282 research outputs found
Light Propagation in Inhomogeneous Universes. V. Gravitational Lensing of Distant Supernovae
We use a series of ray-tracing experiments to determine the magnification
distribution of high-redshift sources by gravitational lensing. We determine
empirically the relation between magnification and redshift, for various
cosmological models. We then use this relation to estimate the effect of
lensing on the determination of the cosmological parameters from observations
of high-z supernovae. We found that, for supernovae at redshifts z<1.8 the
effect of lensing is negligible compared to the intrinsic uncertainty in the
measurements. Using mock data in the range 1.8<z<8, we show that the effect of
lensing can become significant. Hence, if a population of very-high-z
supernovae was ever discovered, it would be crucial to fully understand the
effect of lensing, before these SNe could be used to constrain cosmological
models. We show that the distance moduli m-M for an open CDM universe and a
LCDM universe are comparable at z>2. Therefore if supernovae up to these
redshifts were ever discovered, it is still the ones in the range 0.3<z<1 that
would distinguish these two models.Comment: 15 pages. Accepted for publication in The Astrophysical Journal.
References adde
Chemodynamics of dwarf galaxies under ram-pressure
By implementing a dynamic wind-tunnel model in a smoothed-particle
chemodynamic/hydrodynamic simulation suite, we have investigated the effects of
ram pressure and tidal forces on dwarf galaxies similar to the Magellanic
Clouds, within host galaxies with gas and dark matter halos that are varied, to
compare the relative effects of tides and ram pressure. We concentrate on how
the distributions of metals are affected by interactions. We find that while
ram pressure and tidal forces have some effect on dwarf galaxy outflows, these
effects do not produce large differences in the metal distributions of the
dwarf disks other than truncation in the outer regions in some cases, and that
confinement from the host galaxy gas halo appears to be more significant than
ram pressure stripping. We find that stochastic variations in the star
formation rate can explain the remaining variations in disk metal properties.
This raises questions on the cause of low metallicities in dwarf galaxies.Comment: Submitted to ApJ, under 2nd review (very minor revisions
Explosions and Outflows during Galaxy Formation
We consider an explosion at the center of a halo which forms at the
intersection of filaments inside a cosmological pancake, a convenient test-bed
model for galaxy formation. ASPH/P3M simulations reveal that such explosions
are anisotropic. The energy and metals are channeled into the low density
regions, away from the pancake. The pancake remains essentially undisturbed,
even if the explosion is strong enough to blow away all the gas located inside
the halo and reheat the IGM surrounding the pancake. Infall quickly replenishes
this ejected gas and gradually restores the gas fraction as the halo continues
to grow. Estimates of the collapse epoch and SN energy-release for galaxies of
different mass in the CDM model can relate these results to scale-dependent
questions of blow-out and blow-away and their implication for early IGM heating
and metal enrichment and the creation of gas-poor dwarf galaxies.Comment: To appear in "The 20th Texas Symposium on Relativistic Astrophysics",
eds. H. Martel and J.C. Wheeler, AIP, in press (2001) (3 pages, 2 figures
A Convenient Set of Comoving Cosmological Variables and Their Application
We present a set of cosmological variables, called "supercomoving variables,"
which are particularly useful for describing the gas dynamics of cosmic
structure formation. For ideal gas with gamma=5/3, the supercomoving position,
velocity, density, temperature, and pressure are constant in time in a uniform,
isotropic, adiabatically expanding universe. Expressed in terms of these
supercomoving variables, the cosmological fluid conservation equations and the
Poisson equation closely resemble their noncosmological counterparts. This
makes it possible to generalize noncosmological results and techniques to
cosmological problems, for a wide range of cosmological models. These variables
were initially introduced by Shandarin for matter-dominated models only. We
generalize supercomoving variables to models with a uniform component
corresponding to a nonzero cosmological constant, domain walls, cosmic strings,
a nonclumping form of nonrelativistic matter (e.g. massive nettrinos), or
radiation. Each model is characterized by the value of the density parameter
Omega0 of the nonrelativistic matter component in which density fluctuation is
possible, and the density parameter OmegaX of the additional, nonclumping
component. For each type of nonclumping background, we identify FAMILIES within
which different values of Omega0 and OmegaX lead to fluid equations and
solutions in supercomoving variables which are independent of Omega0 and
OmegaX. We also include the effects of heating, radiative cooling, thermal
conduction, viscosity, and magnetic fields. As an illustration, we describe 3
familiar cosmological problems in supercomoving variables: the growth of linear
density fluctuations, the nonlinear collapse of a 1D plane-wave density
fluctuation leading to pancake formation, and the Zel'dovich approximation.Comment: 38 pages (AAS latex) + 2 figures (postscript) combined in one gzip-ed
tar file. Identical to original posted version, except for addition of 2
references. Monthly Notices of the R.A.S., in pres
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