7 research outputs found
Exploring Optimization for the Random-Field Ising Model
The push-relabel algorithm can be used to calculate rapidly the exact ground states for a given sample with a random-field Ising model (RFIM) Hamiltonian. Although the algorithm is guaranteed to terminate after a time polynomial in the number of spins, implementation details are important for practical performance. Empirical results for the timing in dimensions d=1,2, and 3 are used to determine the fastest among several implementations. Direct visualization of the auxiliary fields used by the algorithm provides insight into its operation and suggests how to optimize the algorithm. Recommendations are given for further study of the RFIM
Exploring optimization for the random-field Ising model
The push-relabel algorithm can be used to calculate rapidly the exact ground
states for a given sample with a random-field Ising model (RFIM) Hamiltonian.
Although the algorithm is guaranteed to terminate after a time polynomial in
the number of spins, implementation details are important for practical
performance. Empirical results for the timing in dimensions d=1,2, and 3 are
used to determine the fastest among several implementations. Direct
visualization of the auxiliary fields used by the algorithm provides insight
into its operation and suggests how to optimize the algorithm. Recommendations
are given for further study of the RFIM.Comment: 10 page
The Effects of X-Ray Feedback from AGN on Host Galaxy Evolution
Hydrodynamic simulations of galaxies with active galactic nuclei (AGN) have
typically employed feedback that is purely local: i.e., an injection of energy
to the immediate neighborhood of the black hole. We perform GADGET-2
simulations of massive elliptical galaxies with an additional feedback
component: an observationally calibrated X-ray radiation field which emanates
from the black hole and heats gas out to large radii from the galaxy center. We
find that including the heating and radiation pressure associated with this
X-ray flux in our simulations enhances the effects which are commonly reported
from AGN feedback. This new feedback model is twice as effective as traditional
feedback at suppressing star formation, produces 3 times less star formation in
the last 6 Gyr, and modestly lowers the final BH mass (30%). It is also
significantly more effective than an X-ray background in reducing the number of
satellite galaxies.Comment: 9 emulateapj pages, 8 figures; accepted to Ap
Topology of structure in the Sloan Digital Sky Survey: model testing
We measure the three-dimensional topology of large-scale structure in the
Sloan Digital Sky Survey (SDSS). This allows the genus statistic to be measured
with unprecedented statistical accuracy. The sample size is now sufficiently
large to allow the topology to be an important tool for testing galaxy
formation models. For comparison, we make mock SDSS samples using several
state-of-the-art N-body simulations: the Millennium run of Springel et al.
(2005)(10 billion particles), Kim & Park (2006) CDM models (1.1 billion
particles), and Cen & Ostriker (2006) hydrodynamic code models (8.6 billion
cell hydro mesh). Each of these simulations uses a different method for
modeling galaxy formation. The SDSS data show a genus curve that is broadly
characteristic of that produced by Gaussian random phase initial conditions.
Thus the data strongly support the standard model of inflation where Gaussian
random phase initial conditions are produced by random quantum fluctuations in
the early universe. But on top of this general shape there are measurable
differences produced by non-linear gravitational effects (cf. Matsubara 1994),
and biasing connected with galaxy formation. The N-body simulations have been
tuned to reproduce the power spectrum and multiplicity function but not
topology, so topology is an acid test for these models. The data show a
``meatball'' shift (only partly due to the Sloan Great Wall of Galaxies; this
shift also appears in a sub-sample not containing the Wall) which differs at
the 2.5\sigma level from the results of the Millennium run and the Kim & Park
dark halo models, even including the effects of cosmic variance.Comment: 13 Apj pages, 7 figures High-resolution stereo graphic available at
http://www.astro.princeton.edu/~dclayh/stereo50.ep