18,175 research outputs found
Feedback Heating by Cosmic Rays in Clusters of Galaxies
Recent observations show that the cooling flows in the central regions of
galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles
are a leading candidate for suppressing cooling, usually via some form of
mechanical heating. At the same time, observed X-ray cavities and synchrotron
emission point toward a significant non-thermal particle population. Previous
studies have focused on the dynamical effects of cosmic-ray pressure support,
but none have built successful models in which cosmic-ray heating is
significant. Here we investigate a new model of AGN heating, in which the
intracluster medium is efficiently heated by cosmic-rays, which are injected
into the ICM through diffusion or the shredding of the bubbles by
Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal
conduction as well. Using numerical simulations, we show that the cooling
catastrophe is efficiently suppressed. The cluster quickly relaxes to a
quasi-equilibrium state with a highly reduced accretion rate and temperature
and density profiles which match observations. Unlike the conduction-only case,
no fine-tuning of the Spitzer conduction suppression factor f is needed. The
cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within
observational bounds. Cosmic ray heating is a very attractive alternative to
mechanical heating, and may become particularly compelling if GLAST detects the
gamma-ray signature of cosmic-rays in clusters.Comment: Revised version accepted for publication in MNRAS. Significantly
expanded discussion and new simulations exploring parameter space/model
robustness; conclusions unchange
From the Tully-Fisher relation to the Fundamental Plane through Mergers
We set up a series of self-consistent N-body simulations to investigate the
fundamental plane of merger remnants of spiral galaxies. These last ones are
obtained from a theoretical Tully-Fisher relation at z=1, assuming a constant
mass-to-light ratio within the LambdaCDM cosmogony. Using a Sersic growth curve
and an orthogonal fitting method, we found that the fundamental plane of our
merger remnants is described by the relation Re ~ sigma^{1.48} Ie^{-0.75} which
is in good agreement with that reported from the Sloan Digital Sky Survey Re ~
sigma^{1.49} Ie^{-0.75}. However, the R^{1/4}-profile leads to a fundamental
plane given by Re ~ sigma^{1.79} Ie^{-0.60}. In general, the correlation found
in our merger remnants arises from homology breaking (V^2 ~ sigma^nu, Rg ~
Re^eta) in combination with a mass scaling relation between the total and
luminous mass, $M ~ ML^gamma. Considering an orthogonal fitting method, it is
found that 1.74<nu<1.79, 0.21<eta<0.52 and 0.80<gamma<0.90 depending on the
adopted profile (Sersic or R^{1/4}).Comment: 5 pages and 2 figures. Accepted version in MNRAS Letter
Characterizing and Subsetting Big Data Workloads
Big data benchmark suites must include a diversity of data and workloads to
be useful in fairly evaluating big data systems and architectures. However,
using truly comprehensive benchmarks poses great challenges for the
architecture community. First, we need to thoroughly understand the behaviors
of a variety of workloads. Second, our usual simulation-based research methods
become prohibitively expensive for big data. As big data is an emerging field,
more and more software stacks are being proposed to facilitate the development
of big data applications, which aggravates hese challenges. In this paper, we
first use Principle Component Analysis (PCA) to identify the most important
characteristics from 45 metrics to characterize big data workloads from
BigDataBench, a comprehensive big data benchmark suite. Second, we apply a
clustering technique to the principle components obtained from the PCA to
investigate the similarity among big data workloads, and we verify the
importance of including different software stacks for big data benchmarking.
Third, we select seven representative big data workloads by removing redundant
ones and release the BigDataBench simulation version, which is publicly
available from http://prof.ict.ac.cn/BigDataBench/simulatorversion/.Comment: 11 pages, 6 figures, 2014 IEEE International Symposium on Workload
Characterizatio
From Filamentary Networks to Dense Cores in Molecular Clouds: Toward a New Paradigm for Star Formation
Recent studies of the nearest star-forming clouds of the Galaxy at
submillimeter wavelengths with the Herschel Space Observatory have provided us
with unprecedented images of the initial and boundary conditions of the star
formation process. The Herschel results emphasize the role of interstellar
filaments in the star formation process and connect remarkably well with nearly
a decade's worth of numerical simulations and theory that have consistently
shown that the ISM should be highly filamentary on all scales and star
formation is intimately related to self-gravitating filaments. In this review,
we trace how the apparent complexity of cloud structure and star formation is
governed by relatively simple universal processes - from filamentary clumps to
galactic scales. We emphasize two crucial and complementary aspects: (i) the
key observational results obtained with Herschel over the past three years,
along with relevant new results obtained from the ground on the kinematics of
interstellar structures, and (ii) the key existing theoretical models and the
many numerical simulations of interstellar cloud structure and star formation.
We then synthesize a comprehensive physical picture that arises from the
confrontation of these observations and simulations.Comment: 24 pages, 15 figures. Accepted for publication as a review chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Hennin
Massive Coronae of Galaxies
There is reason to suspect that about half of the baryons are in
pressure-supported plasma in the halos of normal galaxies, drawn in by gravity
along with about half of the dark matter. To be consistent with the
observations this baryonic component, the corona, would have to be hotter than
the kinetic temperature of the dark matter in the halo so as to produce
acceptable central electron densities. We ascribe this hotter plasma
temperature to the addition of entropy prior to and during assembly of the
system, in an analogy to cluster formation. The plasma cooling time would be
longer than the gravitational collapse time but, in the inner parts, shorter
than the Hubble time, making the corona thermally unstable to the formation of
a cloudy structure that may be in line with what is indicated by quasar
absorption line systems. The corona of an isolated spiral galaxy would be a
source of soft X-ray and recombination radiation, adding to the more commonly
discussed effects of stars and supernovae. In this picture the mass in the
corona is much larger than the mass in condensed baryons in a spiral galaxy.
The corona thus would be a substantial reservoir of diffuse baryons that are
settling and adding to the mass in interstellar matter and stars, so that star
formation in isolated spirals will continue well beyond the present epoch.Comment: 14 pages, 5 figure
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