296 research outputs found

### Radial Mixing due to Spiral-Bar Resonance Overlap: Implications to the Milky Way

We have recently identified a previously unknown radial migration mechanism
resulting from the overlap of spiral and bar resonances in galactic discs
(Minchev & Famaey 2010, Minchev et al. 2010). This new mechanism is much more
efficient than mixing by transient spirals and its presence is unavoidable in
all barred galaxies, such as our own Milky Way. The consequences of this are a
strong flattening in the metallicity gradient in the disc, an extended disc
profile, and the formation of a thick disc component, all taking place in only
a couple of Gyr. This timescale is drastically shorter than previously expected
and thus can put strong constraints on the longevity, strength and pattern
speeds of the Galactic bar and Spiral Structure.Comment: 4 pages, 2 figures, presented at ELSA 2010: Gaia, at the frontiers of
astrometry, 7-11 June 2010, S\`evres, Paris; To published in EAS Series;
Proceedings editors: C. Turon, F. Arenou & F. Meynadie

### Disk heating by more than one spiral density wave

We consider a differentially rotating, 2D stellar disk perturbed by two
steady state spiral density waves moving at different patterns speeds. Our
investigation is based on direct numerical integration of initially circular
test-particle orbits. We examine a range of spiral strengths and spiral speeds
and show that stars in this time dependent gravitational field can be heated
(their random motions increased).This is particularly noticeable in the
simultaneous propagation of a 2-armed spiral density wave near the corotation
resonance (CR), and a weak 4-armed one near the inner and outer 4:1 Lindblad
resonances. In simulations with 2 spiral waves moving at different pattern
speeds we find: (1) the variance of the radial velocity, sigma_R^2, exceeds the
sum of the variances measured from simulations with each individual pattern;
(2) sigma_R^2 can grow with time throughout the entire simulation; (3)
sigma_R^2 is increased over a wider range of radii compared to that seen with
one spiral pattern; (4) particles diffuse radially in real space whereas they
don't when only one spiral density wave is present. Near the CR with the
stronger, 2-armed pattern, test particles are observed to migrate radially.
These effects take place at or near resonances of both spirals so we interpret
them as the result of stochastic motions. This provides a possible 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.Comment: 20 pages, 13 figures. Submitted to MNRA

### Radial migration in galactic disks caused by resonance overlap of multiple patterns: Self-consistent simulations

We have recently identified a new radial migration mechanism resulting from
the overlap of spiral and bar resonances in galactic disks. Here we confirm the
efficiency of this mechanism in fully self-consistent, Tree-SPH simulations, as
well as high-resolution pure N-body simulations. In all barred cases we clearly
identify the effect of spiral-bar resonance overlap by measuring a bimodality
in the changes of angular momentum in the disk, dL, whose maxima are near the
bar's corotation and outer Lindblad resonance. This contrasts with the smooth
distribution of dL for a simulation with no stable bar present, where strong
radial migration is induced by multiple spirals. The presence of a disk gaseous
component appears to increase the rate of angular momentum exchange by about
20%. The efficiency of this mechanism is such that galactic stellar disks can
extend to over 10 scale-lengths within 1-3 Gyr in both Milky Way size and
low-mass galaxies (circular velocity ~100 km/s). We also show that metallicity
gradients can flatten in less than 1 Gyr rendering mixing in barred galaxies an
order of magnitude more efficient than previously thought.Comment: replaced with accepted version: 5 pages, 5 figures (one new figure
added), minor change

### New Constraints on the Galactic Bar

Previous work has related the Galactic Bar to structure in the local stellar
velocity distribution. Here 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 the Bar pattern speed is
Omega_b/Omega_0=1.87\pm0.04, where Omega_0 is the local circular frequency, and
the Bar angle lies within 20<phi_0<45 deg. We find that the Galactic Bar
affects measurements of the Oort constants A and B less than ~2 km/s/kpc for
the hot stars.Comment: 4 pages, 2 figures, Accepted to ApJ Letters. Replaced with accepted
versio

### The Effect of Spiral Structure on the Measurements of the Oort Constants

We perform test-particle simulations in a 2D, differentially rotating stellar
disk, subjected to a two-armed steady state spiral density wave perturbation in
order to estimate the influence of spiral structure on the local velocity
field. By using Levenberg-Marquardt least-squares fit we decompose the local
velocity field (as a result of our simulations) into Fourier components to
fourth order. Thus we obtain simulated measurements of the Oort constants, A,
B, C, and K. We get relations between the Fourier coefficients and some
galactic parameters, such as the phase angle of the Solar neighborhood and the
spiral pattern speed. We show that systematic errors due to the presence of
spiral structure are likely to affect the measurements of the Oort constants.
Moderate strength spiral structure causes errors of order 5 km/s/kpc in A and
B. We find variations of the Fourier coefficients with velocity dispersion,
pattern speed, and sample depth. For a sample at an average heliocentric
distance of 0.8 kpc we can summarize our findings as follows:(i) if our
location in the Galaxy is near corotation then we expect a vanishing value for
C for all phase angles;(ii) for a hot disk, spiral structure induced errors for
all Oort constants vanish at, and just inward of the corotation radius; (iii)
as one approaches the 4:1 indblad resonances |C| increases and so does its
variation with galactic azimuth; (iv) for all simulations |C|, on average, is
larger for lower stellar velocity dispersions, contrary to recent measurements.Comment: 20p., 11 fig. Replaced with accepted version to be published in MNRA

### A New Mechanism for Radial Migration in Galactic Disks: Spiral-Bar Resonance Overlap

While it has long been known that a large number of short-lived transient
spirals can cause stellar migration, here we report that another mechanism is
also effective at mixing disks of barred galaxies. The resonance overlap of the
bar and spiral structure induces a nonlinear response leading to a strong
redistribution of angular momentum in the disk. We find that, depending on the
amplitudes of the perturbers, the changes in angular momentum, dL, could occur
up to an order of magnitude faster than in the case of recurrent spirals. The
signature of this mechanism is a bimodality in dL with maxima near the bar's
corotation and its outer Lindblad resonance; this is independent of the
properties of the spiral structure. For parameters consistent with the Milky
Way the disk mixes in about 3 Gyr and the stellar velocity dispersion increases
with time in a manner roughly consistent with observations. This new mechanism
could account for both the observed age-velocity relation and the absence of
age-metallicity relation in the solar neighborhood. Spiral-bar interaction
could also explain observations showing that strongly barred galaxies have
weaker metallicity gradients than weakly barred or non-barred galaxies.Comment: 10 pages, 7 figures. Substantially expanded. Main results remain the
same. Accepted for publication in Ap

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