1,081 research outputs found
Inversion of low-frequency subsurface data in a finite-depth ocean
AbstractLet:[▿2+k2+k2v(x)]u=−δ(x−y) in L=R2 × [0,h],u= 0 at x3 = 0, ux3 = 0 at x3 = h, u(x,y,k) satisfies the limiting absorption principle. Letu(x,y,k) be known for all x,y∈ P ≔ {x:x3 = d}, where 0< d<h is a small fixed number (subsurface data), and allk∈ (0,k0),k0 > 0 is a small number. These data determine v(x) uniquely and an analytical procedure is given for finding v(x) given the above data. It is assumed that v(x), the inhomogeneity in the refraction coefficient (of the ocean of depth h), is an arbitrary compactly supported square integrable function
Numerical Simulations of High Redshift Star Formation in Dwarf Galaxies
We present first results from three-dimensional hydrodynamic simulations of
the high redshift formation of dwarf galaxies. The simulations use an Eulerian
adaptive mesh refinement technique to follow the non-equilibrium chemistry of
hydrogen and helium with cosmological initial conditions drawn from a popular
Lambda-dominated CDM model. We include the effects of reionization using a
uniform radiation field, a phenomenological description of the effect of star
formation and, in a separate simulation, the effects of stellar feedback. The
results highlight the effects of stellar feedback and photoionization on the
baryon content and star formation of galaxies with virial temperatures of
approximately 10^4K. Dwarf sized dark matter halos that assemble prior to
reionization are able to form stars. Most halos of similar mass that assemble
after reionization do not form stars by redshift of three. Dwarf galaxies that
form stars show large variations in their gas content because of stellar
feedback and photoionization effects. Baryon-to-dark matter mass ratios are
found to lie below the cosmic mean as a result of stellar feedback. The
supposed substructure problem of CDM is critically assessed on the basis of
these results. The star formation histories modulated by radiative and stellar
feedbacks are discussed. In addition, metallicities of individual objects are
shown to be naturally correlated with their mass-to-light ratios as is also
evident in the properties of local dwarf galaxies.Comment: 27 pages, 8 figures, accepted for publication in Ap
Formation of Massive Primordial Stars in a Reionized Gas
We use cosmological hydrodynamic simulations with unprecedented resolution to
study the formation of primordial stars in an ionized gas at high redshifts.
Our approach includes all the relevant atomic and molecular physics to follow
the thermal evolution of a prestellar gas cloud to very high densities of
~10^{18} cm^{-3}. We locate a star-forming gas cloud within a reionized region
in our cosmological simulation. The first run-away collapse is triggered when
the gas cloud's mass is ~40 Msun. We show that the cloud core remains stable
against chemo-thermal instability and also against gravitational deformation
throughout its evolution. Consequently, a single proto-stellar seed is formed,
which accretes the surrounding hot gas at the rate ~10^{-3} Msun/year. We carry
out proto-stellar evolution calculations using the inferred accretion rate. The
resulting mass of the star when it reaches the zero-age main sequence is M_ZAMS
~40 Msun. We argue that, since the obtained M_ZAMS is as large as the mass of
the collapsing parent cloud, the final stellar mass should be close to this
value. Such massive, rather than exceptionally massive, primordial stars are
expected to cause early chemical enrichment of the Universe by exploding as
black hole-forming super/hypernovae, and may also be progenitors of high
redshift gamma-ray bursts. The elemental abundance patterns of recently
discovered hyper metal-poor stars suggest that they might have been born from
the interstellar medium that was metal-enriched by supernovae of these massive
primordial stars.Comment: Revised version. To appear in ApJ
A coupled finite volume and material point method for two-phase simulation of liquid-sediment and gas-sediment flows
Mixtures of fluids and granular sediments play an important role in many
industrial, geotechnical, and aerospace engineering problems, from waste
management and transportation (liquid--sediment mixtures) to dust kick-up below
helicopter rotors (gas--sediment mixtures). These mixed flows often involve
bulk motion of hundreds of billions of individual sediment particles and can
contain both highly turbulent regions and static, non-flowing regions. This
breadth of phenomena necessitates the use of continuum simulation methods, such
as the material point method (MPM), which can accurately capture these large
deformations while also tracking the Lagrangian features of the flow (e.g.\ the
granular surface, elastic stress, etc.).
