836 research outputs found

### The Bar--Halo Interaction--I. From Fundamental Dynamics to Revised N-body Requirements

Only through resonances can non-axisymmetric features such as spiral arms and
bars exert torques over large scales and change the overall structure of a
near-equilibrium galaxy. We describe the resonant interaction mechanism in
detail and derive explicit criteria for the particle number required to
simulate these dynamical processes accurately using N-body simulations and
illustrate them with numerical experiments. To do this, we perform direct
numerical solution of perturbation theory and make detailed comparisons with
N-body simulations. The criteria include: sufficient particle coverage in phase
space near the resonance and enough particles to minimize gravitational
potential fluctuations that will change the dynamics of the resonant encounter.
Some of our more surprising findings are as follows. First, the
Inner-Lindblad-like resonance (ILR), responsible for coupling the bar to the
central halo cusp, requires almost 10^9 equal mass particles within the virial
radius for a Milky-Way-like bar in an NFW profile. Second, orbits that linger
near the resonance receive more angular momentum than orbits that move through
the resonance quickly. Small-scale fluctuations present in state-of-the-art
particle-particle simulations can knock orbits out of resonance, preventing
them from lingering and, thereby, decrease the torque. The required particle
numbers are sufficiently high for scenarios of interest that apparent
convergence in particle number is misleading: the convergence is in the
noise-dominated regime. State-of-the-art simulations are not adequate to follow
all aspects of secular evolution driven by the bar-halo interaction. We present
a procedure to test the requirements for individual N-body codes for the actual
problem of interest. [abridged]Comment: 30 pages, 19 figures, submitted to Monthly Notices. For paper with
figures at full resolution:
http://www.astro.umass.edu/~weinberg/weinberg_katz_1.ps.g

### The Bar-Halo Interaction - II. Secular evolution and the religion of N-body simulations

This paper explores resonance-driven secular evolution between a bar and
dark-matter halo using N-body simulations. We make direct comparisons to our
analytic theory (Weinberg & Katz 2005) to demonstrate the great difficulty that
an N-body simulation has representing these dynamics for realistic astronomical
interactions. In a dark-matter halo, the bar's angular momentum is coupled to
the central density cusp (if present) by the Inner Lindblad Resonance. Owing to
this angular momentum transfer and self-consistent re-equilibration, strong
realistic bars WILL modify the cusp profile, lowering the central densities
within about 30% of the bar radius in a few bar orbits. Past results to the
contrary (Sellwood 2006, McMillan & Dehnen 2005) may be the result of weak bars
or numerical artifacts. The magnitude depends on many factors and we illustrate
the sensitivity of the response to the dark-matter profile, the bar shape and
mass, and the galaxy's evolutionary history. For example, if the bar length is
comparable to the size of a central dark-matter core, the bar may exchange
angular momentum without changing its pattern speed significantly. We emphasise
that this apparently simple example of secular evolution is remarkably subtle
in detail and conclude that an N-body exploration of any astronomical scenario
requires a deep investigation into the underlying dynamical mechanisms for that
particular problem to set the necessary requirements for the simulation
parameters and method (e.g. particle number and Poisson solver). Simply put,
N-body simulations do not divinely reveal truth and hence their results are not
infallible. They are unlikely to provide useful insight on their own,
particularly for the study of even more complex secular processes such as the
production of pseudo-bulges and disk heating.Comment: 23 pages, 18 figures, submitted to Monthly Notices. For paper with
figures at full resolution:
http://www.astro.umass.edu/~weinberg/weinberg_katz_2.ps.g

### On the Destruction and Over-Merging of Dark Halos in Dissipationless N-body Simulations

N-body simulations that follow only a collisionless dark matter component
have failed to produce galaxy halos or substructure within dense environments.
We investigate the `over-merging' problem analytically and with numerical
simulations, by calculating dissolution timescales of halos due to physical and
artificial dynamical effects. The numerical resolution that has recently been
attained is such that mass-loss from two-body relaxation is negligible. We
demonstrate that substructure is destroyed in present simulations as a result
of large force softening combined with the heating sources of tides and
encounters with dissolving substructure. In the limit of infinite numerical
resolution, whether or not individual halos or substructure can survive depends
sensitively on their inner density profiles. Singular isothermal halos will
always survive at some level, however, if halos form with large core radii then
the over-merging problem will always exist within dissipationless N-body
simulations. In this latter case a dissipational component can increase the
halos central density enabling galaxies to survive.Comment: submitted to ApJL. compressed postscript file includes figures

