10,356 research outputs found
The role of the Interstellar Medium in Galaxy Formation Simulations
There is a strong connection between the formation of a disk galaxy and the
properties of the interstellar medium (ISM). Theoretical work has typically
either focused on the cosmological buildup of a galaxy with a relatively crude
model for the gas physics, or examined local processes in the ISM and ignored
the global evolution of the galaxy itself. Here, I briefly review what has been
learned from both of these approaches, and what can be done to bridge the gap
between them. I argue that cosmological simulations need to learn from
observational and theoretical work on local ISM properties and adopt more
sophisticated models for the processes that they cannot resolve. Since the ISM
is still incompletely understood, there are a number of reasonable approaches
for these "subgrid" models, and I will discuss the strengths and limitations of
each.Comment: 10 pages, 4 figures, appeared in the proceedings of the
CRAL-Conference Series I "Chemodynamics: from first stars to local galaxies",
Lyon 10-14 July 2006, France, Eds. Emsellem, Wozniak, Massacrier, Gonzalez,
Devriendt, Champavert, EAS Publications Serie
Generalized Erdos Numbers for network analysis
In this paper we consider the concept of `closeness' between nodes in a
weighted network that can be defined topologically even in the absence of a
metric. The Generalized Erd\H{o}s Numbers (GENs) satisfy a number of desirable
properties as a measure of topological closeness when nodes share a finite
resource between nodes as they are real-valued and non-local, and can be used
to create an asymmetric matrix of connectivities. We show that they can be used
to define a personalized measure of the importance of nodes in a network with a
natural interpretation that leads to a new global measure of centrality and is
highly correlated with Page Rank. The relative asymmetry of the GENs (due to
their non-metric definition) is linked also to the asymmetry in the mean first
passage time between nodes in a random walk, and we use a linearized form of
the GENs to develop a continuum model for `closeness' in spatial networks. As
an example of their practicality, we deploy them to characterize the structure
of static networks and show how it relates to dynamics on networks in such
situations as the spread of an epidemic
Achieving Extreme Resolution in Numerical Cosmology Using Adaptive Mesh Refinement: Resolving Primordial Star Formation
As an entry for the 2001 Gordon Bell Award in the "special" category, we
describe our 3-d, hybrid, adaptive mesh refinement (AMR) code, Enzo, designed
for high-resolution, multiphysics, cosmological structure formation
simulations. Our parallel implementation places no limit on the depth or
complexity of the adaptive grid hierarchy, allowing us to achieve unprecedented
spatial and temporal dynamic range. We report on a simulation of primordial
star formation which develops over 8000 subgrids at 34 levels of refinement to
achieve a local refinement of a factor of 10^12 in space and time. This allows
us to resolve the properties of the first stars which form in the universe
assuming standard physics and a standard cosmological model. Achieving extreme
resolution requires the use of 128-bit extended precision arithmetic (EPA) to
accurately specify the subgrid positions. We describe our EPA AMR
implementation on the IBM SP2 Blue Horizon system at the San Diego
Supercomputer Center.Comment: 23 pages, 5 figures. Peer reviewed technical paper accepted to the
proceedings of Supercomputing 2001. This entry was a Gordon Bell Prize
finalist. For more information visit http://www.TomAbel.com/GB
Cooling and Clusters: When Is Heating Needed?
There are (at least) two unsolved problems concerning the current state of
the thermal gas in clusters of galaxies. The first is identifying the source of
the heating which offsets cooling in the centers of clusters with short cooling
times (the ``cooling flow'' problem). The second is understanding the mechanism
which boosts the entropy in cluster and group gas. Since both of these problems
involve an unknown source of heating it is tempting to identify them with the
same process, particular since AGN heating is observed to be operating at some
level in a sample of well-observed ``cooling flow'' clusters. Here we show,
using numerical simulations of cluster formation, that much of the gas ending
up in clusters cools at high redshift and so the heating is also needed at
high-redshift, well before the cluster forms. This indicates that the same
process operating to solve the cooling flow problem may not also resolve the
cluster entropy problem.Comment: 10 pages, 5 figures, published in Philosophical Transactions A (Royal
Society
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