Strong clustering of underdense regions and the environmental dependence of clustering from Gaussian initial conditions

We discuss two slightly counter-intuitive findings about the environmental dependence of clustering in the Sloan Digital Sky Survey. First, we find that the relation between clustering strength and density is not monotonic: galaxies in the densest regions are more strongly clustered than are galaxies in regions of moderate overdensity; galaxies in moderate overdensities are more strongly clustered than are those in moderate underdensities; but galaxies in moderate underdensities are less clustered than galaxies in the least dense regions. We argue that this is natural if clustering evolved gravitationally from a Gaussian field, since the highest peaks and lowest troughs in Gaussian fields are similarly clustered. The precise non-monotonic dependence of galaxy clustering on density is very well reproduced in a mock catalog which is based on a halo-model decomposition of galaxy clustering. Second, the distribution of galaxy counts in our sample is rather well described by a Poisson cluster model. We show that, despite their Poisson nature, correlations with environment are expected in such models. More remarkably, the expected trends are very like those in standard models of halo bias, despite the fact that correlations with environment in these models arise purely from the fact that dense regions are dense because they happen to host more massive halos. This is in contrast to the usual analysis which assumes that it is the large scale environment which determines the halo mass function. [Abridged]Comment: 10 pages, 5 figures, accepted for publication in MNRA

How does clustering depend on environment?

According to the current paradigm, galaxies form and reside in extended cold dark matter (CDM) halos and in turn are key tracers of cosmological structure. Understanding how different types of galaxies occupy halos of different masses is one of the major challenges facing extragalactic astrophysics. The observed galaxy properties depend on the environment surrounding the galaxy. Within the framework of most galaxy formation models the environmental dependence of the galaxy population is mainly due to the change of the halo mass function with large-scale environment. Such models make precise predictions for how galaxy clustering should depend on environment. We will illustrate this by presenting analytical models of dark matter and galaxy clustering along with results obtained from numerical simulations. With these results we can hope to obtain a better understanding of the link between galaxies and dark matter and thereby constrain galaxy formation models.Comment: 3 pages, 1 figure, uses iaus.cls. To appear in the proceedings of IAU Colloquium 195: "Outskirts of galaxy clusters: intense life in the suburbs", Torino, Italy, March 12-16, 200

The Environmental dependence of Dark Matter and Galaxy Clustering

We have addressed important astrophysical issues of the large scale structure of the universewith the help of analytical and numerical tools. In particular, we have concentrated on obtaininga better understanding of the environmental dependence of clustering of dark matter and galaxies.We will discuss the development of the analytical framework, the generation of mock catalogs of data, and present results from numerical studies of the comparison of the theory with the simulated data samples and subsequently with actual observational data.The N-body simulations from the Virgo consortium and SDSS data were used to study the clusteringproperties of dense and less dense subsamples with the help of the correlation function.The dense sample shows a correlation function that is about a factor of ten times or more higher than the less dense sample at large scales. This discernible difference holds true for mock galaxies, dark matter, and data from the Sloan Digital Sky Survey.By utilizing the halo model and adapting the formalism to different environments, we present the correspondence between theoretical models and simulated samples and real data.The strong agreement between the model and results from simulations and data provides strong support for the hierarchical clustering models. This suggests that unless care is taken to study galaxies at fixed halo mass, observed correlations between gastrophysical effects and environment are dominated by the fact that these effects also correlate with the halo mass, and halo mass correlates with environment

The Global sphere reconstruction (GSR) - Demonstrating an independent implementation of the astrometric core solution for Gaia

