681 research outputs found
Cold Dark Matter I: The Formation of Dark Halos
We use numerical simulations of critically-closed cold dark matter (CDM)
models to study the effects of numerical resolution on observable quantities.
We study simulations with up to particles using the particle-mesh (PM)
method and with up to particles using the adaptive particle-particle
--particle-mesh (PM) method. Comparisons of galaxy halo distributions are
made among the various simulations. We also compare distributions with
observations and we explore methods for identifying halos, including a new
algorithm that finds all particles within closed contours of the smoothed
density field surrounding a peak. The simulated halos show more substructure
than predicted by the Press-Schechter theory. We are able to rule out all
CDM models for linear amplitude \sigma_8\gsim 0.5 because the
simulations produce too many massive halos compared with the observations. The
simulations also produce too many low mass halos. The distribution of halos
characterized by their circular velocities for the PM simulations is in
reasonable agreement with the observations for 150\kms\lsim V_{\rm circ} \lsim
350\kms.}}Comment: 41 pages, plain tex, ApJ, 236, in press; postscript figures available
in ftp://arcturus.mit.edu/Preprints/CDM1_figs.tar.
Variational Integrators for the Gravitational N-Body Problem
This paper describes a fourth-order integration algorithm for the
gravitational N-body problem based on discrete Lagrangian mechanics. When used
with shared timesteps, the algorithm is momentum conserving and symplectic. We
generalize the algorithm to handle individual time steps; this introduces
fifth-order errors in angular momentum conservation and symplecticity. We show
that using adaptive block power of two timesteps does not increase the error in
symplecticity. In contrast to other high-order, symplectic, individual
timestep, momentum-preserving algorithms, the algorithm takes only forward
timesteps. We compare a code integrating an N-body system using the algorithm
with a direct-summation force calculation to standard stellar cluster
simulation codes. We find that our algorithm has about 1.5 orders of magnitude
better symplecticity and momentum conservation errors than standard algorithms
for equivalent numbers of force evaluations and equivalent energy conservation
errors.Comment: 31 pages, 8 figures. v2: Revised individual-timestepping description,
expanded comparison with other methods, corrected error in predictor
equation. ApJ, in pres
Reheating of the Universe and Population III
We note that current observational evidence strongly favors a conventional
recombination of ionized matter subsequent to redshift z=1200, followed by
reionization prior to redshift z=5 and compute how this would have occurred in
a standard scenario for the growth of structure. Extending prior semi-analytic
work, we show by direct, high-resolution numerical simulations (of a COBE
normalized CDM+Lambda model) that reheating, will occur in the interval 15>z>7,
followed by reionization and accompanied by a significant increase in the Jeans
mass. However, the evolution of the Jeans mass does not significantly affect
star formation in dense, self-shielded clumps of gas, which are detached from
the thermal evolution of the rest of the universe. On average, the growth of
the Jeans mass tracks the growth of the nonlinear mass scale, a result we
suspect is due to nonlinear feedback effects. Cooling on molecular hydrogen
leads to a burst of star formation prior to reheating which produces Population
III stars with Omega_* reaching 10^{-5.5} and Z/Z_sun reaching 10^{-3.7} by
z=14. Star formation subsequently slows down as molecular hydrogen is depleted
by photo-destruction and the rise of the temperature. At later times, z<10,
when the characteristic virial temperature of gas clumps reach 10,000 degrees,
star formation increases again as hydrogen line cooling become efficient.
Objects containing Pop III stars accrete mass with time and, as soon as they
reach 10,000 K virial temperature, they engage in renewed star formation and
turn into normal Pop II objects having an old Pop III metal poor component.Comment: six postscript figures included, submitted to ApJ
Cosmological Reionization by Stellar Sources
I use cosmological simulations that incorporate a physically motivated
approximation to three-dimensional radiative transfer that recovers correct
asymptotic ionization front propagation speeds for some cosmologically relevant
density distributions transfer to investigate the process of the reionization
of the universe by ionizing radiation from proto-galaxies. Reionization
proceeds in three stages and occupies a large redshift range from z~15 until
z~5. During the first, ``pre-overlap'' stage, HII regions gradually expand into
the low density IGM, leaving behind neutral high density protrusions. During
the second, ``overlap'' stage, that occurs in about 10% of the Hubble time, HII
regions merge and the ionizing background rises by a large factor. During the
third, ``post-overlap'' stage, remaining high density regions are being
gradually ionized as the required ionizing photons are being produced.
