187 research outputs found
Long-Lived Double-Barred Galaxies: Critical Mass and Length Scales
A substantial fraction of disk galaxies is double-barred. We analyze the
dynamical stability of such nested bar systems by means of Liapunov
exponents,by fixing a generic model and varying the inner (secondary) bar mass.
We show that there exists a critical mass below which the secondary bar cannot
sustain its own orbital structure, and above which it progressively destroys
the outer (primary) bar-supporting orbits. In this critical state, a large
fraction of the trajectories (regular and chaotic) are aligned with either bar,
suggesting the plausibility of long-lived dynamical states when
secondary-to-primary bar mass ratio is of the order of a few percent.
Qualitatively similar results are obtained by varying the size of the secondary
bar, within certain limits, while keeping its mass constant. In both cases, an
important role appears to be played by chaotic trajectories which are trapped
around (especially) the primary bar for long periods of time.Comment: 7 pages, 1 figure, to be published in Astrophysical Journal Letters
(Vol. 595, 9/20/03 issue). Replaced by revised figure and corrected typo
Density Waves Inside Inner Lindblad Resonance: Nuclear Spirals in Disk Galaxies
We analyze formation of grand-design two-arm spiral structure in the nuclear
regions of disk galaxies. Such morphology has been recently detected in a
number of objects using high-resolution near-infrared observations. Motivated
by the observed (1) continuity between the nuclear and kpc-scale spiral
structures, and by (2) low arm-interarm contrast, we apply the density wave
theory to explain the basic properties of the spiral nuclear morphology. In
particular, we address the mechanism for the formation, maintenance and the
detailed shape of nuclear spirals. We find, that the latter depends mostly on
the shape of the underlying gravitational potential and the sound speed in the
gas. Detection of nuclear spiral arms provides diagnostics of mass distribution
within the central kpc of disk galaxies. Our results are supported by 2D
numerical simulations of gas response to the background gravitational potential
of a barred stellar disk. We investigate the parameter space allowed for the
formation of nuclear spirals using a new method for constructing a
gravitational potential in a barred galaxy, where positions of resonances are
prescribed.Comment: 18 pages, 9 figures, higher resolution available at
http://www.pa.uky.edu/~ppe/papers/nucsp.ps.g
Collisionless evaporation from cluster elliptical galaxies
We describe a particular aspect of the effects of the parent cluster tidal
field (CTF) on stellar orbits inside cluster Elliptical galaxies. In particular
we discuss, with the aid of a simple numerical model, the possibility that
collisionless stellar evaporation from elliptical galaxies is an effective
mechanism for the production of the recently discovered intracluster stellar
populations. A preliminary investigation, based on very idealized galaxy
density profiles (Ferrers density distributions), showed that over an Hubble
time, the amount of stars lost by a representative galaxy may sum up to the 10%
of the initial galaxy mass, a fraction in interesting agreement with
observational data. The effectiveness of this mechanism is due to the fact that
the galaxy oscillation periods near equilibrium configurations in the CTF are
comparable to stellar orbital times in the external galaxy regions. Here we
extend our previous study to more realistic galaxy density profiles, in
particular by adopting a triaxial Hernquist model.Comment: 6 pages, 2 figures. To appear on "Lecture Notes in Physics",
proceedings of the Workshop on "Galaxies and Chaos. Theory and Observations",
Athens (September 16-19, 2002), G. Contopoulos and N. Voglis, ed
Star cluster evolution in barred disc galaxies. I. Planar periodic orbits
The dynamical evolution of stellar clusters is driven to a large extent by
their environment. Several studies so far have considered the effect of tidal
fields and their variations, such as, e.g., from giant molecular clouds,
galactic discs, or spiral arms. In this paper we will concentrate on a tidal
field whose effects on star clusters have not yet been studied, namely that of
bars. We present a set of direct N-body simulations of star clusters moving in
an analytic potential representing a barred galaxy. We compare the evolution of
the clusters moving both on different planar periodic orbits in the barred
potential and on circular orbits in a potential obtained by axisymmetrising its
mass distribution. We show that both the shape of the underlying orbit and its
stability have strong impact on the cluster evolution as well as the morphology
and orientation of the tidal tails and the sub-structures therein. We find that
the dissolution time-scale of the cluster in our simulations is mainly
determined by the tidal forcing along the orbit and, for a given tidal forcing,
only very little by the exact shape of the gravitational potential in which the
cluster is moving.Comment: 15 pages, 17 figures, 5 tables; accepted for publication in MNRAS.
Complementary movies can be be found at this http URL
http://lam.oamp.fr/research/dynamique-des-galaxies/scientific-results/star-cluster-evolution
Two-Dimensional Magnetohydrodynamic Simulations of Barred Galaxies
Barred galaxies are known to possess magnetic fields that may affect the
properties of bar substructures such as dust lanes and nuclear rings. We use
two-dimensional high-resolution magnetohydrodynamic (MHD) simulations to
investigate the effects of magnetic fields on the formation and evolution of
such substructures as well as on the mass inflow rates to the galaxy center.
