3,982 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
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
The Backstroke framework for source level reverse computation applied to parallel discrete event simulation
This report introduces Backstroke, a new open source framework for the automatic generation of reverse code for functions written in C++. Backstroke enables reverse computation for optimistic parallel discrete event simulations. It is built over the ROSE open- source compiler infrastructure, and handles complex C++ features including pointers and pointer types, arrays, function and method calls, class types. inheritance, polymorphism, virtual functions, abstract classes, templated classes and containers. Backstroke also introduces new program inversion techniques based on advanced compiler analysis tools built into ROSE. We explore and illustrate some of the complex language and semantic issues that arise in generating correct reverse code for C++ functions
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
Sub-femtosecond absolute timing precision with a 10 GHz hybrid photonic-microwave oscillator
We present an optical-electronic approach to generating microwave signals
with high spectral purity. By circumventing shot noise and operating near
fundamental thermal limits, we demonstrate 10 GHz signals with an absolute
timing jitter for a single hybrid oscillator of 420 attoseconds (1Hz - 5 GHz)
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
Inducing safer oblique trees without costs
Decision tree induction has been widely studied and applied. In safety applications, such as determining whether a chemical process is safe or whether a person has a medical condition, the cost of misclassification in one of the classes is significantly higher than in the other class. Several authors have tackled this problem by developing cost-sensitive decision tree learning algorithms or have suggested ways of changing the
distribution of training examples to bias the decision tree learning process so as to take account of costs. A prerequisite for applying such algorithms is the availability of costs of misclassification.
Although this may be possible for some applications, obtaining reasonable estimates of costs of misclassification is not easy in the area of safety.
This paper presents a new algorithm for applications where the cost of misclassifications cannot be quantified, although the cost of misclassification in one class is known to be significantly higher than in another class. The algorithm utilizes linear discriminant analysis to identify oblique relationships between continuous attributes and then carries out an appropriate modification to ensure that the resulting tree errs on the side of safety. The algorithm is evaluated with respect to one of the best known cost-sensitive algorithms (ICET), a well-known oblique decision tree algorithm (OC1) and an algorithm that utilizes robust linear programming
Collisional Dark Matter and the Origin of Massive Black Holes
If the cosmological dark matter is primarily in the form of an elementary
particle which has cross section and mass for self-interaction having a ratio
similar to that of ordinary nuclear matter, then seed black holes (formed in
stellar collapse) will grow in a Hubble time, due to accretion of the dark
matter, to a mass range 10^6 - 10^9 solar masses. Furthermore, the dependence
of the final black hole mass on the galaxy velocity dispersion will be
approximately as observed and the growth rate will show a time dependence
consistent with observations. Other astrophysical consequences of collisional
dark matter and tests of the idea are noted.Comment: 7 pages, no figures, LaTeX2e, Accepted for publication in Phys. Rev.
Lett. Changed conten
Infrared conductivity of a d_{x^2-y^2}-wave superconductor with impurity and spin-fluctuation scattering
Calculations are presented of the in-plane far-infrared conductivity of a
d_{x^2-y^2}-wave superconductor, incorporating elastic scattering due to
impurities and inelastic scattering due to spin fluctuations. The impurity
scattering is modeled by short-range potential scattering with arbitrary phase
shift, while scattering due to spin fluctuations is calculated within a
weak-coupling Hubbard model picture. The conductivity is characterized by a
low-temperature residual Drude feature whose height and weight are controlled
by impurity scattering, as well as a broad peak centered at 4 Delta_0 arising
from clean-limit inelastic processes. Results are in qualitative agreement with
experiment despite missing spectral weight at high energies.Comment: 29 pages (11 tar-compressed-uuencoded Postscript figures), REVTeX 3.0
with epsf macro
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