an intelligent assistant for exploratory data analysis (EDA) of social science survey data that incorporates a range of data mining techniques. EDA has much in common with existing data mining techniques: its main objective is to help an investigator reach an understanding of the important relationships ina data set rather than simply develop predictive models for selectd variables. Brief descriptions of a number of novel techniques developed for use in SNOUT are presented. These include heuristic variable level inference and classification, automatic category formation, the use of similarity trees to identify groups of related variables, interactive decision tree construction and model selection using a genetic algorithm
Dynamical Evolution of Elliptical Galaxies with Central Singularities
We study the effect of a massive central singularity on the structure of a
triaxial galaxy using N-body simulations. Starting from a single initial model,
we grow black holes with various final masses Mh and at various rates, ranging
from impulsive to adiabatic. In all cases, the galaxy achieves a final shape
that is nearly spherical at the center and close to axisymmetric throughout.
However, the rate of change of the galaxy's shape depends strongly on the ratio
Mh/Mg of black hole mass to galaxy mass. When Mh/Mg < 0.3%, the galaxy evolves
in shape on a timescale that exceeds 100 orbital periods, or roughly a galaxy
lifetime. When Mh/Mg > 2%, the galaxy becomes axisymmetric in little more than
a crossing time. We propose that the rapid evolution toward axisymmetric shapes
that occurs when Mh/Mg > 2% provides a negative feedback mechanism which limits
the mass of central black holes by cutting off their supply of fuel.Comment: 27 Latex pages, 9 Postscript figures, uses aastex.sty. Accepted for
Publication in The Astrophysical Journal, Nov. 26, 199
Robust Machine Learning Applied to Astronomical Datasets I: Star-Galaxy Classification of the SDSS DR3 Using Decision Trees
We provide classifications for all 143 million non-repeat photometric objects
in the Third Data Release of the Sloan Digital Sky Survey (SDSS) using decision
trees trained on 477,068 objects with SDSS spectroscopic data. We demonstrate
that these star/galaxy classifications are expected to be reliable for
approximately 22 million objects with r < ~20. The general machine learning
environment Data-to-Knowledge and supercomputing resources enabled extensive
investigation of the decision tree parameter space. This work presents the
first public release of objects classified in this way for an entire SDSS data
release. The objects are classified as either galaxy, star or nsng (neither
star nor galaxy), with an associated probability for each class. To demonstrate
how to effectively make use of these classifications, we perform several
important tests. First, we detail selection criteria within the probability
space defined by the three classes to extract samples of stars and galaxies to
a given completeness and efficiency. Second, we investigate the efficacy of the
classifications and the effect of extrapolating from the spectroscopic regime
by performing blind tests on objects in the SDSS, 2dF Galaxy Redshift and 2dF
QSO Redshift (2QZ) surveys. Given the photometric limits of our spectroscopic
training data, we effectively begin to extrapolate past our star-galaxy
training set at r ~ 18. By comparing the number counts of our training sample
with the classified sources, however, we find that our efficiencies appear to
remain robust to r ~ 20. As a result, we expect our classifications to be
accurate for 900,000 galaxies and 6.7 million stars, and remain robust via
extrapolation for a total of 8.0 million galaxies and 13.9 million stars.
[Abridged]Comment: 27 pages, 12 figures, to be published in ApJ, uses emulateapj.cl
Swift J164449.3+573451 event: generation in the collapsing star cluster?
We discuss the multiband energy release in a model of a collapsing galactic
nucleus, and we try to interpret the unique super-long cosmic gamma-ray event
Swift J164449.3+573451 (GRB 110328A by early classification) in this scenario.
Neutron stars and stellar-mass black holes can form evolutionary a compact
self-gravitating subsystem in the galactic center. Collisions and merges of
these stellar remnants during an avalanche contraction and collapse of the
cluster core can produce powerful events in different bands due to several
mechanisms. Collisions of neutron stars and stellar-mass black holes can
generate gamma-ray bursts (GRBs) similar to the ordinary models of short GRB
origin. The bright peaks during the first two days may also be a consequence of
multiple matter supply (due to matter release in the collisions) and accretion
onto the forming supermassive black hole. Numerous smaller peaks and later
quasi-steady radiation can arise from gravitational lensing, late accretion of
gas onto the supermassive black hole, and from particle acceleration by shock
waves. Even if this model will not reproduce exactly all the Swift
J164449.3+573451 properties in future observations, such collapses of galactic
nuclei can be available for detection in other events.Comment: 7 pages, replaced by the final versio
An Extensible Open-Source Compiler Infrastructure for Testing
Testing forms a critical part of the development process for large-scale software, and there is growing need for automated tools that can read, represent, analyze, and transform the application's source code to help carry out testing tasks. However, the support required to compile applications written in common general purpose languages is generally inaccessible to the testing research community. In this paper, we report on an extensible, open-source compiler infrastructure called ROSE, which is currently in development at Lawrence Livermore National Laboratory. ROSE specifically targets developers who wish to build source-based tools that implement customized analyses and optimizations for large-scale C, C++, and Fortran90 scientific computing applications (on the order of a million lines of code or more). However, much of this infrastructure can also be used to address problems in testing, and ROSE is by design broadly accessible to those without a formal compiler background. This paper details the interactions between testing of applications and the ways in which compiler technology can aid in the understanding of those applications. We emphasize the particular aspects of ROSE, such as support for the general analysis of whole programs, that are particularly well-suited to the testing research community and the scale of the problems that community solves
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