4 research outputs found
Jeans Instability of Palomar 5's Tidal Tail
Tidal tails composed of stars should be unstable to the Jeans instability and
this can cause them to look like beads on a string. The Jeans wavelength and
tail diameter determine the wavelength and growth rate of the fastest growing
unstable mode. Consequently the distance along the tail to the first clump and
spacing between clumps can be used to estimate the mass density in the tail and
its longitudinal velocity dispersion. Clumps in the tidal tails of the globular
cluster Palomar 5 could be due to Jeans instability. We find that their spacing
is consistent with the fastest growing mode if the velocity dispersion in the
tail is similar to that in the cluster itself. While all tidal tails should
exhibit gravitational instability, we find that clusters or galaxies with low
concentration parameters are most likely to exhibit short wavelength rapidly
growing Jeans modes in their tidal tails.Comment: sumbmitted to MNRA
Jeans Instability in a Tidally Disrupted Halo Satellite Galaxy
We use a hybrid test particle/N-body simulation to integrate 4 million
massless test particle trajectories within a fully self-consistent 10^5
particle N-body simulation. The number of massless particles allows us to
resolve fine structure in the spatial distribution and phase space of a dwarf
galaxy as it is disrupted in the tidal field of a Milky Way type galaxy. The
tidal tails exhibit nearly periodic clumping or a smoke-like appearance. By
running simulations with different satellite particle mass, halo particle mass,
number of massive and massless particles and with and without a galaxy disk, we
have determined that the instabilities are not due to numerical noise,
amplification of structure in the halo, or shocking as the satellite passes
through the disk of the Galaxy. We measure Jeans wavelengths and growth
timescales in the tidal tail and show that the Jeans instability is a viable
explanation for the clumps. We find that the instability causes velocity
perturbations of order 10 km/s. Clumps in tidal tails present in the Milky Way
could be seen in stellar radial velocity surveys as well as number counts. We
find that the unstable wavelength growth is sensitive to the simulated mass of
dark matter halo particles. A simulation with a smoother halo exhibits colder
and thinner tidal tails with more closely spaced clumps than a simulation with
more massive dark matter halo particles. Heating by the halo particles
increases the Jeans wavelength in the tidal tail affecting substructure
development, suggesting an intricate connection between tidal tails and dark
matter halo substructure.Comment: 15 pages, 7 figures, submitted to MNRAS, May 25 201
Structure in phase space associated with spiral and bar density waves in an N-body galactic disk
An N-body hybrid simulation, integrating both massive and tracer particles,
of a Galactic disk is used to study the stellar phase space distribution or
velocity distributions in different local neighborhoods. Pattern speeds
identified in Fourier spectrograms suggest that two-armed and three-armed
spiral density waves, a bar and a lopsided motion are coupled in this
simulation, with resonances of one pattern lying near resonances of other
patterns. We construct radial and tangential (uv) velocity distributions from
particles in different local neighborhoods. More than one clump is common in
these local velocity distributions regardless of the position in the disk.
Features in the velocity distribution observed at one galactic radius are also
seen in nearby neighborhoods (at larger and smaller radii) but with shifted
mean v values. This is expected if the v velocity component of a clump sets the
mean orbital galactic radius of its stars. We find that gaps in the velocity
distribution are associated with the radii of kinks or discontinuities in the
spiral arms. These gaps also seem to be associated with Lindblad resonances
with spiral density waves and so denote boundaries between different dominant
patterns in the disk. We discuss implications for interpretations of the Milky
Way disk based on local velocity distributions. Velocity distributions created
from regions just outside the bar's Outer Lindblad resonance and with the bar
oriented at 45 degrees from the Sun-Galactic center line more closely resemble
that seen in the solar neighborhood (triangular in shape at lower uv and with a
Hercules like stream) when there is a strong nearby spiral arm, consistent with
the observed Centaurus Arm tangent, just interior to the solar neighborhood.Comment: accepted for publication in MNRA