Context. The Gaia ESA mission will estimate the astrometric and physical data of more than one billion objects, providing the largest and most precise catalog of absolute astrometry in the history of Astronomy. The core of this process, the so-called global sphere reconstruction, is represented by the reduction of a subset of these objects which will be used to define the celestial reference frame. As the Hipparcos mission showed, and as is inherent to all kinds of absolute measurements, possible errors in the data reduction can hardly be identified from the catalog, thus potentially introducing systematic errors in all derived work. Aims. Following up on the lessons learned from Hipparcos, our aim is thus to develop an independent sphere reconstruction method that contributes to guarantee the quality of the astrometric results without fully reproducing the main processing chain. Methods. Indeed, given the unfeasibility of a complete replica of the data reduction pipeline, an astrometric verification unit (AVU) was instituted by the Gaia Data Processing and Analysis Consortium (DPAC). One of its jobs is to implement and operate an independent global sphere reconstruction (GSR), parallel to the baseline one (AGIS, namely Astrometric Global Iterative Solution) but limited to the primary stars and for validation purposes, to compare the two results, and to report on any significant differences. Results. Tests performed on simulated data show that GSR is able to reproduce at the sub-$\mu$as level the results of the AGIS demonstration run presented in Lindegren et al. (2012). Conclusions. Further development is ongoing to improve on the treatment of real data and on the software modules that compare the AGIS and GSR solutions to identify possible discrepancies above the tolerance level set by the accuracy of the Gaia catalog.Comment: Accepted for publication on Astronomy & Astrophysic

Differential astrometric framework for the Jupiter relativistic experiment with Gaia

We employ differential astrometric methods to establish a small field reference frame stable at the microarcsecond (μas) level on short time-scales using high-cadence simulated observations taken by Gaia in 2017 February of a bright star close to the limb of Jupiter, as part of the relativistic experiment on Jupiter's quadrupole. We achieve subμas-level precision along scan through a suitable transformation of the field angles into a small-field tangent plane and a least-squares fit over several overlapping frames for estimating the plate and geometric calibration parameters with tens of reference stars that lie within ∼0.5 deg from the target star, assuming perfect knowledge of stellar proper motions and parallaxes. Furthermore, we study the effects of unmodelled astrometric parameters on the residuals and find that proper motions have a stronger effect than unmodelled parallaxes, e.g. unmodelled Gaia DR2 proper motions introduce extra residuals of ∼23 μas (AL) and 69 μas (AC) versus the ∼5 μas (AL) and 17 μas (AC) due to unmodelled parallaxes. On the other hand, assuming catalogue errors in the proper motions and parallaxes such as those from Gaia DR2 has a minimal impact on the stability introducing subμas and μas level residuals in the along and across scanning direction, respectively. Finally, the effect of a coarse knowledge in the satellite velocity components (with time-dependent errors of 10 μas s-1) is capable of enlarging the size of the residuals to roughly 0.2 mas

Fecal Calprotectin Associated with Spondyloarthritis Disease Activity

Background: Fecal calprotectin (FC) is one of fecal biomarkers that has validation to distinguish the organic and functional bowel disease but the relationship with disease activity in Spondyloarthritis (SpA) is still questionable. We determined the correlation of FC to the degree of disease activity on SpA. Material and Method: This cross sectional analytic study involved thirty-two subjects diagnosed with SpA. FC levels were analyzed using ELISA method. The Spondylartritis disease activities were assessed by ASDAS-CRP scores. Results: The median age was 52.5 (23.0-71.0) years with the average ASDAS-CRP score of 2.53 ± 0.61. The median level of FC increased by 58.15 (5.20-269.20) µg/g and was found to be positive (=50 µg/g) in 56.3% subjects. FC levels was significantly correlated with the degree of SpA activity according to ASDAS-CRP scores (r = 0.563; P = 0.001) Conclusion: Fecal Calprotectin associated with the degree of activity in the SpA measured by ASDAS-CRP scores

The environmental dependence of clustering in hierarchical models

In hierarchical models, density fluctuations on different scales are correlated. This induces correlations between dark halo masses, their formation histories, and their larger-scale environments. In turn, this produces a correlation between galaxy properties and environment. This correlation is entirely statistical in nature. We show how the observed clustering of galaxies can be used to quantify the importance of this statistical correlation relative to other physical effects which may also give rise to correlations between the properties of galaxies and their surroundings. We also develop a halo model description of this environmental dependence of clustering.Comment: 11 pages, 6 figures, MNRAS in pres

