6,055 research outputs found
Towards a complete accounting of energy and momentum from stellar feedback in galaxy formation simulations
Stellar feedback plays a key role in galaxy formation by regulating star
formation, driving interstellar turbulence and generating galactic scale
outflows. Although modern simulations of galaxy formation can resolve scales of
10-100 pc, star formation and feedback operate on smaller, "subgrid" scales.
Great care should therefore be taken in order to properly account for the
effect of feedback on global galaxy evolution. We investigate the momentum and
energy budget of feedback during different stages of stellar evolution, and
study its impact on the interstellar medium using simulations of local star
forming regions and galactic disks at the resolution affordable in modern
cosmological zoom-in simulations. In particular, we present a novel subgrid
model for the momentum injection due to radiation pressure and stellar winds
from massive stars during early, pre-supernova evolutionary stages of young
star clusters. Early injection of momentum acts to clear out dense gas in star
forming regions, hence limiting star formation. The reduced gas density
mitigates radiative losses of thermal feedback energy from subsequent supernova
explosions, leading to an increased overall efficiency of stellar feedback. The
detailed impact of stellar feedback depends sensitively on the implementation
and choice of parameters. Somewhat encouragingly, we find that implementations
in which feedback is efficient lead to approximate self-regulation of global
star formation efficiency. We compare simulation results using our feedback
implementation to other phenomenological feedback methods, where thermal
feedback energy is allowed to dissipate over time scales longer than the formal
gas cooling time. We find that simulations with maximal momentum injection
suppress star formation to a similar degree as is found in simulations adopting
adiabatic thermal feedback.Comment: ApJ submitted. For a high-resolution version of the paper, see
http://kicp.uchicago.edu/~agertz
Introduction: Urban revolutions in the age of global urbanism
This special issue, papers presented at an Urban Studies Foundation-funded conference in Jakarta (March 2011), examines the current ‘urban century’ in terms of three revolutions. Revolutions from above index the logics and norms of mainstream global urbanism, particularly the form they have taken as policymakers work with municipal officials worldwide to organise urban development around neoliberal norms. Revolutions from below refer to the multifaceted contestations of global urbanism that take place in and around cities, ranging from urban street demonstrations and occupations (such as those riveting the world in early 2011 when these papers were written) to the quotidian actions of those pursuing politics and livelihoods that subvert the norms of mainstream global urbanism. It also highlights conceptual revolutions, referencing the ongoing challenge of reconceptualising urban theory from the South – not simply as a hemispheric location or geopolitical category but an epistemological stance, staged from many different locations but always fraught with the differentials of power and the weight of historical geographies. Drawing on the insights of scholars writing from, and not just about, such locations, a further iteration in this ‘southern’ turn of urban theorising is proposed. This spatio-temporal conjunctural approach emphasises how the specificity of cities – their existence as entities that are at once singular and universal – emerges from spatio-temporal dynamics, connectivities and horizontal and vertical relations. Practically, such scholarship entails taking the field seriously through collaborative work that is multi-sited, engages people along the spectrum of academics and activists, and is presented before and scrutinised by multiple publics
Time resolution of the Atlas Tile calorimeter and its performance for a measurement of heavy stable particles
Time resolution of the Atlas Tile calorimeter modules has been measured using the test beam data. The resolution depends on an energy deposited in a given cell and is equal to about 1.5 at = 1 , 270 at = 25 (high gain), 700 at = 25 (low gain) and 170 at = 150 . These values have to be compared to the time of flight of relativistic particles to reach first samples of Tile calorimeter (from 8.3 to 20 ). Time of Flight measurement using Tile Calorimeter can be used to identify, and combined with momentum measurements by Atlas inner detector, to measure the mass of exotic heavy stable particles. The results are compared to previous analysis using the Atlas muon spectrometercite{KRAAN2}
A side-by-side comparison of Daya Bay antineutrino detectors
The Daya Bay Reactor Neutrino Experiment is designed to determine precisely the neutrino mixing angle θ_(13) with a sensitivity better than 0.01 in the parameter sin^22θ_(13) at the 90% confidence level. To achieve this goal, the collaboration will build eight functionally identical antineutrino detectors. The first two detectors have been constructed, installed and commissioned in Experimental Hall 1, with steady data-taking beginning September 23, 2011. A comparison of the data collected over the subsequent three months indicates that the detectors are functionally identical, and that detector-related systematic uncertainties are smaller than requirements
