817 research outputs found
Magnetic susceptibility of a neutron star crust
The magnetic susceptibility of the degenerate free electrons in the crust of a neutron star is computed for a range of densities, temperatures, and field strengths. It is shown that when the temperature is low enough (typically less than 10^7K for densities of about 10^7 g cm^(-3) and 10^(12) G fields), the susceptibility undergoes large de Haas-van Alphen oscillations. The crust is then unstable to the formation of layers of domains of alternating magnetisation. Associated with these domains are magnetic field fluctuations of a few per cent amplitude and anisotropic magnetostrictive stresses which may be large enough to crumble the crust. It is argued that these domains are unlikely to directly influence the surface properties of the neutron star but may possibly be coupled indirectly to observable effects
Global Properties of Multiple Merger Remnants
Merger remnants of small groups of galaxies are contrasted with relics of
mergers of pairs of galaxies to determine which process produces objects that
best compare to real ellipticals. In both cases, the progenitors consist of
self-gravitating disks, halos, and, sometimes, bulges. Pairs of galaxies merge
from orbits that initially have zero--energy. The systems that produce multiple
merger remnants are dense, six--member groups in virial equilibrium with low
velocity dispersions. Multiple and pair mergers produce remnants which differ
in both their spatial and kinematic properties. Multiple merger remnants have
small triaxialities and are most likely to appear nearly round from most
viewing angles. They possess cores, with sizes of a few tenths of an effective
radius, that are more extended than pair remnant cores, even when bulges are
included in the progenitors. In multiple mergers, the spin of all components --
halo, disk, and bulge -- increases and, while velocity dispersion dominates in
the central regions, outside an effective radius in some
projections. The angular momentum and minor axis vectors are aligned for
multiple merger remnants. During merging of multiple progenitors, about half of
the orbital angular momentum in each luminous component is converted into
internal rotation in that component. Material is prevented from accumulating in
the center of multiple merger remnants as efficiently as it does in pair
mergers. In pair mergers that include gas, unrealistically steep surface
brightness profiles have been produced in center of the remnants; in multiple
mergers the formation of overdense nuclei may be impeded, thus allowing more
successful comparison with real elliptical galaxies.Comment: 43 pages of uuencoded compressed postscript with 17 figures.
Postscript figures 1,2,3,17, 3Mb total, available upon request from
[email protected]. Accepted to the Astrophysical Journa
First Structure Formation: A Simulation of Small Scale Structure at High Redshift
We describe the results of a simulation of collisionless cold dark matter in
a LambdaCDM universe to examine the properties of objects collapsing at high
redshift (z=10). We analyze the halos that form at these early times in this
simulation and find that the results are similar to those of simulations of
large scale structure formation at low redshift. In particular, we consider
halo properties such as the mass function, density profile, halo shape, spin
parameter, and angular momentum alignment with the minor axis. By understanding
the properties of small scale structure formation at high redshift, we can
better understand the nature of the first structures in the universe, such as
Population III stars.Comment: 31 pages, 14 figures; accepted for publication in ApJ. Figure 1 can
also be viewed at http://cfa-www.harvard.edu/~hjang/research
Gas distribution and starbursts in shell galaxies
Detailed maps of most elliptical galaxies reveal that, whereas the greatest part of their luminous mass originates from a smooth distribution with a surface brightness approximated by a de Vaucouleurs law, a small percentage of their light is contributed by low surface brightness distortions termed 'fine structures'. The sharp-edged features called 'shells' are successfully reproduced by merger and infall models involving accretion from less massive companions. In this context, dwarf spheroidal and compact disk galaxies are likely progenitors of these stellar phenomena. However, it is probable that the sources of shell-forming material also contain significant amounts of gas. This component may play an important role in constraining the formation and evolution of shell galaxies. To investigate the effects of the gaseous component, numerical simulations were performed to study the tidal disruption of dwarf galaxies containing both gas and stars by more massive primaries, and the evolution of the ensuing debris. The calculations were performed with a hybrid N-body/hydrodynamics code. Collisionless matter is evolved using a conventional N-body technique and gas is treated using smoothed particle hydrodynamics in which self-gravitating fluid elements are represented as particles evolving according to Lagrangian hydrodynamic equations. An isothermal equation of state is employed so the gas remains at a temperature 104 K. Owing to the large mass ratio between the primary and companion, the primary is modeled as a rigid potential and the self-gravity of both galaxies is neglected
Self-similarity and scaling behavior of scale-free gravitational clustering
We measure the scaling properties of the probability distribution of the
smoothed density field in -body simulations of expanding universes with
scale-free initial power-spectra, with particular attention to the predictions
of the stable clustering hypothesis. We concentrate our analysis on the ratios
, , where is the averaged -body correlation function over a cell of radius
. The behavior of the higher order correlations is studied through that
of the void probability distribution function.
As functions of , the quantities , ,
exhibit two plateaus separated by a smooth transition around . In the weakly nonlinear regime, {\bar \xi}_2 \la 1, the results are in
reasonable agreement with the predictions of perturbation theory. In the
nonlinear regime, , the function is
larger than in the weakly nonlinear regime, and increasingly so with . It
is well-fitted by the expression $S_Q= ({\bar \xi}_2/100)^{0.045(Q-2)}\
{\widetilde S}_Qn. This weak dependence on scale proves {\em a
small, but significant departure from the stable clustering predictions} at
least for n=0n=+1P_0S_Qn=-2n=-1$. In these two cases, our measurements are not accurate enough to be
discriminant.Comment: 31 pages, postscript file, figure 1 missing. Postscript file
including figure 1 available at
ftp://ftp-astro-theory.fnal.gov:/pub/Publications/Pub-95-256-
Extended Perturbation Theory for the Local Density Distribution Function
Perturbation theory makes it possible to calculate the probability
distribution function (PDF) of the large scale density field in the small
variance limit. For top hat smoothing and scale-free Gaussian initial
fluctuations, the result depends only on the linear variance, sigma_linear, and
its logarithmic derivative with respect to the filtering scale
-(n_linear+3)=dlog sigma_linear^2/dlog L (Bernardeau 1994). In this paper, we
measure the PDF and its low-order moments in scale-free simulations evolved
well into the nonlinear regime and compare the results with the above
predictions, assuming that the spectral index and the variance are adjustable
parameters, n_eff and sigma_eff=sigma, where sigma is the true, nonlinear
variance. With these additional degrees of freedom, results from perturbation
theory provide a good fit of the PDFs, even in the highly nonlinear regime. The
value of n_eff is of course equal to n_linear when sigma << 1, and it decreases
with increasing sigma. A nearly flat plateau is reached when sigma >> 1. In
this regime, the difference between n_eff and n_linear increases when n_linear
decreases. For initial power-spectra with n_linear=-2,-1,0,+1, we find n_eff ~
-9,-3,-1,-0.5 when sigma^2 ~ 100.Comment: 13 pages, 6 figures, Latex (MN format), submitted to MNRA
Simulations of shell galaxies with GADGET-2: Multi-generation shell systems
As the missing complement to existing studies of shell galaxies, we carried
out a set of self-consistent N-body simulations of a minor merger forming a
stellar shell system within a giant elliptical galaxy. We discuss the effect of
a phenomenon possibly associated with the galaxy merger simulations --- a
presence of multiple generations of shells.Comment: 2 pages, 1 figure, to appear in the Proceedings of JENAM 2010,
Symposium 2: "Environment and the formation of galaxies: 30 years later
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