Gaia Reference frame determination: the AVU/GSR pipeline

The main goal of the Gaia ESA mission is the production of a 5 parameters astrometric catalog - i.e. including positions, parallaxes and the two components of the proper motions - of about 1 billion stars of our Galaxy at an accuracy level going from the few micro-arcseconds of the brightest objects to some ~100 micro-arcseconds of the faintests. This goal will be reached by means of high-precision astrometric measurements conducted by a satellite sweeping continuously the celestial sphere during its 5-years mission.A fundamental step toward the realization of this catalog is the so-called ''Sphere Reconstruction'', which determines the celestial reference frame using the observations of a subset of up to 100 million ''primary stars'' among those observed by Gaia.From a mathematical point of view, these observations translate into a large number of equations, linearized with respect to the unknown parameters around known initial values, whose solution in the least-squares sense eventually provides the catalog with its errors, and determines the Gaia reference frame.This represents an extremely challenging problem because of the high accuracy of the observations and of the large number of unknowns involved. The former issue implies that an adequately accurate relativistic astrometric model has to be used, while the huge number of unknowns and observations puts this task at the forefront of the High-Performance Computing problems.These challenges, and the absolute character of the Gaia measurements and results, calls for a careful scientific validation of the sphere reconstruction, as it was done for the previous HIPPARCOS mission. For these reasons the Gaia Data Processing and Analysis Consortium (DPAC) decided to replicate the baseline process, named AGIS (Astrometric Global Iterative Solution) with another independent solution, named GSR (Global Sphere Reconstruction) which uses a different astrometric model and different algorithms for the system solution.This talk will show the goals of GSR, describing its implementation, its results for complete sphere reconstructions and its first tests with real data. <P /

Diagnostic values of helicobacter pylori stool antigen immunochromatographic method compared to histopathology in dyspepsia patient

Background: Helicobacter pylori infection often leads to complaints of dyspepsia. Enforcement of infection still relies on invasive histopathological methods through endoscopic and biopsy procedures. Helicobacter pylori stool antigen (HpSA) is a method of rapid immunochromatography that is not invasive and relatively inexpensive. We determined the diagnostic value of HpSA examination of immunochromatographic methods compared to histopathological examination as the gold standard for diagnosing H. pylori infection. Methods: HpSA examination was used to identify H. pylori infection by its ability to detect H. pylori antigen from stool of dysepeptic patients .Its diagnostic values including sensitivity, specificity, positive predictive value and negative predictive value was determined by comparing them to those of histopathologic examination as gold standard. Results: From 93 dyspeptic patients, pre-test probability of H. pylori infection using histopathologic examination showed result as much as 17.2 . The sensitivity, specificity, positive predictive value and negative predictive value of HpSA immunochromatographic methods were 38, 94, 55 and 88, respectively. A positive probability ratio of 5.78 increased the post-test probability for H. pylori infection by 37.8. A negative probability ratio of 0.68 increased the post-test probability of not being infected with H. pylori by 5.4. Conclusion: The diagnostic value of HpSA examination of immunochromatographic methods was not good enough to exclude or diagnose H. pylori infection in dyspeptic patients. © 2019, Yerevan State Medical University. All rights reserved

Helicobacter pylori density and expression of gastric mucosal interleukin-8 in dyspeptic patients

Background: The density of the H. pylori colonies is associated with more severe clinical manifestations, chronic infections, ineffective therapy responses and malignant events. Interleukin-8 (IL-8) is one of cytokines that plays a role in the inflammatory process that results in gastric mucosal damage. We determined the association between H. pylori density and gastric mucosal IL-8 expression in dyspeptic patients. Methods: H. pylori density was determined using histopathology based on Updated Sydney System. IL-8 levels were measured using immunohistochemistry from the gastric biopsy. Results: There was significant difference between gastric mucosal IL-8 expression obtained in gastric mucosal gland epithelium (p = 0.028) at mild and high density of H. pylori (p = 0.013). There was a correlation between H. pylori density with gastric mucosal IL-8 expression of glandular epithelium (r = 0.622; p = 0.004), surface epithelium (r = 0.510; p = 0.026), and inflammatory cells (r = 0.054; = 0.028). Conclusion: We found a significant positive correlation between H. pylori density and gastric mucosal IL-8 expression in dyspeptic patients. © 2019, Yerevan State Medical University. All rights reserved