431 research outputs found
Social Links from Latent Topics in Microblogs
Language use is overlaid on a network of social con-nections, which exerts an influence on both the top-ics of discussion and the ways that these topics can be expressed (Halliday, 1978). In the past, efforts t
Diffusion of Lexical Change in Social Media
Computer-mediated communication is driving fundamental changes in the nature
of written language. We investigate these changes by statistical analysis of a
dataset comprising 107 million Twitter messages (authored by 2.7 million unique
user accounts). Using a latent vector autoregressive model to aggregate across
thousands of words, we identify high-level patterns in diffusion of linguistic
change over the United States. Our model is robust to unpredictable changes in
Twitter's sampling rate, and provides a probabilistic characterization of the
relationship of macro-scale linguistic influence to a set of demographic and
geographic predictors. The results of this analysis offer support for prior
arguments that focus on geographical proximity and population size. However,
demographic similarity -- especially with regard to race -- plays an even more
central role, as cities with similar racial demographics are far more likely to
share linguistic influence. Rather than moving towards a single unified
"netspeak" dialect, language evolution in computer-mediated communication
reproduces existing fault lines in spoken American English.Comment: preprint of PLOS-ONE paper from November 2014; PLoS ONE 9(11) e11311
TopicViz: Semantic Navigation of Document Collections
When people explore and manage information, they think in terms of topics and
themes. However, the software that supports information exploration sees text
at only the surface level. In this paper we show how topic modeling -- a
technique for identifying latent themes across large collections of documents
-- can support semantic exploration. We present TopicViz, an interactive
environment for information exploration. TopicViz combines traditional search
and citation-graph functionality with a range of novel interactive
visualizations, centered around a force-directed layout that links documents to
the latent themes discovered by the topic model. We describe several use
scenarios in which TopicViz supports rapid sensemaking on large document
collections
Economic analysis of endovascular repair versus surveillance for patients with small abdominal aortic aneurysms
BackgroundThe Positive Impact of EndoVascular Options for Treating Aneurysms Early (PIVOTAL) trial enrolled individuals with small (4.0- to 5.0-cm diameter) abdominal aortic aneurysms (AAA) and reported no difference in rupture or aneurysm-related death for patients who received early endovascular repair (EVAR) vs surveillance with serial imaging studies. We evaluated resource use, medical cost, and quality of life outcomes associated with the PIVOTAL treatment strategies.MethodsThis prospective economic and quality of life study was conducted within a randomized trial, with PIVOTAL sites participating in the quality of life (n = 67) and economic (n = 63) studies. The PIVOTAL trial randomized 728 patients (366 early EVAR and 362 surveillance). We used information from 701 quality of life (351 early EVAR and 350 surveillance) and 614 economic (314 early EVAR and 300 surveillance) study participants enrolled in the PIVOTAL trial. The main outcome measures were total medical costs and the aneurysm repair rate at 48 months.ResultsAfter 6 months, the rate of aneurysm repair was 96 vs 10 per 100 patients in the early EVAR and surveillance groups, respectively (difference, 86; 95% confidence interval [CI], 82-90; P < .0001), and total medical costs were greater in the early EVAR group (5520; difference, 25,156-40,592 vs 25,394; 95% CI, 35,605; P < .0001). At 48 months' follow-up, early EVAR patients had greater cumulative use of AAA repair (97 vs 64 per 100 patients; difference, 34; 95% CI, 21-46; P < .0001), but there was no difference in total medical costs (46,112; difference, 8043 to 47,765 vs 5561 to $14,025; P = .40). There were no treatment-related differences in quality of life at 24 months.ConclusionsA treatment strategy involving early repair of smaller AAA with EVAR is associated with no difference in total medical costs at 48 months vs surveillance with serial imaging studies. Longer follow-up is required to determine whether the late medical cost increases observed for surveillance will persist beyond 48 months
Structure and Turbulence in Simulated Galaxy Clusters and the Implications for the Formation of Radio Halos
We track the histories of massive clusters of galaxies formed within a
cosmological hydrodynamic simulation. Specifically, we track the time evolution
of the energy in random bulk motions of the intracluster medium and X-ray
measures of cluster structure and their relationship to cluster mergers. We aim
to assess the viability of the turbulent re-acceleration model for the
generation of giant radio halos by comparing the level of turbulent kinetic
energy in simulated clusters with the observed properties of radio halo
clusters, giving particular attention to the association of radio halos to
clusters with disturbedX-ray structures. The evolution of X-ray cluster
structure and turbulence kinetic energy, k, in simulations can then inform us
about the expected lifetime of radio halos and the fraction of clusters as a
function of redshift expected to host them. We find strong statistical
correlation of disturbed structure measures and the presence of enhancements in
k. Specifically, quantitatively "disturbed", radio halo-like X-ray morphology
in our sample indicates a 92% chance of the cluster in question having k
elevated to more than twice its minimum value over the cluster's life. The
typical lifetime of episodes of elevated turbulence is on the order of 1 Gyr,
though these periods can last 5 Gyrs or more. This variation reflects the wide
range of cluster histories; while some clusters undergo complex and repeated
mergers spending a majority of their time in elevated k states, other clusters
are relaxed over nearly their entire history. We do not find a bimodal
relationship between cluster X-ray luminosity and the total energy in
turbulence that might account directly for a bimodal L_X-P_{1.4 GHz} relation.
