Title: The effect of spiral and bar structure on the Milky Way velocity distribution
Abstract: Using particle integrations of more than a million
particles we explore the effect of spiral arms and the Galactic bar
on the velocity distribution in the Milky Way disk. In this thesis
we explore four topics: Heating due to
multiple spiral patterns: We find that stochastic heating
(increased random motions with time) is induced by interactions
between two steady-state spiral density waves moving at different
pattern speeds. We examine a range of spiral strengths and spiral
speeds and show that stars in this time dependent gravitational
field can be heated. This is a new mechanism for increasing the
stellar velocity dispersion in galactic disks. If multiple spiral
patterns are present in the Galaxy we predict that there should be
large variations in the stellar velocity dispersion as a function
of radius. The effect of spiral structure perturbations
on Oort constant measurements: We find that a two-armed spiral
density wave affects measurements of the Oort constants introducing
systematic errors in A and B
of order 5 km s−1
kpc−1. Oort’s |C|, on
average, is larger for lower stellar velocity dispersions, contrary
to recent measurements (Olling & Dehnen 2003). We conclude that
spiral structure alone cannot account for this observation. This
discrepancy is resolved by considering the effect of the Galactic
bar on the local stellar velocity distribution (see (3)).
The effect of the Galactic bar on Oort’s
C-value: Previous work has related the Galactic
bar to structure in the local stellar velocity distribution. We
show that the bar also influences the spatial gradients of the
velocity vector via the Oort constants. By numerical integration of
test-particles we simulate measurements of the Oort
C-value in a gravitational potential including
the Galactic bar. We account for the observed trend that
C is increasingly negative for stars with higher
velocity dispersion. By comparing measurements of
C with our simulations we improve on previous
models of the bar, estimating that its pattern speed is
[omega]b/[omega] 0 =
1.87 ± 0.04, where [omega]0 is the local
circular frequency, and the bar angle lies within 20◦ ≤
φ0 ≤ 45◦. We find that the Galactic bar
causes biases in measurements of the Oort constants
A and B less than ~2 km
s−1 kpc−1 for the hot
stars. Signatures of spiral structure in Galactic
pencil-beam and large-scale radial velocity surveys: By simulating
pencil-beam and all-sky Galactic radial velocity surveys we show
how the statistical kinematic properties of stars can be used to
infer spiral structure parameters. Spiral pattern speeds can be
estimated from the location of rings of increased velocity
dispersion caused by Lindblad resonances. This information,
however, must be combined with information related to the
velocities and stellar number density in order to distinguish from
a four-armed structure. The mean line-of-sight velocity and…
Publication Year: 2008
Publication Date: 2008-06-01
Language: en
Type: article
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