### New insight on galaxy structure from GALPHAT I. Motivation, methodology, and benchmarks for Sersic models

We introduce a new galaxy image decomposition tool, GALPHAT (GALaxy
PHotometric ATtributes), to provide full posterior probability distributions
and reliable confidence intervals for all model parameters. GALPHAT is designed
to yield a high speed and accurate likelihood computation, using grid
interpolation and Fourier rotation. We benchmark this approach using an
ensemble of simulated Sersic model galaxies over a wide range of observational
conditions: the signal-to-noise ratio S/N, the ratio of galaxy size to the PSF
and the image size, and errors in the assumed PSF; and a range of structural
parameters: the half-light radius $r_e$ and the Sersic index $n$. We
characterise the strength of parameter covariance in Sersic model, which
increases with S/N and $n$, and the results strongly motivate the need for the
full posterior probability distribution in galaxy morphology analyses and later
inferences.
The test results for simulated galaxies successfully demonstrate that, with a
careful choice of Markov chain Monte Carlo algorithms and fast model image
generation, GALPHAT is a powerful analysis tool for reliably inferring
morphological parameters from a large ensemble of galaxies over a wide range of
different observational conditions. (abridged)Comment: Submitted to MNRAS. The submitted version with high resolution
figures can be downloaded from
http://www.astro.umass.edu/~iyoon/GALPHAT/galphat1.pd

### Photoionization, Numerical Resolution, and Galaxy Formation

Using cosmological simulations that incorporate gas dynamics and
gravitational forces, we investigate the influence of photoionization by a UV
radiation background on the formation of galaxies. In our highest resolution
simulations, we find that photoionization has essentially no effect on the
baryonic mass function of galaxies at $z=2$, down to our resolution limit of
5e9 M_\sun. We do, however, find a strong interplay between the mass
resolution of a simulation and the microphysics included in the computation of
heating and cooling rates. At low resolution, a photoionizing background can
appear to suppress the formation of even relatively massive galaxies. However,
when the same initial conditions are evolved with a factor of eight better mass
resolution, this effect disappears. Our results demonstrate the need for care
in interpreting the results of cosmological simulations that incorporate
hydrodynamics and radiation physics. For example, we conclude that a simulation
with limited resolution may yield more realistic results if it ignores some
relevant physical processes, such as photoionization. At higher resolution, the
simulated population of massive galaxies is insensitive to the treatment of
photoionization and star formation, but it does depend significantly on the
amplitude of the initial density fluctuations. By $z=2$, an $\Omega=1$ cold
dark matter model normalized to produce the observed masses of present-day
clusters has already formed galaxies with baryon masses exceeding 1e11
M_\sun.Comment: 25 pages, w/ embedded figures. Submitted to ApJ. Also available at
http://www-astronomy.mps.ohio-state.edu/~dhw/Docs/preprints.htm

### Cosmological Simulations with TreeSPH

We describe numerical methods for incorporating gas dynamics into
cosmological simulations and present illustrative applications to the cold dark
matter (CDM) scenario. Our evolution code, a version of TreeSPH (Hernquist \&
Katz 1989) generalized to handle comoving coordinates and periodic boundary
conditions, combines smoothed--particle hydrodynamics (SPH) with the
hierarchical tree method for computing gravitational forces. The Lagrangian
hydrodynamics approach and individual time steps for gas particles give the
algorithm a large dynamic range, which is essential for studies of galaxy
formation in a cosmological context. The code incorporates radiative cooling
for an optically thin, primordial composition gas in ionization equilibrium
with a user-specified ultraviolet background. We adopt a phenomenological
prescription for star formation that gradually turns cold, dense,
Jeans-unstable gas into collisionless stars, returning supernova feedback
energy to the surrounding medium. In CDM simulations, some of the baryons that
fall into dark matter potential wells dissipate their acquired thermal energy
and condense into clumps with roughly galactic masses. The resulting galaxy
population is insensitive to assumptions about star formation; we obtain
similar baryonic mass functions and galaxy correlation functions from
simulations with star formation and from simulations without star formation in
which we identify galaxies directly from the cold, dense gas.Comment: compressed postscript, 38 pages including 6 out of 7 embedded
figures. Submitted to ApJ Supplements. Version with all 7 figures available
from ftp://bessel.mps.ohio-state.edu/pub/dhw/Preprint

