7 research outputs found
Transient chaos and resonant phase mixing in violent relaxation
This paper explores how orbits in a galactic potential can be impacted by
large amplitude time-dependences of the form that one might associate with
galaxy or halo formation or strong encounters between pairs of galaxies. A
period of time-dependence with a strong, possibly damped, oscillatory component
can give rise to large amounts of transient chaos, and it is argued that
chaotic phase mixing associated with this transient chaos could play a major
role in accounting for the speed and efficiency of violent relaxation. Analysis
of simple toy models involving time-dependent perturbations of an integrable
Plummer potential indicates that this chaos results from a broad, possibly
generic, resonance between the frequencies of the orbits and harmonics thereof
and the frequencies of the time-dependent perturbation. Numerical computations
of orbits in potentials exhibiting damped oscillations suggest that, within a
period of 10 dynamical times t_D or so, one could achieve simultaneously both
`near-complete' chaotic phase mixing and a nearly time-independent, integrable
end state.Comment: 11 pages and 12 figures: an extended version of the original
manuscript, containing a modified title, one new figure, and approximately
one page of additional text, to appear in Monthly Notices of the Royal
Astronomical Societ
Evolution of the Dark Matter Phase-Space Density Distributions of LCDM Halos
We study the evolution of phase-space density during the hierarchical
structure formation of LCDM halos. We compute both a spherically-averaged
surrogate for phase-space density (Q) and the coarse-grained distribution
function f(x,v) for dark matter particles that lie within~2 virial radii of
four Milky-Way-sized dark matter halos. The estimated f(x,v) spans over four
decades at any radius. Dark matter particles that end up within two virial
radii of a Milky-Way-sized DM halo at have an approximately Gaussian
distribution in log(f) at early redshifts, but the distribution becomes
increasingly skewed at lower redshifts. The value corresponding to the peak of
the Gaussian decreases as the evolution progresses and is well described by a
power-law in (1+z). The highest values of f are found at the centers of dark
matter halos and subhalos, where f can be an order of magnitude higher than in
the center of the main halo. The power-law Q(r) profile likely reflects the
distribution of entropy (K = sigma^2/rho^{2/3} \propto r^{1.2}), which dark
matter acquires as it is accreted onto a growing halo. The estimated f(x, v),
on the other hand, exhibits a more complicated behavior. Although the median
coarse-grained phase-space density profile F(r) can be approximated by a
power-law in the inner regions of halos and at larger radii the profile
flattens significantly. This is because phase-space density averaged on small
scales is sensitive to the high-f material associated with surviving subhalos,
as well as relatively unmixed material (probably in streams) resulting from
disrupted subhalos, which contribute a sizable fraction of matter at large
radii. (ABRIDGED)Comment: Closely matches version accepted for publicatio