Investigation of top mass measurements with the ATLAS detector at LHC
Several methods for the determination of the mass of the top quark with the
ATLAS detector at the LHC are presented. All dominant decay channels of the top
quark can be explored. The measurements are in most cases dominated by
systematic uncertainties. New methods have been developed to control those
related to the detector. The results indicate that a total error on the top
mass at the level of 1 GeV should be achievable.Comment: 47 pages, 40 figure
Manifestation of the Arnol'd Diffusion in Quantum Systems
We study an analog of the classical Arnol'd diffusion in a quantum system of
two coupled non-linear oscillators one of which is governed by an external
periodic force with two frequencies. In the classical model this very weak
diffusion happens in a narrow stochastic layer along the coupling resonance,
and leads to an increase of total energy of the system. We show that the
quantum dynamics of wave packets mimics, up to some extent, global properties
of the classical Arnol'd diffusion. This specific diffusion represents a new
type of quantum dynamics, and may be observed, for example, in 2D semiconductor
structures (quantum billiards) perturbed by time-periodic external fields.Comment: RevTex, 4 pages including 7 ps-figures, corrected forma
Weight filtration on the cohomology of complex analytic spaces
We extend Deligne's weight filtration to the integer cohomology of complex
analytic spaces (endowed with an equivalence class of compactifications). In
general, the weight filtration that we obtain is not part of a mixed Hodge
structure. Our purely geometric proof is based on cubical descent for
resolution of singularities and Poincar\'e-Verdier duality. Using similar
techniques, we introduce the singularity filtration on the cohomology of
compactificable analytic spaces. This is a new and natural analytic invariant
which does not depend on the equivalence class of compactifications and is
related to the weight filtration.Comment: examples added + minor correction
Heliospheric Evolution of Magnetic Clouds
Interplanetary evolution of eleven magnetic clouds (MCs) recorded by at least
two radially aligned spacecraft is studied. The in situ magnetic field
measurements are fitted to a cylindrically symmetric Gold-Hoyle force-free
uniform-twist flux-rope configuration. The analysis reveals that in a
statistical sense the expansion of studied MCs is compatible with self-similar
behavior. However, individual events expose a large scatter of expansion rates,
ranging from very weak to very strong expansion. Individually, only four events
show an expansion rate compatible with the isotropic self-similar expansion.
The results indicate that the expansion has to be much stronger when MCs are
still close to the Sun than in the studied 0.47 - 4.8 AU distance range. The
evolution of the magnetic field strength shows a large deviation from the
behavior expected for the case of an isotropic self-similar expansion. In the
statistical sense, as well as in most of the individual events, the inferred
magnetic field decreases much slower than expected. Only three events show a
behavior compatible with a self-similar expansion. There is also a discrepancy
between the magnetic field decrease and the increase of the MC size, indicating
that magnetic reconnection and geometrical deformations play a significant role
in the MC evolution. About half of the events show a decay of the electric
current as expected for the self-similar expansion. Statistically, the inferred
axial magnetic flux is broadly consistent with it remaining constant. However,
events characterized by large magnetic flux show a clear tendency of decreasing
flux.Comment: 64 pages, 10 figure
Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
This paper explores the mapping between the observable properties of a
stellar halo in phase- and abundance-space and the parent galaxy's accretion
history in terms of the characteristic epoch of accretion and mass and orbits
of progenitor objects. The study utilizes a suite of eleven stellar halo models
constructed within the context of a standard LCDM cosmology. The results
demonstrate that coordinate-space studies are sensitive to the recent (0-8
Gyears ago) merger histories of galaxies (this timescale corresponds to the
last few to tens of percent of mass accretion for a Milky-Way-type galaxy).
Specifically, the {\it frequency, sky coverage} and {\it fraction of stars} in
substructures in the stellar halo as a function of surface brightness are
indicators of the importance of recent merging and of the luminosity function
of infalling dwarfs. The {\it morphology} of features serves as a guide to the
orbital distribution of those dwarfs. Constraints on the earlier merger history
(> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars:
within our models, dramatic differences in the dominant epoch of accretion or
luminosity function of progenitor objects leave clear signatures in the
[alpha/Fe] and [Fe/H] distributions of the stellar halo - halos dominated by
very early accretion have higher average [alpha/Fe], while those dominated by
high luminosity satellites have higher [Fe/H]. This intuition can be applied to
reconstruct much about the merger histories of nearby galaxies from current and
future data sets.Comment: 21 pages, 20 figures. To appear in the Astrophysical Journa
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