25,818 research outputs found
The Formation of Galactic Disks
We study the population of galactic disks expected in current hierarchical
clustering models for structure formation. A rotationally supported disk with
exponential surface density profile is assumed to form with a mass and angular
momentum which are fixed fractions of those of its surrounding dark halo. We
assume that haloes respond adiabatically to disk formation, and that only
stable disks can correspond to real systems. With these assumptions the
predicted population can match both present-day disks and the damped Lyman
alpha absorbers in QSO spectra. Good agreement is found provided: (i) the
masses of disks are a few percent of those of their haloes; (ii) the specific
angular momenta of disks are similar to those of their haloes; (iii)
present-day disks were assembled recently (at z<1). In particular, the observed
scatter in the size-rotation velocity plane is reproduced, as is the slope and
scatter of the Tully-Fisher relation. The zero-point of the TF relation is
matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent
with observational values derived from disk dynamics. High redshift disks are
predicted to be small and dense, and could plausibly merge together to form the
observed population of elliptical galaxies. In many (but not all) currently
popular cosmogonies, disks with rotation velocities exceeding 200 km/s can
account for a third or more of the observed damped Lyman alpha systems at
z=2.5. Half of the lines-of-sight to such systems are predicted to intersect
the absorber at r>3kpc/h and about 10% at r>10kpc/h. The cross-section for
absorption is strongly weighted towards disks with large angular momentum and
so large size for their mass. The galaxy population associated with damped
absorbers should thus be biased towards low surface brightness systems.Comment: 47 pages, Latex, aaspp4.sty, 14 figs included, submitted to MNRA
The Structure and Clustering of Lyman Break Galaxies
The number density and clustering properties of Lyman-break galaxies (LBGs)
are consistent with them being the central galaxies of the most massive dark
halos present at z~3. This conclusion holds in all currently popular
hierarchical models for structure formation, and is almost independent of the
global cosmological parameters. We examine whether the sizes, luminosities,
kinematics and star-formation rates of LBGs are also consistent with this
identification. Simple formation models tuned to give good fits to low redshift
galaxies can predict the distribution of these quantities in the LBG
population. The LBGs should be small (with typical half-light radii of 0.6-2
kpc/h), should inhabit haloes of moderately high circular velocity (180-290
km/s) but have low stellar velocity dispersions (70-120 km/s) and should have
substantial star formation rates (15-100 Msun/yr). The numbers here refer to
the predicted median values in the LBG sample of Adelberger et al. (1998); the
first assumes an Omega=1 universe and the second a flat universe with
Omega=0.3. For either cosmology these predictions are consistent with the
current (rather limited) observational data. Following the work of Kennicutt
(1998) we assume stars to form more rapidly in gas of higher surface density.
This predicts that LBG samples should preferentially contain objects with low
angular momentum, and so small size, for their mass. In contrast, samples of
damped Lyman alpha systems (DLSs), should be biased towards objects with large
angular momentum. Bright LBGs and DLSs may therefore form distinct populations,
with very different sizes and star formation rates, LBGs being smaller and more
metal-rich than DLSs of similar mass and redshift.Comment: 27 pages, 9 figures, MNRAS submitte
Neutron star matter in the quark-meson coupling model in strong magnetic fields
The effects of strong magnetic fields on neutron star matter are investigated
in the quark-meson coupling (QMC) model. The QMC model describes a nuclear
many-body system as nonoverlapping MIT bags in which quarks interact through
self-consistent exchange of scalar and vector mesons in the mean-field
approximation. The results of the QMC model are compared with those obtained in
a relativistic mean-field (RMF) model. It is found that quantitative
differences exist between the QMC and RMF models, while qualitative trends of
the magnetic field effects on the equation of state and composition of neutron
star matter are very similar.Comment: 16 pages, 4 figure
Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions
Accurate models of alkali and halide ions in aqueous solution are necessary
for computer simulations of a broad variety of systems. Previous efforts to
develop ion force fields have generally focused on reproducing experimental
measurements of aqueous solution properties such as hydration free energies and
ion-water distribution functions. This dependency limits transferability of the
resulting parameters because of the variety and known limitations of water
models. We present a solvent-independent approach to calibrating ion parameters
based exclusively on crystal lattice properties. Our procedure relies on
minimization of lattice sums to calculate lattice energies and interionic
distances instead of equilibrium ensemble simulations of dense fluids. The gain
in computational efficiency enables simultaneous optimization of all parameters
for Li+, Na+, K+, Rb+, Cs+, F-, Cl-, Br-, and I- subject to constraints that
enforce consistency with periodic table trends. We demonstrate the method by
presenting lattice-derived parameters for the primitive model and the
Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting
parameters successfully reproduce the lattice properties used to derive them
and are free from the influence of any water model. To assess the
transferability of the Lennard-Jones parameters to aqueous systems, we used
them to estimate hydration free energies and found that the results were in
quantitative agreement with experimentally measured values. These
lattice-derived parameters are applicable in simulations where coupling of ion
parameters to a particular solvent model is undesirable. The simplicity and low
computational demands of the calibration procedure make it suitable for
parametrization of crystallizable ions in a variety of force fields.Comment: 9 pages, 5 table
Constraints on the average magnetic field strength of relic radio sources 0917+75 and 1401-33 from XMM-Newton observations
We observed two relic radio sources, 0917+75 and 1401-33, with the XMM-Newton
X-ray observatory. We did not detect any X-ray emission, thermal or
non-thermal, in excess of the local background level from either target. This
imposes new upper limits on the X-ray flux due to inverse Compton scattering of
photons from the cosmic microwave background by relativistic electrons in the
relic sources, and new lower limits on the magnetic field strength from the
relative strength of the radio and X-ray emission. The combination of radio and
X-ray observations provides a measure of the magnetic field independent of
equipartition or minimum energy assumptions. Due to increasing sensitivity of
radio observations, the known population of cluster relics has been growing;
however, studies of non-thermal X-ray emission from relics remain scarce. Our
study adds to the small sample of relics studied in X-rays. In both relics, our
field strength lower limits are slightly larger than estimates of the
equipartition magnetic field.Comment: 11 pages, 5 figures. Accepted by MNRA
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