115 research outputs found
Cosmological simulations of galaxy clusters with feedback from active galactic nuclei: profiles and scaling relations
We present results from a new set of 30 cosmological simulations of galaxy clusters, including the effects of radiative cooling, star formation, supernova feedback, black hole growth and AGN feedback. We first demonstrate that our AGN model is capable of reproducing the observed cluster pressure profile at redshift, z ≃ 0, once the AGN heating temperature of the targeted particles is made to scale with the final virial temperature of the halo. This allows the ejected gas to reach larger radii in higher mass clusters than would be possible had a fixed heating temperature been used. Such a model also successfully reduces the star formation rate in brightest cluster galaxies and broadly reproduces a number of other observational properties at low redshift, including baryon, gas and star fractions, entropy profiles outside the core and the X-ray luminosity–mass relation. Our results are consistent with the notion that the excess entropy is generated via selective removal of the densest material through radiative cooling; supernova and AGN feedback largely serve as regulation mechanisms, moving heated gas out of galaxies and away from cluster cores. However, our simulations fail to address a number of serious issues; for example, they are incapable of reproducing the shape and diversity of the observed entropy profiles within the core region. We also show that the stellar and black hole masses are sensitive to numerical resolution, particularly the gravitational softening length; a smaller value leads to more efficient black hole growth at early times and a smaller central galaxy
Fractal Reconnection in Solar and Stellar Environments
Recent space based observations of the Sun revealed that magnetic
reconnection is ubiquitous in the solar atmosphere, ranging from small scale
reconnection (observed as nanoflares) to large scale one (observed as long
duration flares or giant arcades). Often the magnetic reconnection events are
associated with mass ejections or jets, which seem to be closely related to
multiple plasmoid ejections from fractal current sheet. The bursty radio and
hard X-ray emissions from flares also suggest the fractal reconnection and
associated particle acceleration. We shall discuss recent observations and
theories related to the plasmoid-induced-reconnection and the fractal
reconnection in solar flares, and their implication to reconnection physics and
particle acceleration. Recent findings of many superflares on solar type stars
that has extended the applicability of the fractal reconnection model of solar
flares to much a wider parameter space suitable for stellar flares are also
discussed.Comment: Invited chapter to appear in "Magnetic Reconnection: Concepts and
Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016),
33 pages, 18 figure
VERTIGO (VERtical Transport In the Global Ocean) : a study of particle sources and flux attenuation in the North Pacific
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1522-1539, doi:10.1016/j.dsr2.2008.04.024.The VERtical Transport In the Global Ocean (VERTIGO) study examined particle sources and
fluxes through the ocean’s “twilight zone” (defined here as depths below the euphotic zone to
1000 m). Interdisciplinary process studies were conducted at contrasting sites off Hawaii
(ALOHA) and in the NW Pacific (K2) during 3 week occupations in 2004 and 2005, respectively.
We examine in this overview paper the contrasting physical, chemical and biological settings and
how these conditions impact the source characteristics of the sinking material and the transport
efficiency through the twilight zone. A major finding in VERTIGO is the considerably lower
transfer efficiency (Teff) of particulate organic carbon (POC), POC flux 500 / 150 m, at ALOHA
(20%) vs. K2 (50%). This efficiency is higher in the diatom-dominated setting at K2 where
silica-rich particles dominate the flux at the end of a diatom bloom, and where zooplankton and
their pellets are larger. At K2, the drawdown of macronutrients is used to assess export and
suggests that shallow remineralization above our 150 m trap is significant, especially for N
relative to Si. We explore here also surface export ratios (POC flux/primary production) and
possible reasons why this ratio is higher at K2, especially during the first trap deployment. When
we compare the 500 m fluxes to deep moored traps, both sites lose about half of the sinking POC
by >4000 m, but this comparison is limited in that fluxes at depth may have both a local and
distant component. Certainly, the greatest difference in particle flux attenuation is in the
mesopelagic, and we highlight other VERTIGO papers that provide a more detailed examination
of the particle sources, flux and processes that attenuate the flux of sinking particles. Ultimately,
we contend that at least three types of processes need to be considered: heterotrophic degradation
of sinking particles, zooplankton migration and surface feeding, and lateral sources of suspended
and sinking materials. We have evidence that all of these processes impacted the net attenuation
of particle flux vs. depth measured in VERTIGO and would therefore need to be considered and
quantified in order to understand the magnitude and efficiency of the ocean’s biological pump.Funding for VERTIGO was provided primarily by research grants
from the US National Science Foundation Programs in Chemical and Biological Oceanography
(KOB, CHL, MWS, DKS, DAS). Additional US and non-US grants included: US Department
of Energy, Office of Science, Biological and Environmental Research Program (JKBB); the
Gordon and Betty Moore Foundation (DMK); the Australian Cooperative Research Centre
program and Australian Antarctic Division (TWT); Chinese NSFC and MOST programs (NZJ);
Research Foundation Flanders and Vrije Universiteit Brussel (FD, ME); JAMSTEC (MCH); New
Zealand Public Good Science Foundation (PWB); and internal WHOI sources and a contribution
from the John Aure and Cathryn Ann Hansen Buesseler Foundation (KOB)
Novel genetic loci associated with hippocampal volume
The hippocampal formation is a brain structure integrally involved in episodic memory, spatial navigation, cognition and stress responsiveness. Structural abnormalities in hippocampal volume and shape are found in several common neuropsychiatric disorders. To identify the genetic underpinnings of hippocampal structure here we perform a genome-wide association study (GWAS) of 33,536 individuals and discover six independent loci significantly associated with hippocampal volume, four of them novel. Of the novel loci, three lie within genes (ASTN2, DPP4 and MAST4) and one is found 200 kb upstream of SHH. A hippocampal subfield analysis shows that a locus within the MSRB3 gene shows evidence of a localized effect along the dentate gyrus, subiculum, CA1 and fissure. Further, we show that genetic variants associated with decreased hippocampal volume are also associated with increased risk for Alzheimer's disease (rg =-0.155). Our findings suggest novel biological pathways through which human genetic variation influences hippocampal volume and risk for neuropsychiatric illness
Destination branding case study: Tracking brand equity for an emerging destination between 2003 and 2007
Place branding has become a major focus of operations for destination marketing organizations (DMOs) striving for differentiation in cluttered markets. The topic of destination branding has only received attention in the tourism literature since the late 1990s, and there has been relatively little research reported in relations to analyzing destination brand effectiveness over time. This article reports an attempt to oprationalize the concept of consumer-based brand equity (CBBE) for an emerging destination over two points in time. The purpose of the project was to track the effectiveness of the brand in 2007 against benchmarks that were established in a 2003 student at the commencement of a new destination brand campaign. The key finding was there was no change in perceived performance for the destination across the brand's performance indicators and CBBE dimensions. Because of the common challenges faced by DMOs worldwide, it is suggested the CBBE hierarchy provides destination marketers with a practical tool for evaluation brand performance over time
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