Recent works using two-phase MPM frameworks to simulate these mixtures have
shown substantial promise; however, these approaches are hindered by the
numerical limitations of MPM when simulating pure fluids. In addition to the
well-known particle ringing instability and difficulty defining inflow/outflow
boundary conditions, MPM has a tendency to accumulate quadrature errors as
materials deform, increasing the rate of overall error growth as simulations
progress. In this work, we present an improved, two-phase continuum simulation
framework that uses the finite volume method (FVM) to solve the fluid phase
equations of motion and MPM to solve the solid phase equations of motion,
substantially reducing the effect of these errors and providing better accuracy
and stability for long-duration simulations of these mixtures
A New Algorithm for Computing Statistics of Weak Lensing by Large-Scale Structure
We describe an efficient algorithm for calculating the statistics of weak
lensing by large-scale structure based on a tiled set of independent
particle-mesh N-body simulations which telescope in resolution along the line
of sight. This efficiency allows us to predict not only the mean properties of
lensing observables such as the power spectrum, skewness and kurtosis of the
convergence, but also their sampling errors for finite fields of view, which
are themselves crucial for assessing the cosmological significance of
observations. We find that the nongaussianity of the distribution substantially
increases the sampling errors for the skewness and kurtosis in the several to
tens of arcminutes regime, whereas those for the power spectrum are only
fractionally increased even out to wavenumbers where shot noise from the
intrinsic ellipticities of the galaxies will likely dominate the errors.Comment: 12 pages, 13 figures; minor changes reflect accepted versio
Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?
We present a smoothed particle hydrodynamic (SPH) simulation that reproduces
a galaxy that is a moderate facsimile of those observed. The primary failing
point of previous simulations of disk formation, namely excessive transport of
angular momentum from gas to dark matter, is ameliorated by the inclusion of a
supernova feedback algorithm that allows energy to persist in the model ISM for
a period corresponding to the lifetime of stellar associations. The inclusion
of feedback leads to a disk at a redshift , with a specific angular
momentum content within 10% of the value required to fit observations. An
exponential fit to the disk baryon surface density gives a scale length within
17% of the theoretical value. Runs without feedback, with or without star
formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Supermassive Black Holes and Galaxy Formation
The formation of supermassive black holes (SMBH) is intimately related to
galaxy formation, although precisely how remains a mystery. I speculate that
formation of, and feedback from, SMBH may alleviate problems that have arisen
in our understanding of the cores of dark halos of galaxies.Comment: Talk at conference on Matter in the Universe, March 2001, ISSI Ber
Metal Enrichment and Temperature of the Intergalactic Medium
Hydrodynamic simulations of Lyman alpha clouds based on ab inito cosmological
models have produced results that are in broad agreement with observations.
However, further analyses have revealed that, with progressively higher
numerical resolution, the median or cutoff line width of the simulated Lyman
alpha clouds (i.e. the Doppler parameter) appears to converge to a value
significantly below what is observed at z~3 (by about a factor of 1.5). These
convergence test simulations do not include feedback from star formation. Given
the observed metallicity in the Lyman alpha clouds we suggest that supernovae,
which presumably polluted the IGM with metals, may have deposited a sufficient
amount of energy in the IGM to reconcile the theory with observations. Simple
arguments immediately narrow the redshift range of pollution down to
4<z_{dep}<9. It seems quite certain that dwarf and sub-dwarf galaxies with
total masses in the range 10^{6.5-9.0}Msun have to be largely responsible for
the pollution. Furthermore, it is implied that either star formation is very
efficient or metal yield is very high for these early dwarf galaxies, if the
mean metallicity in the universe at z=3 is as high as =0.01Zsun. Finally,
assuming the specific supernova heating energy is proportional to the
metallicity of a gas, we note that the picture proposed here would be
consistent with supernovae being the apparently needed heating source for the
intra-cluster gas, if the required heating of the intra-cluster gas is no
greater than 1 keV per particle.Comment: accepted to ApJ Letter
The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations
By means of high-resolution cosmological hydrodynamical simulations of Milky
Way-like disc galaxies, we conduct an analysis of the associated stellar
metallicity distribution functions (MDFs). After undertaking a kinematic
decomposition of each simulation into spheroid and disc sub-components, we
compare the predicted MDFs to those observed in the solar neighbourhood and the
Galactic bulge. The effects of the star formation density threshold are visible
in the star formation histories, which show a modulation in their behaviour
driven by the threshold. The derived MDFs show median metallicities lower by
0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic
bulge. Possible reasons for this apparent discrepancy include the use of low
stellar yields and/or centrally-concentrated star formation. The dispersions
are larger than the one of the observed MDF; this could be due to simulated
discs being kinematically hotter relative to the Milky Way. The fraction of low
metallicity stars is largely overestimated, visible from the more negatively
skewed MDF with respect to the observational sample. For our fiducial Milky Way
analog, we study the metallicity distribution of the stars born "in situ"
relative to those formed via accretion (from disrupted satellites), and
demonstrate that this low-metallicity tail to the MDF is populated primarily by
accreted stars. Enhanced supernova and stellar radiation energy feedback to the
surrounding interstellar media of these pre-disrupted satellites is suggested
as an important regulator of the MDF skewness.Comment: 20 pages, 14 figures, MNRAS, accepte
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