2,123 research outputs found
Radial Mixing in Galactic Discs
We show that spiral waves in galaxy discs churn the stars and gas in a manner
that largely preserves the overall angular momentum distribution and leads to
little increase in random motion. Changes in the angular momenta of individual
stars are typically as large as ~50% over the lifetime of the disk. The changes
are concentrated around the corotation radius for an individual spiral wave,
but since transient waves with a wide range of pattern speeds develop in rapid
succession, the entire disk is affected. This behaviour has profound
consequences for the metallicity gradients with radius in both stars and gas.
The ISM is also stirred by the same mechanism. We find observational support
for stirring, propose a simple model for the distribution of stars over
metallicity and age, and discuss other possible consequences.Comment: Accepted to appear in MNRAS, 13 pages, 16 figures, LaTeX uses
mn2e.cls Minor additions to text and one extra figur
Is Galactic Structure Compatible with Microlensing Data?
We generalize to elliptical models the argument of Kuijken (1997), which
connects the microlensing optical depth towards the Galactic bulge to the
Galactic rotation curve. When applied to the latest value from the MACHO
collaboration for the optical depth for microlensing of bulge sources, the
argument implies that the Galactic bar cannot plausibly reconcile the measured
values of the optical depth, the rotation curve and the local mass density.
Either there is a problem with the interpretation of the microlensing data, or
our line of sight to the Galactic centre is highly atypical in that it passes
through a massive structure that wraps only a small distance around the
Galactic centre.Comment: Submitted to ApJ Letters. 8 pages LaTeX, 3 figures. Corrected error
in description of microlensing observation
Stellar Orbits and the Interstellar Gas Temperature in Elliptical Galaxies
We draw attention to the close relationship between the anisotropy parameter
beta(r) for stellar orbits in elliptical galaxies and the temperature profile
T(r) of the hot interstellar gas. For nearly spherical galaxies the gas density
can be accurately determined from X-ray observations and the stellar luminosity
density can be accurately found from the optical surface brightness. The Jeans
equation and hydrostatic equilibrium establish a connection between beta(r) and
T(r) that must be consistent with the observed stellar velocity dispersion.
Purely optical observations of the bright elliptical galaxy NGC 4472 indicate
beta(r) < 0.35 within the effective radius. However, the X-ray gas temperature
profile T(r) for NGC 4472 requires significantly larger anisotropy, beta = 0.6
- 0.7, about twice the optical value. This strong preference for radial stellar
orbits must be understood in terms of the formation history of massive
elliptical galaxies. Conversely, if the smaller, optically determined
anisotropy is indeed correct, we are led to the important conclusion that the
temperature profile T(r) of the hot interstellar gas in NGC 4472 must differ
from that indicated by X-ray observations, or that the hot gas is not in
hydrostatic equilibrium.Comment: 6 pages (emulateapj5) with 4 figures; accepted by The Astrophysical
Journa
Majority-vote model on (3,4,6,4) and (3^4,6) Archimedean lattices
On Archimedean lattices, the Ising model exhibits spontaneous ordering. Two
examples of these lattices of the majority-vote model with noise are considered
and studied through extensive Monte Carlo simulations. The order/disorder phase
transition is observed in this system. The calculated values of the critical
noise parameter are q_c=0.091(2) and q_c=0.134(3) for (3,4,6,4) and (3^4,6)
Archimedean lattices, respectively. The critical exponents beta/nu, gamma/nu
and 1/nu for this model are 0.103(6), 1.596(54), 0.872(85) for (3,4,6,4) and
0.114(3), 1.632(35), 0.978(104) for (3^4,6) Archimedean lattices. These results
differs from the usual Ising model results and the majority-vote model on
so-far studied regular lattices or complex networks. The effective
dimensionality of the system [D_{eff}(3,4,6,4)=1.802(55) and
D_{eff}(3^4,6)=1.860(34)] for these networks are reasonably close to the
embedding dimension two.Comment: 6 pages, 7 figures in 12 eps files, RevTex
The Anisotropic Distribution of Galactic Satellites
We present a study of the spatial distribution of subhalos in galactic dark
matter halos using dissipationless cosmological simulations of the concordance
LCDM model. We find that subhalos are distributed anisotropically and are
preferentially located along the major axes of the triaxial mass distributions
of their hosts. The Kolmogorov-Smirnov probability for drawing our simulated
subhalo sample from an isotropic distribution is P_KS \simeq 1.5 \times
10^{-4}. An isotropic distribution of subhalos is thus not the correct null
hypothesis for testing the CDM paradigm. The nearly planar distribution of
observed Milky Way (MW) satellites is marginally consistent (probability \simeq
0.02) with being drawn randomly from the subhalo distribution in our
simulations. Furthermore, if we select the subhalos likely to be luminous, we
find a distribution that is consistent with the observed MW satellites. In
fact, we show that subsamples of the subhalo population with a
centrally-concentrated radial distribution, similar to that of the MW dwarfs,
typically exhibit a comparable degree of planarity. We explore the origin of
the observed subhalo anisotropy and conclude that it is likely due to (1)
preferential accretion of subhalos along filaments, often closely aligned with
the major axis of the host halo, and (2) evolution of satellite orbits within
the prolate, triaxial potentials typical of CDM halos. Agreement between
predictions and observations requires the major axis of the outer dark matter
halo of the Milky Way to be nearly perpendicular to the disk. We discuss
possible observational tests of such disk-halo alignment with current large
galaxy surveys.Comment: 14 pages (including appendix), 9 figures. Accepted for Publication in
ApJ. Minor changes to reflect referee's comment
The Pattern Speed of the Galactic Bar
Most late-type stars in the solar neighborhood have velocities similar to the
local standard of rest (LSR), but there is a clearly separated secondary
component corresponding to a slower rotation and a mean outward motion.
