29,840 research outputs found
Is J 133658.3-295105 a Radio Source at z >= 1.0 or at the Distance of M 83?
We present Gemini optical imaging and spectroscopy of the radio source J
133658.3-295105. This source has been suggested to be the core of an FR II
radio source with two detected lobes. J 133658.3-295105 and its lobes are
aligned with the optical nucleus of M 83 and with three other radio sources at
the M 83 bulge outer region. These radio sources are neither supernova remnants
nor H II regions. This curious configuration prompted us to try to determine
the distance to J 133658.3-295105. We detected H_alpha emission redshifted by ~
130 km s^-1 with respect to an M 83 H II region 2.5" east-southeast of the
radio source. We do not detect other redshifted emission lines of an optical
counterpart down to m_i = 22.2 +/- 0.8. Two different scenarios are proposed:
the radio source is at z >= 2.5, a much larger distance than the previously
proposed lower limit z >= 1.0, or the object was ejected by a gravitational
recoil event from the M 83 nucleus. This nucleus is undergoing a strong
dynamical evolution, judging from previous three-dimensional spectroscopy.Comment: 6 pages, 4 figure
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
Heat conductivity in the presence of a quantized degree of freedom
We propose a model with a quantized degree of freedom to study the heat
transport in quasi-one dimensional system. Our simulations reveal three
distinct temperature regimes. In particular, the intermediate regime is
characterized by heat conductivity with a temperature exponent much
greater than 1/2 that was generally found in systems with point-like particles.
A dynamical investigation indicates the occurrence of non-equipartition
behavior in this regime. Moreover, the corresponding Poincar\'e section also
shows remarkably characteristic patterns, completely different from the cases
of point-like particles.Comment: 7 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
Expanding the thermodynamical potential and the analysis of the possible phase diagram of deconfinement in FL model
The deconfinement phase transition is studied in the FL model at finite
temperature and chemical potential. At MFT approximation, the phase transition
can only be the first order in the whole phase plane. By a Landau
expansion we further study the phase transition order and the possible phase
diagram of deconfinement. We discuss the possibilities of second order phase
transitions in FL model. By our analysis the cubic term in the Landau expansion
could be cancelled by the high order fluctuations. By an ansatz of the Landau
parameters, we obtain the possible phase diagram with both first and second
order phase transition including the tricritical point which is similar to that
of the chiral phase transition.Comment: 7 pages, 8 figures, submitted to Chinese Physics
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