The gaseous medium is assumed to be infinitesimally-thin, isothermal,
non-self-gravitating, and threaded by initially uniform, azimuthal magnetic
fields. We find that there exists an outermost x1-orbit relative to which
gaseous responses to an imposed stellar bar potential are completely different
between inside and outside. Inside this orbit, gas is shocked into dust lanes
and infalls to form a nuclear ring. Magnetic fields are compressed in dust
lanes, reducing their peak density. Magnetic stress removes further angular
momentum of the gas at the shocks, temporarily causing the dust lanes to bend
into an 'L' shape and eventually leading to a smaller and more centrally
distributed ring than in unmagnetized models. The mass inflow rates in
magnetized models correspondingly become larger, by more than two orders of
magnitude when the initial fields have an equipartition value with thermal
energy, than in the unmagnetized counterparts. Outside the outermost x1-orbit,
on the other hand, an MHD dynamo due to the combined action of the bar
potential and background shear operates near the corotation and bar-end
regions, efficiently amplifying magnetic fields. The amplified fields shape
into trailing magnetic arms with strong fields and low density. The base of the
magnetic arms has a thin layer in which magnetic fields with opposite polarity
reconnect via a tearing-mode instability. This produces numerous magnetic
islands with large density which propagate along the arms to turn the outer
disk into a highly chaotic state.Comment: 22 pages, 19 figures, 3 tables; Accepted for publication in the ApJ;
Version with full-resolution figures available at
http://mirzam.snu.ac.kr/~wkim/Bar/mhdbar.pd
Rings and spirals in barred galaxies. I Building blocks
In this paper we present building blocks which can explain the formation and
properties both of spirals and of inner and outer rings in barred galaxies. We
first briefly summarise the main results of the full theoretical description we
have given elsewhere, presenting them in a more physical way, aimed to an
understanding without the requirement of extended knowledge of dynamical
systems or of orbital structure. We introduce in this manner the notion of
manifolds, which can be thought of as tubes guiding the orbits. The dynamics of
these manifolds can govern the properties of spirals and of inner and outer
rings in barred galaxies. We find that the bar strength affects how unstable
the L1 and L2 Lagrangian points are, the motion within the 5A5A5Amanifold tubes
and the time necessary for particles in a manifold to make a complete turn
around the galactic centre. We also show that the strength of the bar, or, to
be more precise, of the non-axisymmetric forcing at and somewhat beyond the
corotation region, determines the resulting morphology. Thus, less strong bars
give rise to R1 rings or pseudorings, while stronger bars drive R2, R1R2 and
spiral morphologies. We examine the morphology as a function of the main
parameters of the bar and present descriptive two dimensional plots to that
avail. We also derive how the manifold morphologies and properties are modified
if the L1 and L2 Lagrangian points become stable. Finally, we discuss how
dissipation affects the manifold properties and compare the manifolds in
gas-like and in stellar cases. Comparison with observations, as well as clear
predictions to be tested by observations will be given in an accompanying
paper.Comment: Typos corrected to match the version in press in MNRA
Gaseous Structures in Barred Galaxies: Effects of the Bar Strength
Using hydrodynamic simulations, we investigate the physical properties of
gaseous substructures in barred galaxies and their relationships with the bar
strength. The gaseous medium is assumed to be isothermal and unmagnetized. The
bar potential is modeled as a Ferrers prolate with index n. To explore
situations with differing bar strength, we vary the bar mass fbar relative to
the spheroidal component as well as its aspect ratio. We derive expressions as
functions of fbar and the aspect ratio for the bar strength Qb and the radius
r(Qb) where the maximum bar torque occurs. When applied to observations, these
expressions suggest that bars in real galaxies are most likely to have
fbar=0.25-0.5 and n<1. Dust lanes approximately follow one of x1-orbits and
tend to be more straight under a stronger and more elongated bar, but are
insensitive to the presence of self-gravity. A nuclear ring of a conventional
x2 type forms only when the bar is not so massive or elongated. The radius of
an x2-type ring is generally smaller than the inner Lindblad resonance,
decreases systematically with increasing Qb, and slightly larger when
self-gravity is included. This evidences that the ring position is not
determined by the resonance but by the amount of angular momentum loss at
dust-lane shocks. Nuclear spirals exist only when the ring is of the x2-type
and sufficiently large in size. Unlike the other features, nuclear spirals are
transient in that they start out as being tightly-wound and weak, and then due
to the nonlinear effect unwind and become stronger until turning into shocks,
with an unwinding rate higher for larger Qb. The mass inflow rate to the galaxy
center is found to be less than 0.01 Msun/yr for models with Qb<0.2, while
becoming larger than 0.1 Msun/yr when Qb>0.2 and self-gravity is included.Comment: 24 pages, 17 figures, 5 tables; Accepted for publication in the ApJ;
Version with full-resolution figures available at
http://mirzam.snu.ac.kr/~wkim/Bar/barHDn.pd
Nested Bars in Disk Galaxies: No Offset Dust Lanes in Secondary Nuclear Bars
Under certain conditions, sub-kpc nuclear bars form inside large-scale
stellar bars of disk galaxies. These secondary bars spend a fraction of their
lifetime in a dynamically-decoupled state, tumbling in the gravitational field
of the outer bars. We analyze the flow pattern in such nested bar systems and
find that secondary bars differ fundamentally from their large-scale
counterparts. In particular the gas flow across the bar-bar interface in these
systems can be more chaotic or more regular in nature, and, contrary to
predictions, has no difficulty in penetrating the secondary bars. The outer
parts of both short and long nuclear bars (with respect to their corotation)
appear to be depopulated of gas, while deep inside them the flow exhibits low
Mach numbers and follows ovally-shaped orbits with little dissipation. We find
that gas-dominated and star-dominated nuclear bars avoid the bar-bar interface,
making both types of bars short relative to their corotation. Furthermore, our
earlier work has shown that dynamically-coupled secondary bars exhibit a
similarly relaxed low-dissipation flow as well. Therefore, no large-scale
shocks form in the nuclear bars, and consequently, no offset dust lanes are
expected there. We find that offset dust lanes cannot be used in the search for
secondary (nuclear) bars.Comment: 13 pages, 8 figures, to be published in ApJ 564, January 10, 200
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