Detailed simulations of the response of a stellar disk to a central bar show
that such a bi-modality is expected from outer-Lindblad resonant scattering.
When constraining the run of the rotation curve by the proper motion of Sgr A*
and the terminal gas velocities, the value observed for the rotation velocity
separating the two components results in a value of (53+/-3)km/s/kpc for the
pattern speed of the bar, only weakly dependent on the precise values for Ro
and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter
On the kinematic signature of a central Galactic bar in observed star samples
A quasi self-consistent model for a barred structure in the central regions
of our Galaxy is used to calculate the signature of such a triaxial structure
on the kinematical properties of star samples. We argue that, due to the
presence of a velocity dispersion, such effects are much harder to detect in
the stellar component than in the gas. It might be almost impossible to detect
stellar kinematical evidence for a bar using only l-v diagrams, if there is no
a priori knowledge of the potential. Therefore, we propose some test parameters
that can easily be applied to observed star samples, and that also incorporate
distances or proper motions. We discus the diagnostic power of these tests as a
function of the sample size and the bar strength. We conclude that about 1000
stars would be necessary to diagnose triaxiality with some statistical
confidence.Comment: 9 pages + 8 PS figures, uses aas2pp4.sty. Accepted by Ap
Spectral Simplicity of Apparent Complexity, Part I: The Nondiagonalizable Metadynamics of Prediction
Virtually all questions that one can ask about the behavioral and structural
complexity of a stochastic process reduce to a linear algebraic framing of a
time evolution governed by an appropriate hidden-Markov process generator. Each
type of question---correlation, predictability, predictive cost, observer
synchronization, and the like---induces a distinct generator class. Answers are
then functions of the class-appropriate transition dynamic. Unfortunately,
these dynamics are generically nonnormal, nondiagonalizable, singular, and so
on. Tractably analyzing these dynamics relies on adapting the recently
introduced meromorphic functional calculus, which specifies the spectral
decomposition of functions of nondiagonalizable linear operators, even when the
function poles and zeros coincide with the operator's spectrum. Along the way,
we establish special properties of the projection operators that demonstrate
how they capture the organization of subprocesses within a complex system.
Circumventing the spurious infinities of alternative calculi, this leads in the
sequel, Part II, to the first closed-form expressions for complexity measures,
couched either in terms of the Drazin inverse (negative-one power of a singular
operator) or the eigenvalues and projection operators of the appropriate
transition dynamic.Comment: 24 pages, 3 figures, 4 tables; current version always at
http://csc.ucdavis.edu/~cmg/compmech/pubs/sdscpt1.ht
Local Kinematics and the Local Standard of Rest
We re-examine the stellar kinematics of the Solar neighbourhood in terms of
the velocity of the Sun with respect to the local standard of rest. We show
that the classical determination of its component V_sun in the direction of
Galactic rotation via Stroemberg's relation is undermined by the metallicity
gradient in the disc, which introduces a correlation between the colour of a
group of stars and the radial gradients of its properties. Comparing the local
stellar kinematics to a chemodynamical model which accounts for these effects,
we obtain (U,V,W)_sun = (11.1 +/- 0.74, 12.24 +/- 0.47, 7.25 +/-0.37) km/s,
with additional systematic uncertainties of ~ (1,2,0.5) km/s. In particular,
V_sun is 7 km/s larger than previously estimated. The new values of solar
motion are extremely insensitive to the metallicity gradient within the disc.Comment: 5 pages, submitted to MNRA
The Fundamental Plane of Gravitational Lens Galaxies and The Evolution of Early-Type Galaxies in Low Density Environments
Most gravitational lenses are early-type galaxies in relatively low density
environments -- a ``field'' rather than a ``cluster'' population. We show that
field early-type galaxies with 0 < z < 1, as represented by the lens galaxies,
lie on the same fundamental plane as those in rich clusters at similar
redshifts. We then use the fundamental plane to measure the combined
evolutionary and K-corrections for early-type galaxies in the V, I and H bands.
Only for passively evolving stellar populations formed at z > 2 (H_0=65 km/s
Mpc, Omega_0=0.3, Lambda_0=0.7) can the lens galaxies be matched to the local
fundamental plane. The high formation epoch and the lack of significant
differences between the field and cluster populations contradict many current
models of the formation history of early-type galaxies. Lens galaxy colors and
the fundamental plane provide good photometric redshift estimates with an
empirical accuracy of -0.03 +/- 0.11 for the 17 lenses with known redshifts. A
mass model dominated by dark matter is more consistent with the data than
either an isotropic or radially anisotropic constant M/L mass model, and a
radially anisotropic model is better than an isotropic model.Comment: 36 pages, 9 figures, 6 tables. ApJ in press. Final version contains
more observational dat
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