50 research outputs found
Mark correlations: relating physical properties to spatial distributions
Mark correlations provide a systematic approach to look at objects both
distributed in space and bearing intrinsic information, for instance on
physical properties. The interplay of the objects' properties (marks) with the
spatial clustering is of vivid interest for many applications; are, e.g.,
galaxies with high luminosities more strongly clustered than dim ones? Do
neighbored pores in a sandstone have similar sizes? How does the shape of
impact craters on a planet depend on the geological surface properties? In this
article, we give an introduction into the appropriate mathematical framework to
deal with such questions, i.e. the theory of marked point processes. After
having clarified the notion of segregation effects, we define universal test
quantities applicable to realizations of a marked point processes. We show
their power using concrete data sets in analyzing the luminosity-dependence of
the galaxy clustering, the alignment of dark matter halos in gravitational
-body simulations, the morphology- and diameter-dependence of the Martian
crater distribution and the size correlations of pores in sandstone. In order
to understand our data in more detail, we discuss the Boolean depletion model,
the random field model and the Cox random field model. The first model
describes depletion effects in the distribution of Martian craters and pores in
sandstone, whereas the last one accounts at least qualitatively for the
observed luminosity-dependence of the galaxy clustering.Comment: 35 pages, 12 figures. to be published in Lecture Notes of Physics,
second Wuppertal conference "Spatial statistics and statistical physics
Correlations in the orientations of galaxy clusters
The relative orientation of clusters' major elongation axes and clusters'
angular momentum is studied using a large N-body simulation in a box of 500
Mpc/h base length for a standard LambdaCDM model. Employing the technique of
mark correlation functions, we successfully separated the correlations in the
orientation from the well known clustering signal traced by the two-point
correlation function. The correlations in the orientation are highly
significant for our sample of 3000 clusters. We found an alignment of
neighboring clusters, i.e. an enhanced probability of the major elongation axes
of neighboring cluster pairs to be in parallel with each other. At 10 Mpc/h
separation the amplitude of this signal is ~10% above the value expected from
random orientations, and it vanishes on scales larger than 15 Mpc/h. The
``filamentary'' alignment between clusters' major elongation axes and the lines
pointing towards neighboring clusters shows even stronger deviations from
random orientation, which can be detected out to scales of 100 Mpc/h, both in
2D and 3D analyses. Similarly, strong correlations of the angular momentum were
seen. Also a clear signal in the scalar correlation of the absolute value of
the angular momentum, the spin parameter and the mass was found. They extend up
to 50 Mpc/h and have an amplitude of 40%, 15%, and 10% above a random
distribution at 10 Mpc/h separation, respectively.Comment: 10 pages, 10 figures, A&A, accepted 28/08/0
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
Cosmological parameters from the clustering of AGN
We attempt to put constraints on different cosmological and biasing models by
combining the recent clustering results of X-ray sources in the local () and distant universe ().Comment: 9 pages, 3 figures, to be published in the proceedings of the ''2nd
Hellenic Cosmology Workshop'', Athens 2001, eds, Manolis Plionis & Spiros
Kotsaki
The XMM Cluster Survey: X-ray analysis methodology
The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters
using all publicly available data in the XMM-Newton Science Archive. Its main
aims are to measure cosmological parameters and trace the evolution of X-ray
scaling relations. In this paper we describe the data processing methodology
applied to the 5,776 XMM observations used to construct the current XCS source
catalogue. A total of 3,675 > 4-sigma cluster candidates with > 50
background-subtracted X-ray counts are extracted from a total non-overlapping
area suitable for cluster searching of 410 deg^2. Of these, 993 candidates are
detected with > 300 background-subtracted X-ray photon counts, and we
demonstrate that robust temperature measurements can be obtained down to this
count limit. We describe in detail the automated pipelines used to perform the
spectral and surface brightness fitting for these candidates, as well as to
estimate redshifts from the X-ray data alone. A total of 587 (122) X-ray
temperatures to a typical accuracy of < 40 (< 10) per cent have been measured
to date. We also present the methodology adopted for determining the selection
function of the survey, and show that the extended source detection algorithm
is robust to a range of cluster morphologies by inserting mock clusters derived
from hydrodynamical simulations into real XMM images. These tests show that the
simple isothermal beta-profiles is sufficient to capture the essential details
of the cluster population detected in the archival XMM observations. The
redshift follow-up of the XCS cluster sample is presented in a companion paper,
together with a first data release of 503 optically-confirmed clusters.Comment: MNRAS accepted, 45 pages, 38 figures. Our companion paper describing
our optical analysis methodology and presenting a first set of confirmed
clusters has now been submitted to MNRA
Simulating Chandra observations of galaxy clusters
Numerical hydro-N-body simulations are very important tools for making
theoretical predictions for the formation of galaxy clusters. They show that
the atmospheres of clusters of galaxies have quite complex angular and thermal
structures. The full understanding of the physical processes behind these
features can be only achieved by direct comparison of observations to
hydro-N-body simulations. Although simple in principle, these comparisons are
not always trivial. In fact, real data are convolved with the instrument
response which may substantially influence the apparent properties of the
studied features. To overcome this problem we build the software package X-MAS
devoted to simulate X-ray observations of galaxy clusters obtained from
hydro-N-body simulations. In this paper we present how this software package
works and discuss its application to the simulation of Chandra ACIS-S3
observations. We compare some of the main physical properties of the input data
to the ones derived from simulated observations after performing a standard
imaging and spectral analysis. We show that, if the thermal structure of the
cluster along a particular line of sight is quite complex, the projected
spectroscopic temperature obtained from the observation is significantly lower
than the emission-weighed value inferred directly from hydrodynamical
simulation. This implies that much attention must be paid in the theoretical
interpretation of observational temperatures.Comment: Submitted for publication in MNRAS; 9 pages, 8 color figures and 2 BW
figures,mn2e.cl
Global patient outcomes after elective surgery: prospective cohort study in 27 low-, middle- and high-income countries.
BACKGROUND: As global initiatives increase patient access to surgical treatments, there remains a need to understand the adverse effects of surgery and define appropriate levels of perioperative care. METHODS: We designed a prospective international 7-day cohort study of outcomes following elective adult inpatient surgery in 27 countries. The primary outcome was in-hospital complications. Secondary outcomes were death following a complication (failure to rescue) and death in hospital. Process measures were admission to critical care immediately after surgery or to treat a complication and duration of hospital stay. A single definition of critical care was used for all countries. RESULTS: A total of 474 hospitals in 19 high-, 7 middle- and 1 low-income country were included in the primary analysis. Data included 44 814 patients with a median hospital stay of 4 (range 2-7) days. A total of 7508 patients (16.8%) developed one or more postoperative complication and 207 died (0.5%). The overall mortality among patients who developed complications was 2.8%. Mortality following complications ranged from 2.4% for pulmonary embolism to 43.9% for cardiac arrest. A total of 4360 (9.7%) patients were admitted to a critical care unit as routine immediately after surgery, of whom 2198 (50.4%) developed a complication, with 105 (2.4%) deaths. A total of 1233 patients (16.4%) were admitted to a critical care unit to treat complications, with 119 (9.7%) deaths. Despite lower baseline risk, outcomes were similar in low- and middle-income compared with high-income countries. CONCLUSIONS: Poor patient outcomes are common after inpatient surgery. Global initiatives to increase access to surgical treatments should also address the need for safe perioperative care. STUDY REGISTRATION: ISRCTN5181700