671 research outputs found
The Structure of Isothermal, Self-gravitating Gas Spheres for Softened Gravity
A theory for the structure of isothermal, self-gravitating gas spheres in
pressure equilibrium in a softened gravitational field is developed. The one
parameter spline softening proposed by Hernquist & Katz (1989) is used. We show
that the addition of this extra scale parameter implies that the set of
equilibrium solutions constitute a one-parameter family, rather than the one
and only one isothermal sphere solution for Newtonian gravity. We demonstrate
the perhaps somewhat surprising result that for any finite choice of softening
length and temperature, it is possible to deposit an arbitrarily large mass of
gas in pressure equilibrium and with a non-singular density distribution inside
of r_0 for any r_0 > 0. The theoretical predictions of our models are compared
with the properties of the small, massive, quasi-isothermal gas clumps which
typically form in numerical Tree-SPH simulations of 'passive' galaxy formation
of Milky Way sized galaxies. We find reasonable agreement despite the neglect
of rotational support in the models. We comment on whether the hydrodynamical
resolution in our numerical simulation of galaxy formation is sufficient, and
finally we conclude that one should be cautious, when comparing results of
numerical simulations involving gravitational softening and hydrodynamical
smoothing, with reality.Comment: 22 pages Latex + 12 figure
Dark resonances as a probe for the motional state of a single ion
Single, rf-trapped ions find various applications ranging from metrology to
quantum computation. High-resolution interrogation of an extremely weak
transition under best observation conditions requires an ion almost at rest. To
avoid line-broadening effects such as the second order Doppler effect or rf
heating in the absence of laser cooling, excess micromotion has to be
eliminated as far as possible. In this work the motional state of a confined
three-level ion is probed, taking advantage of the high sensitivity of observed
dark resonances to the trapped ion's velocity. Excess micromotion is controlled
by monitoring the dark resonance contrast with varying laser beam geometry. The
influence of different parameters such as the cooling laser intensity has been
investigated experimentally and numerically
Conservation Laws in Smooth Particle Hydrodynamics: the DEVA Code
We describe DEVA, a multistep AP3M-like-SPH code particularly designed to
study galaxy formation and evolution in connection with the global cosmological
model. This code uses a formulation of SPH equations which ensures both energy
and entropy conservation by including the so-called \bn h terms. Particular
attention has also been paid to angular momentum conservation and to the
accuracy of our code. We find that, in order to avoid unphysical solutions, our
code requires that cooling processes must be implemented in a non-multistep
way.
We detail various cosmological simulations which have been performed to test
our code and also to study the influence of the \bn h terms. Our results
indicate that such correction terms have a non-negligible effect on some
cosmological simulations, especially on high density regions associated either
to shock fronts or central cores of collapsed objects. Moreover, they suggest
that codes paying a particular attention to the implementation of conservation
laws of physics at the scales of interest, can attain good accuracy levels in
conservation laws with limited computational resources.Comment: 36 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
MCG+00-32-16: An Irregular Galaxy Close to the Lowest Redshift Absorber on the 3C 273 Line of Sight
MCG+00-32-16 is the galaxy closest in position-velocity space to the lowest
redshift Ly absorber along the line-of-sight to the quasar 3C 273. Its
projected separation is 204 (d/19 Mpc) kpc, where d is the distance from the
Milky Way to the galaxy, and the redshift difference is only 94 km/s; HI
1225+01 is slightly closer in projected separation to the absorber, but has a
greater redshift difference. We present HI synthesis array mapping and CCD
photometry in B and R for MCG+00-32-16. The HI disk is rotating in such a way
that the side of the galaxy closer to the sight-line to the quasar has the
larger velocity difference from the absorber. The absorber may be a ``failed
dwarf'' member of a poor galaxy group of which MCG+00-32-16 and HI 1225+01 are
the only members to have formed stars.Comment: 14 pages, 9 figures, accepted by Astrophysical Journa
Cosmological SPH simulations with four million particles: statistical properties of X-ray clusters in a low-density universe
We present results from a series of cosmological SPH (smoothed particle
hydrodynamics) simulations coupled with the P3M
(Particle-Particle-Particle-Mesh) solver for the gravitational force. The
simulations are designed to predict the statistical properties of X-ray
clusters of galaxies as well as to study the formation of galaxies. We have
seven simulation runs with different assumptions on the thermal state of the
intracluster gas. Following the recent work by Pearce et al., we modify our SPH
algorithm so as to phenomenologically incorporate the galaxy formation by
decoupling the cooled gas particles from the hot gas particles. All the
simulations employ 128^3 particles both for dark matter and for gas components,
and thus constitute the largest systematic catalogues of simulated clusters in
the SPH method performed so far. These enable us to compare the analytical
predictions on statistical properties of X-ray clusters against our direct
simulation results in an unbiased manner. We find that the luminosities of the
simulated clusters are quite sensitive to the thermal history and also to the
numerical resolution of the simulations, and thus are not reliable. On the
other hand, the mass-temperature relation for the simulated clusters is fairly
insensitive to the assumptions of the thermal state of the intracluster gas,
robust against the numerical resolution, and in fact agrees well with the
analytic prediction. Therefore the prediction for the X-ray temperature
function of clusters on the basis of the Press-Schechter mass function and the
virial equilibrium is fairly reliable.Comment: Accepted for publication in The Astrophysical Journal. 18 pages with
7 embedded figure
- âŠ