### Simulating Cosmic Structure Formation

We describe cosmological simulation techniques and their application to
studies of cosmic structure formation, with particular attention to recent
hydrodynamic simulations of structure in the high redshift universe.
Collisionless N-body simulations with Gaussian initial conditions produce a
pattern of sheets, filaments, tunnels, and voids that resembles the observed
large scale galaxy distribution. Simulations that incorporate gas dynamics and
dissipation form dense clumps of cold gas with sizes and masses similar to the
luminous parts of galaxies. Models based on inflation and cold dark matter
predict a healthy population of high redshift galaxies, including systems with
star formation rates of 20 M_{\sun}/year at z=6. At z~3, most of the baryons in
these models reside in the low density intergalactic medium, which produces
fluctuating Lyman-alpha absorption in the spectra of background quasars. The
physical description of this ``Lyman-alpha forest'' is particularly simple if
the absorption spectrum is viewed as a 1-dimensional map of a continuous medium
instead of a collection of lines. The combination of superb observational data
and robust numerical predictions makes the Lyman-alpha forest a promising tool
for testing cosmological models.Comment: Latex w/ paspconf.sty, 25 pages, 8 ps figs. To appear in Origins,
eds. J. M. Shull, C. E. Woodward, & H. Thronson (ASP Conference Series

### A remarkably simple and accurate method for computing the Bayes Factor from a Markov chain Monte Carlo Simulation of the Posterior Distribution in high dimension

Weinberg (2012) described a constructive algorithm for computing the marginal
likelihood, Z, from a Markov chain simulation of the posterior distribution.
Its key point is: the choice of an integration subdomain that eliminates
subvolumes with poor sampling owing to low tail-values of posterior
probability. Conversely, this same idea may be used to choose the subdomain
that optimizes the accuracy of Z. Here, we explore using the simulated
distribution to define a small region of high posterior probability, followed
by a numerical integration of the sample in the selected region using the
volume tessellation algorithm described in Weinberg (2012). Even more promising
is the resampling of this small region followed by a naive Monte Carlo
integration. The new enhanced algorithm is computationally trivial and leads to
a dramatic improvement in accuracy. For example, this application of the new
algorithm to a four-component mixture with random locations in 16 dimensions
yields accurate evaluation of Z with 5% errors. This enables Bayes-factor model
selection for real-world problems that have been infeasible with previous
methods.Comment: 14 pages, 3 figures, submitted to Bayesian Analysi

### Group Scaling Relations From a Cosmological Hydrodynamic Simulation: No Pre-heating Required?

We investigate the X-ray vs. optical scaling relations of poor groups to
small clusters (sigma~100-700 km/s) identified in a cosmological hydrodynamic
simulation of a Lambda-CDM universe, with cooling and star formation but no
pre-heating. We find that the scaling relations between X-ray luminosity, X-ray
temperature, and velocity dispersion show significant departures from the
relations predicted by simple hydrostatic equilibrium models or simulations
without cooling, having steeper L_X-sigma and L_X-T_X slopes and a "break" at
\~200 km/s (~0.3 keV). These departures arise because the hot (X-ray emitting)
gas fraction varies substantially with halo mass in this regime. Our
predictions roughly agree with observations. Thus radiative cooling is a
critical physical process in modeling galaxy groups, and may present an
alternative to ad hoc models such as pre-heating or entropy floors for
explaining X-ray group scaling relations.Comment: 4 pages, to be puplished in the proceedings of the ''Sesto
2001-Tracing Cosmic Evolution with Galaxy Clusters'

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