Cluster Structure in Cosmological Simulations I: Correlation to Observables, Mass Estimates, and Evolution

We use Enzo, a hybrid Eulerian AMR/N-body code including non-gravitational heating and cooling, to explore the morphology of the X-ray gas in clusters of galaxies and its evolution in current generation cosmological simulations. We employ and compare two observationally motivated structure measures: power ratios and centroid shift. Overall, the structure of our simulated clusters compares remarkably well to low-redshift observations, although some differences remain that may point to incomplete gas physics. We find no dependence on cluster structure in the mass-observable scaling relations, T_X-M and Y_X-M, when using the true cluster masses. However, estimates of the total mass based on the assumption of hydrostatic equilibrium, as assumed in observational studies, are systematically low. We show that the hydrostatic mass bias strongly correlates with cluster structure and, more weakly, with cluster mass. When the hydrostatic masses are used, the mass-observable scaling relations and gas mass fractions depend significantly on cluster morphology, and the true relations are not recovered even if the most relaxed clusters are used. We show that cluster structure, via the power ratios, can be used to effectively correct the hydrostatic mass estimates and mass-scaling relations, suggesting that we can calibrate for this systematic effect in cosmological studies. Similar to observational studies, we find that cluster structure, particularly centroid shift, evolves with redshift. This evolution is mild but will lead to additional errors at high redshift. Projection along the line of sight leads to significant uncertainty in the structure of individual clusters: less than 50% of clusters which appear relaxed in projection based on our structure measures are truly relaxed.Comment: 57 pages, 18 figures, accepted to ApJ, updated definition of T_X and M_gas but results unchanged, for version with full resolution figures, see http://www.ociw.edu/~tesla/sims.ps.g

AGN heating and dissipative processes in galaxy clusters

Recent X-ray observations reveal growing evidence for heating by active galactic nuclei (AGN) in clusters and groups of galaxies. AGN outflows play a crucial role in explaining the riddle of cooling flows and the entropy problem in clusters. Here we study the effect of AGN on the intra-cluster medium in a cosmological simulation using the adaptive mesh refinement FLASH code. We pay particular attention to the effects of conductivity and viscosity on the dissipation of weak shocks generated by the AGN activity in a realistic galaxy cluster. Our 3D simulations demonstrate that both viscous and conductive dissipation play an important role in distributing the mechanical energy injected by the AGN, offsetting radiative cooling and injecting entropy to the gas. These processes are important even when the transport coefficients are at a level of 10% of the Spitzer value. Provided that both conductivity and viscosity are suppressed by a comparable amount, conductive dissipation is likely to dominate over viscous dissipation. Nevertheless, viscous effects may still affect the dynamics of the gas and contribute a significant amount of dissipation compared to radiative cooling. We also present synthetic Chandra observations. We show that the simulated buoyant bubbles inflated by the AGN, and weak shocks associated with them, are detectable with the Chandra observatory.Comment: accepted to ApJ, minor change

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

Food Recalls and Food Safety Perceptions: The September 2006 Spinach Recall Case

This paper examines public perceptions on food safety particularly relating to spinach, which was subject of countrywide recall in 2006. Results indicate that food safety perceptions may be driven by public trust/confidence in institutions whose activities may be directly or indirectly related to food safety. The results further suggest that food safety perceptions may also be related to the type of the product; for example, the public perceives frozen spinach differently from bagged fresh spinach. Additionally, the results show that low levels of objective knowledge about food pathogens and the resulting illnesses have implications on overall food safety. Results further indicate that females and Caucasians perceived the four types of spinach as safe for consumption. This outcome contrasts with views held by young people, people with education below high school and those belonging to the lower incomes groups, who viewed the four types of spinach as unsafe. More attention should be directed toward public education and outreach efforts on overall food safety targeting the youth, low income groups and those with education below high school. In addition, there is need for the regulatory agencies to put their act together, given current low levels of public trust in their role of safeguarding the food supply.Food Consumption/Nutrition/Food Safety,

Food Recalls and Food Safety Perceptions: The September 2006 Spinach Recall Case

This study analyzes public perceptions of food safety using a national survey conducted in November 2006, soon after the September 2006 nationwide spinach recall. We explore relationships between peoples’ perceived risks of food contamination (spinach in this case) and their trust in the institutions in charge of safeguarding/ensuring safety. Finally, we examine relationships between individual observance of basic good food handling practices and food safety. Trust in institutions through which food passes and regulatory agencies were shown to be critical in determining food safety perceptions. For example, confidence in the USDA as a regulatory agent was viewed positively, and hence contributed toward viewing the four types of spinach as safe for consumption. Conversely, skepticism with which the public views food corporations (processors, transporters, or retailers) impacted food safety perceptions negatively.food recalls, food safety, public perceptions, spinach, Agribusiness, Food Consumption/Nutrition/Food Safety,

Post-harvest entomology research in the United States Department of Agriculture–Agricultural Research Service

This is a review of current post-harvest entomology research conducted by the Agricultural Research Service, the research branch of the US Department of Agriculture. The review covers both durable and perishable commodities. Research on biochemistry, genetics, physiology, monitoring and control of insects infesting stored grain, dried fruits and nuts, and processed commodities is reviewed. Research on development of quarantine treatments, particularly for fruit flies, is also reviewed, including research on thermal and irradiation treatments and a discussion of risk management for quarantine pests. Two areas of research are covered more extensively: a project to map the genome of the red flour beetle, Tribolium castaneum, and the use of near-infrared spectroscopy for detection of hidden infestations in grain, quantification of insect fragments in food, determination of quality in dried fruits, identification of insect species and age-grading insects. Future research directions are identified

Post-harvest entomology research in the United States Department of Agriculture–Agricultural Research Service

This is a review of current post-harvest entomology research conducted by the Agricultural Research Service, the research branch of the US Department of Agriculture. The review covers both durable and perishable commodities. Research on biochemistry, genetics, physiology, monitoring and control of insects infesting stored grain, dried fruits and nuts, and processed commodities is reviewed. Research on development of quarantine treatments, particularly for fruit flies, is also reviewed, including research on thermal and irradiation treatments and a discussion of risk management for quarantine pests. Two areas of research are covered more extensively: a project to map the genome of the red flour beetle, Tribolium castaneum, and the use of near-infrared spectroscopy for detection of hidden infestations in grain, quantification of insect fragments in food, determination of quality in dried fruits, identification of insect species and age-grading insects. Future research directions are identified

NON-CONTRACTIBILITY AND ASSET SPECIFICITY IN REVERSE AUCTIONS: “MOVE TO THE MIDDLE ” OR “EFFICIENT MARKETS”?

Masten, Marshall Van Alstyne and seminar participants at the University of Michigan Busines