358 research outputs found
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
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
Chern classes and extraspecial groups
The mod-p cohomology ring of the extraspecial p-group of exponent p is
studied for odd p. We investigate the subquotient ch(G) generated by Chern
classes modulo the nilradical. The subring of ch(G) generated by Chern classes
of one-dimensional representations was studied by Tezuka and Yagita. The
subring generated by the Chern classes of the faithful irreducible
representations is a polynomial algebra. We study the interplay between these
two families of generators, and obtain some relations between them
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
Rigidification of quasi-categories
We give a new construction for rigidifying a quasi-category into a simplicial
category, and prove that it is weakly equivalent to the rigidification given by
Lurie. Our construction comes from the use of necklaces, which are simplicial
sets obtained by stringing simplices together. As an application of these
methods, we use our model to reprove some basic facts from Lurie's "Higher
Topos Theory" regarding the rigidification process.Comment: 26 page
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
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
Gravitational torques in spiral galaxies: gas accretion as a driving mechanism of galactic evolution
The distribution of gravitational torques and bar strengths in the local
Universe is derived from a detailed study of 163 galaxies observed in the
near-infrared. The results are compared with numerical models for spiral galaxy
evolution. It is found that the observed distribution of torques can be
accounted for only with external accretion of gas onto spiral disks. Accretion
is responsible for bar renewal - after the dissolution of primordial bars - as
well as the maintenance of spiral structures. Models of isolated, non-accreting
galaxies are ruled out. Moderate accretion rates do not explain the
observational results: it is shown that galactic disks should double their mass
in less than the Hubble time. The best fit is obtained if spiral galaxies are
open systems, still forming today by continuous gas accretion, doubling their
mass every 10 billion years.Comment: 4 pages, 2 figures, Astronomy and Astrophysics Letters (accepted
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