However, our result may be consistent with the observed bimodality, as here we
are not including a full treatment of cosmic rays sources and magnetic fields.Comment: 15 pages, 12 figures, MNRAS Submitte
Testing Standard Cosmology with Large Scale Structure
The galaxy power spectrum contains information on the growth of structure,
the growth rate through redshift space distortions, and the cosmic expansion
through baryon acoustic oscillation features. We study the ability of two
proposed experiments, BigBOSS and JDEM-PS, to test the cosmological model and
general relativity. We quantify the latter result in terms of the gravitational
growth index \gamma, whose value in general relativity is \gamma\approx 0.55.
Significant deviations from this value could indicate new physics beyond the
standard model of cosmology. The results show that BigBOSS (JDEM-PS) would be
capable of measuring \gamma with an uncertainty \sigma(\gamma) = 0.043 (0.054),
which tightens to \sigma(\gamma) = 0.031 (0.038) if we include Stage III data
priors, marginalizing over neutrino mass, time varying dark energy equation of
state, and other parameters. For all dark energy parameters and related figures
of merit the two experiments give comparable results. We also carry out some
studies of the influence of redshift range, resolution, treatment of
nonlinearities, and bias evolution to enable further improvement.Comment: 9 pages, 12 tables, 1 figure; v3 matches MNRAS accepted versio
The Nature of the Warm/Hot Intergalactic Medium I. Numerical Methods, Convergence, and OVI Absorption
We perform a series of cosmological simulations using Enzo, an Eulerian
adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the
warm/hot intergalactic medium. The WHIM may be an important component of the
baryons missing observationally at low redshift. We investigate the dependence
of the global star formation rate and mass fraction in various baryonic phases
on spatial resolution and methods of incorporating stellar feedback. Although
both resolution and feedback significantly affect the total mass in the WHIM,
all of our simulations find that the WHIM fraction peaks at z ~ 0.5, declining
to 35-40% at z = 0. We construct samples of synthetic OVI absorption lines from
our highest-resolution simulations, using several models of oxygen ionization
balance. Models that include both collisional ionization and photoionization
provide excellent fits to the observed number density of absorbers per unit
redshift over the full range of column densities (10^13 cm-2 <= N_OVI <= 10^15
cm^-2). Models that include only collisional ionization provide better fits for
high column density absorbers (N_OVI > 10^14 cm^-2). The distribution of OVI in
density and temperature exhibits two populations: one at T ~ 10^5.5 K
(collisionally ionized, 55% of total OVI) and one at T ~ 10^4.5 K
(photoionized, 37%) with the remainder located in dense gas near galaxies.
While not a perfect tracer of hot gas, OVI provides an important tool for a
WHIM baryon census.Comment: 22 pages, 21 figures, emulateapj, accepted for publication in Ap
The Santa Fe Light Cone Simulation Project: II. The Prospects for Direct Detection of the WHIM with SZE Surveys
Detection of the Warm-Hot Intergalactic Medium (WHIM) using Sunyaev-Zeldovich
effect (SZE) surveys is an intriguing possibility, and one that may allow
observers to quantify the amount of "missing baryons" in the WHIM phase. We
estimate the necessary sensitivity for detecting low density WHIM gas with the
South Pole Telescope (SPT) and Planck Surveyor for a synthetic 100 square
degree sky survey. This survey is generated from a very large, high dynamic
range adaptive mesh refinement cosmological simulation performed with the Enzo
code. We find that for a modest increase in the SPT survey sensitivity (a
factor of 2-4), the WHIM gas makes a detectable contribution to the integrated
sky signal. For a Planck-like satellite, similar detections are possible with a
more significant increase in sensitivity (a factor of 8-10). We point out that
for the WHIM gas, the kinematic SZE signal can sometimes dominate the thermal
SZE where the thermal SZE decrement is maximal (150 GHz), and that using the
combination of the two increases the chance of WHIM detection using SZE
surveys. However, we find no evidence of unique features in the thermal SZE
angular power spectrum that may aid in its detection. Interestingly, there are
differences in the power spectrum of the kinematic SZE, which may not allow us
to detect the WHIM directly, but could be an important contaminant in
cosmological analyses of the kSZE-derived velocity field. Corrections derived
from numerical simulations may be necessary to account for this contamination.Comment: 9 pages, submitted to Astrophysical Journa
The Properties of X-ray Cold Fronts in a Statistical Sample of Simulated Galaxy Clusters
We examine the incidence of cold fronts in a large sample of galaxy clusters
extracted from a (512h^-1 Mpc) hydrodynamic/N-body cosmological simulation with
adiabatic gas physics computed with the Enzo adaptive mesh refinement code.
This simulation contains a sample of roughly 4000 galaxy clusters with M >
10^14 M_sun at z=0. For each simulated galaxy cluster, we have created mock
0.3-8.0 keV X-ray observations and spectroscopic-like temperature maps. We have
searched these maps with a new automated algorithm to identify the presence of
cold fronts in projection. Using a threshold of a minimum of 10 cold front
pixels in our images, corresponding to a total comoving length L_cf > 156h^-1
kpc, we find that roughly 10-12% of all projections in a mass-limited sample
would be classified as cold front clusters. Interestingly, the fraction of
clusters with extended cold front features in our synthetic maps of a
mass-limited sample trends only weakly with redshift out to z=1.0. However,
when using different selection functions, including a simulated flux limit, the
trending with redshift changes significantly. The likelihood of finding cold
fronts in the simulated clusters in our sample is a strong function of cluster
mass. In clusters with M>7.5x10^14 M_sun the cold front fraction is 40-50%. We
also show that the presence of cold fronts is strongly correlated with
disturbed morphology as measured by quantitative structure measures. Finally,
we find that the incidence of cold fronts in the simulated cluster images is
strongly dependent on baryonic physics.Comment: 16 pages, 21 figures, Accepted to Ap
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