563 research outputs found
Saddle stresses for generic theories with a preferred acceleration scale
We show how scaling arguments may be used to generate templates for the tidal
stresses around saddles for a vast class of MONDian theories {\it detached from
their obligations as dark matter alternatives}. Such theories are to be seen
simply as alternative theories of gravity with a preferred acceleration scale,
and could be tested in the solar system by extending the LISA Pathfinder (LPF)
mission. The constraints thus obtained may then be combined, if one wishes,
with requirements arising from astrophysical and cosmological applications, but
a clear separation of the issues is achieved. The central technical content of
this paper is the derivation of a scaling prescription allowing complex
numerical work to be bypassed in the generation of templates. We find that LPF
could constrain very tightly the acceleration and the free parameter
present in these theories. As an application of our technique we also
produce predictions for the moon saddle (for which a similar scaling argument
is applicable) with the result that we recommend that it should be included in
orbit design.Comment: Analysis of the lunar saddle added to version to appear in Physical
Review
Did Massive Primordial Stars Preenrich the Lyman Alpha Forest?
We examine the dynamical evolution and statistical properties of the
supernova ejecta of massive primordial stars in a cosmological framework to
determine whether this first population of stars could have enriched the
universe to the levels and dispersions seen by the most recent observations of
the Lyman-Alpha forest. We evolve a lambda CDM model in a 1 Mpc^3 volume to a
redshift of z = 15 and add ``bubbles'' of metal corresponding to the supernova
ejecta of the first generation of massive stars in all dark matter halos with
masses greater than 5 times 10^5 solar masses. These initial conditions are
then evolved to z = 3 and the distribution and levels of metals are compared to
observations. In the absence of further star formation the primordial metal is
initially contained in halos and filaments. Photoevaporation of metal-enriched
gas due to the metagalactic ultraviolet background radiation at the epoch of
reionization (z ~ 6) causes a sharp increase of the metal volume filling
factor. At z = 3, ~ 2.5% of the simulation volume (approx. 20% of the total gas
mass) is filled with gas enriched above a metallicity of 10^-4 Z_solar, and
less than 0.6% of the volume is enriched above a metallicity of 10^-3 Z_solar.
This suggests that, even with the most optimistic prescription for placement of
primordial supernova and the amount of metals produced by each supernova, this
population of stars cannot entirely be responsible for the enrichment of the
Lyman- forest to the levels and dispersions seen by current
observations unless we have severely underestimated the duration of the Pop III
epoch. However, comparison to observations show that Pop III supernovae can be
significant contributors to the very low overdensity Lyman-Alpha forest.Comment: 4 pages, 3 figures (color). Accepted to ApJ Letters. Replaced version
has some correction
The Fate of the First Galaxies. I. Self-Consistent Cosmological Simulations with Radiative Transfer
In cold dark matter (CDM) cosmogonies, low-mass objects play an important
role in the evolution of the universe. Not only are they the first luminous
objects to shed light in a previously dark universe, but, if their formation is
not inhibited by their own feedback, they dominate the galaxy mass function
until redshift z \sim 5. In this paper we present and discuss the
implementation of a 3D cosmological code that includes most of the needed
physics to simulate the formation and evolution of the first galaxies with a
self-consistent treatment of radiative feedback. The simulation includes
continuum radiative transfer using the ``Optically Thin Variable Eddington
Tensor'' (OTVET) approximation and line-radiative transfer in the H_2
Lyman-Werner bands of the background radiation. We include detailed chemistry
for H_2 formation/destruction, molecular and atomic cooling/heating processes,
ionization by secondary electrons, and heating by Ly\alpha resonant scattering.
We find that the first galaxies ("small-halos") are characterized by a
bursting star formation, self-regulated by a feedback process that acts on
cosmological scales. Their formation is not suppressed by feedback processes;
therefore, their impact on cosmic evolution cannot be neglected. The main focus
of this paper is on the methodology of the simulations, and we only briefly
introduce some of the results. An extensive discussion of the results and the
nature of the feedback mechanism are the focus of a companion paper.Comment: Accepted for publication on ApJ, 33 pages, including 14 figures and 2
tables. Movies and a higher quality version of the paper (figures) are
available at: http://casa.colorado.edu/~ricotti/MOVIES.htm
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
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
One Gravitational Potential or Two? Forecasts and Tests
The metric of a perturbed Robertson-Walker spacetime is characterized by
three functions: a scale-factor giving the expansion history and two potentials
which generalize the single potential of Newtonian gravity. The Newtonian
potential induces peculiar velocities and, from these, the growth of matter
fluctuations. Massless particles respond equally to the Newtonian potential and
to a curvature potential. The difference of the two potentials, called the
gravitational slip, is predicted to be very small in general relativity but can
be substantial in modified gravity theories. The two potentials can be
measured, and gravity tested on cosmological scales, by combining weak
gravitational lensing or the Integrated Sachs-Wolfe effect with galaxy peculiar
velocities or clustering.Comment: 15 pages, invited research article for Theo Murphy Meeting "Testing
general relativity with cosmology
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
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
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