Cluster magnetic fields from active galactic nuclei

Active galactic nuclei (AGN) found at the centers of clusters of galaxies are a possible source for weak cluster-wide magnetic fields. To evaluate this scenario, we present 3D adaptive mesh refinement MHD simulations of a cool-core cluster that include injection of kinetic, thermal, and magnetic energy via an AGN-powered jet. Using the MHD solver in FLASH 2, we compare several sub-resolution approaches that link the estimated accretion rate as measured on the simulation mesh to the accretion rate onto the central black hole and the resulting feedback. We examine the effects of magnetized outflows on the accretion history of the black hole and discuss the ability of these models to magnetize the cluster medium.Comment: 4 pages, 2 figures, submitted to conference proceedings "The Monster's Fiery Breath: Feedback in Groups, Galaxies, and Clusters

The Influence of AGN Feedback on Galaxy Cluster Observables

Galaxy clusters are valuable cosmological probes. However, cluster mass estimates rely on observable quantities that are affected by complicated baryonic physics in the intracluster medium (ICM), including feedback from active galactic nuclei (AGN). Cosmological simulations have started to include AGN feedback using subgrid models. In order to make robust predictions, the systematics of different implementations and parametrizations need to be understood. We have developed an AGN subgrid model in FLASH that supports a few different black hole accretion models and feedback models. We use this model to study the effect of AGN on X-ray cluster observables and its dependence on model variations.Comment: minor error corrected, to appear in proceedings of the conference "The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", June 2009, Madison, Wisconsi

Response to reflected-force feedback to fingers in teleoperations

Reflected-force feedback is an important aspect of teleoperations. The objective is to determine the ability of the human operator to respond to that force. Telerobotics operation is simulated by computer control of a motor-driven device with capabilities for programmable force feedback and force measurement. A computer-controlled motor drive is developed that provides forces against the fingers as well as (angular) position control. A load cell moves in a circular arc as it is pushed by a finger and measures reaction forces on the finger. The force exerted by the finger on the load cell and the angular position are digitized and recorded as a function of time by the computer. Flexure forces of the index, long and ring fingers of the human hand in opposition to the motor driven load cell are investigated. Results of the following experiments are presented: (1) Exertion of maximum finger force as a function of angle; (2) Exertion of target finger force against a computer controlled force; and (3) Test of the ability to move to a target force against a force that is a function of position. Averaged over ten individuals, the maximum force that could be exerted by the index or long finger is about 50 Newtons, while that of the ring finger is about 40 Newtons. From the tests of the ability of a subject to exert a target force, it was concluded that reflected-force feedback can be achieved with the direct kinesthetic perception of force without the use of tactile or visual clues

A response to arXiv:1310.2791: A self-consistent public catalogue of voids and superclusters in the SDSS Data Release 7 galaxy surveys

Recently, Nadathur & Hotchkiss (2013) submitted a paper discussing a new cosmic void catalog. This paper includes claims about the void catalog described in Sutter et al. (2012). In this note, we respond to those claims, clarify some discrepancies between the text of Sutter et al. (2012) and the most recent version of the catalog, and provide some comments on the differences between our catalog and that of Nadathur & Hotchkiss (2013). All updates and documentation for our catalog are available at http://www.cosmicvoids.net.Comment: 3 pages, 1 figure, public catalog available at http://www.cosmicvoids.ne

Sparse sampling, galaxy bias, and voids

To study the impact of sparsity and galaxy bias on void statistics, we use a single large-volume, high-resolution N-body simulation to compare voids in multiple levels of subsampled dark matter, halo populations, and mock galaxies from a Halo Occupation Distribution model tuned to different galaxy survey densities. We focus our comparison on three key observational statistics: number functions, ellipticity distributions, and radial density profiles. We use the hierarchical tree structure of voids to interpret the impacts of sampling density and galaxy bias, and theoretical and empirical functions to describe the statistics in all our sample populations. We are able to make simple adjustments to theoretical expectations to offer prescriptions for translating from analytics to the void properties measured in realistic observations. We find that sampling density has a much larger effect on void sizes than galaxy bias. At lower tracer density, small voids disappear and the remaining voids are larger, more spherical, and have slightly steeper profiles. When a proper lower mass threshold is chosen, voids in halo distributions largely mimic those found in galaxy populations, except for ellipticities, where galaxy bias leads to higher values. We use the void density profile of Hamaus et al. (2014) to show that voids follow a self-similar and universal trend, allowing simple translations between voids studied in dark matter and voids identified in galaxy surveys. We have added the mock void catalogs used in this work to the Public Cosmic Void Catalog at http://www.cosmicvoids.net.Comment: 11 pages, 7 figures, MNRAS accepted. Minor changes from previous version. Public catalog available at http://www.cosmicvoids.ne

Methane Emissions of Differently Fed Dairy Cows and Corresponding Methane and Nitrogen Emissions from their Manure during Storage

This study investigated the effects of supplementing 40 g lauric acid (C12) kg-1 dry matter (DM) in feed on methane emissions from early-lactating dairy cows and the associated effects on methane, nitrous oxide and ammonia release from the manure during storage. Stearic acid (C18), a fatty acid without assumed methane-suppressing potential in the digestive tract of ruminants, was added at 40 g kg-1 DM to a control diet. The complete feed consisted of forage and concentrate in a ratio of 1.5:1 (DM basis). The manure was stored for 14 weeks either as complete slurry or, separately, as urine-rich slurry and farmyard manure representing two common storage systems. Methane release of the cows, as measured in respiratory chambers, was lower with C12 by about 20%, but this was mostly resulting from a reduced feed intake and, partly, from a lower rate of fibre digestion. As milk yield declined less than feed intake, methane emission per kg of milk was significantly lower with C12 (11.4 g) than with C18 (14.0 g). Faeces of C12-fed cows had a higher proportion of undigested fibre and accordingly methane release from their manure was higher compared with the manure obtained from the C18-fed cows. Overall, manure-derived methane accounted for8.2% and 15.4% of total methane after 7 and 14 weeks of storage, respectively. The evolution of methane widely differed between manure types and dietary treatments, with a retarded onset of release in complete slurry particularly in the C12 treatment. Emissions of nitrous oxide were lower in the manures from the C12 treatment. This partially compensated for the higher methane release from the C12 manure with respect to the greenhouse gas potential. The total greenhouse gas potential (cow and manure together) accounted for 8.7 and 10.5 kg equivalents of CO2 cow-1 d-1with C12 and C18, respectively. At unaffected urine-N proportion ammonia and total nitrogen losses from stored manure were lower with C12 than with C18 corresponding to the differences in feed and nitrogen intake. The present results suggest that manure storage significantly contributes to total methane emission from dairy husbandry, and that the identification of effective dietary mitigation strategies has to consider both the digestive tract of the animals and the corresponding manur