Residual fluctuations in the ionizing background reach significant (more than
10%) levels for the Lyman-alpha forest absorption systems with column densities
above 10^14 - 10^15 cm^-2 at z=3 to 4.Comment: Revised version accepted for publication in ApJ. Color versions of
Fig. 3a-h in GIF format, full (unbinned) versions of Fig. 5, 6, and 13, as
well as MPEG animations are available at
http://casa.colorado.edu/~gnedin/GALLERY/rei_p.htm
Cold Dark Matter II: Spatial and Velocity Statistics
We examine high-resolution gravitational N-body simulations of the
cold dark matter (CDM) model in order to determine whether there is any
normalization of the initial density fluctuation spectrum that yields
acceptable results for galaxy clustering and velocities. Dense dark matter
halos in the evolved mass distribution are identified with luminous galaxies;
the most massive halos are also considered as sites for galaxy groups, with a
range of possibilities explored for the group mass to light ratios. We verify
the earlier conclusions of White et al. (1987) for the low amplitude (high
bias) CDM model --- the galaxy correlation function is marginally acceptable
but that there are too many galaxies. We also show that the peak biasing method
does not accurately reproduce the results obtained using dense halos identified
in the simulations themselves. The COBE anisotropy implies a higher
normalization, resulting in problems with excessive pairwise galaxy velocity
dispersion unless a strong velocity bias is present. Although we confirm the
strong velocity bias of halos reported by Couchman \& Carlberg (1992), we show
that the galaxy motions are still too large on small scales. We find no
amplitude for which the CDM model can reconcile simultaneously the galaxy
correlation function, the low pairwise velocity dispersion, and the richness
distribution of groups and clusters. With the normalization implied by COBE,
the CDM spectrum has too much power on small scales if .Comment: 31 pages, plain tex, ApJ, 236, in press; postscript figures available
in ftp://arcturus.mit.edu/Preprints/CDM2_figs.tar.
Kinematic Density Waves in Accretion Disks
When thin accretion disks around black holes are perturbed, the main
restoring force is gravity. If gas pressure, magnetic stresses, and radiation
pressure are neglected, the disk remains thin as long as orbits do not
intersect. Intersections would result in pressure forces which limit the growth
of perturbations. We find that a discrete set of perturbations is possible for
which orbits remain non-intersecting for arbitrarily long times. These modes
define a discrete set of frequencies. We classify all long-lived perturbations
for arbitrary potentials and show how their mode frequencies are related to
pattern speeds computed from the azimuthal and epicyclic frequencies. We show
that modes are concentrated near radii where the pattern speed has vanishing
radial derivative. We explore these modes around Kerr black holes as a possible
explanation for the high-frequency quasi-periodic oscillations of black hole
binaries such as GRO J1655-40. The long-lived modes are shown to coincide with
diskoseismic waves in the limit of small sound speed. While the waves have long
lifetime, they have the wrong frequencies to explain the pairs of
high-frequency quasi-periodic oscillations observed in black hole binaries.Comment: 28 pages, 6 figures; extended comparison with diskoseismology; added
reference to astro-ph/060368
Dependence of the Inner DM Profile on the Halo Mass
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and
2 tables. The revised version has an additional discussion section and work
on the velocity dispersion anisotrop
Structure Formation With a Long-Range Scalar Dark Matter Interaction
Numerical simulations show that a long-range scalar interaction in a single
species of massive dark matter particles causes voids between the
concentrations of large galaxies to be more nearly empty, suppresses accretion
of intergalactic matter onto galaxies at low redshift, and produces an early
generation of dense dark matter halos. These three effects, in moderation, seem
to be improvements over the Lambda CDM model predictions for cosmic structure
formation. Because the scalar interaction in this model has negligible effect
on laboratory physics and the classical cosmological tests, it offers an
observationally attractive example of cosmology with complicated physics in the
dark sector, notably a large violation of the weak equivalence principle.Comment: 10 pages, 7 figures, revtex4. v2: minor improvements, refs added,
version to appear in PR
Non-uniqueness of the Dirac theory in a curved spacetime
We summarize a recent work on the subject title. The Dirac equation in a
curved spacetime depends on a field of coefficients (essentially the Dirac
matrices), for which a continuum of different choices are possible. We study
the conditions under which a change of the coefficient fields leads to an
equivalent Hamiltonian operator H, or to an equivalent energy operator E. In
this paper, we focus on the standard version of the gravitational Dirac
equation, but the non-uniqueness applies also to our alternative versions. We
find that the changes which lead to an equivalent operator H, or respectively
to an equivalent operator E, are determined by initial data, or respectively
have to make some point-dependent antihermitian matrix vanish. Thus, the vast
majority of the possible coefficient changes lead neither to an equivalent
operator H, nor to an equivalent operator E, whence a lack of uniqueness. We
show that even the Dirac energy spectrum is not unique.Comment: 13 pages (standard 12pt article format). Text of a talk given at the
1st Mediterranean Conference on Classical and Quantum Gravity, Kolymbari
(Greece), Sept. 14-18, 200
A New Way to Conserve Total Energy for Eulerian Hydrodynamic Simulations with Self-Gravity
We propose a new method to conserve the total energy to round-off error in
grid-based codes for hydrodynamic simulations with self-gravity. A formula for
the energy flux due to the work done by the the self-gravitational force is
given, so the change in total energy can be written in conservative form.
Numerical experiments with the code Athena show that the total energy is indeed
conserved with our new algorithm and the new algorithm is second order
accurate. We have performed a set of tests that show the numerical errors in
the traditional, non-conservative algorithm can affect the dynamics of the
system. The new algorithm only requires one extra solution of the Poisson
equation, as compared to the traditional algorithm which includes self-gravity
as a source term. If the Poisson solver takes a negligible fraction of the
total simulation time, such as when FFTs are used, the new algorithm is almost
as efficient as the original method. This new algorithm is useful in Eulerian
hydrodynamic simulations with self-gravity, especially when results are
sensitive to small energy errors, as for radiation pressure dominated flow.Comment: 19 pages, 8 figures, 2 tables, add journal information in New
Astronom
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