1,261 research outputs found
Hydrodynamical Simulations of the Barred Spiral Galaxy NGC 6782
NGC 6782 is an early-type barred spiral galaxy exhibiting a rich and complex
morphology with multiple ring patterns. To provide a physical understanding of
its structure and kinematical properties, two-dimensional hydrodynamical
simulations have been carried out. Numerical calculations reveal that the
striking features in NGC 6782 can be reproduced provided that the gas flow is
governed by the gravitational potential associated with a slowly rotating
strong bar. In particular, the response of the gaseous disk to the bar
potential leads to the excitation of spiral density waves at the inner Lindblad
resonance giving rise to the appearance of a nearly circular nuclear ring with
a pair of dust lanes. For a sufficiently strong bar potential, the inner 4:1
spiral density waves are also excited. The interaction of the higher harmonic
waves with the waves excited at the inner Lindblad resonance and confined by
the outer Lindblad resonance results in the observed diamond-shaped (or pointy
oval) inner ring structure. The overall gas morphology and kinematical features
are both well reproduced by the model provided that the pattern speed of the
bar is km s kpc.Comment: 35 pages, 14 figure
A Dust-Penetrated Classification Scheme for Bars as Inferred from their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spirals is a
fundamental aspect of the Hubble galaxy classification system. This ``tuning
fork'' view was revised by de Vaucouleurs, whose classification volume
recognized apparent ``bar strength'' (SA, SAB, SB) as a continuous property of
galaxies called the ``family''. However, the SA, SAB, and SB families are
purely visual judgments that can have little bearing on the actual bar strength
in a given galaxy. Until very recently, published bar judgments were based
exclusively on blue light images, where internal extinction or star formation
can either mask a bar completely or give the false impression of a bar in a
nonbarred galaxy. Near-infrared camera arrays, which principally trace the old
stellar populations in both normal and barred galaxies, now facilitate a
quantification of bar strength in terms of their gravitational potentials and
force fields. In this paper, we show that the maximum value, Qb, of the ratio
of the tangential force to the mean radial force is a quantitative measure of
the strength of a bar. Qb does not measure bar ellipticity or bar shape, but
rather depends on the actual forcing due to the bar embedded in its disk. We
show that a wide range of true bar strengths characterizes the category ``SB'',
while de Vaucouleurs category ``SAB'' corresponds to a much narrower range of
bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar
classes into a dust-penetrated classification system for spiral galaxies.Comment: Accepted for publication in the Astrophysical Journal (LaTex, 30
pages + 3 figures); Figs. 1 and 3 are in color and are also available at
http://bama.ua.edu/~rbuta/bars
OB Stars in the Solar Neighborhood I: Analysis of their Spatial Distribution
We present a newly-developed, three-dimensional spatial classification
method, designed to analyze the spatial distribution of early type stars within
the 1 kpc sphere around the Sun. We propose a distribution model formed by two
intersecting disks -the Gould Belt (GB) and the Local Galactic Disk (LGD)-
defined by their fundamental geometric parameters. Then, using a sample of
about 550 stars of spectral types earlier than B6 and luminosity classes
between III and V, with precise photometric distances of less than 1 kpc, we
estimate for some spectral groups the parameters of our model, as well as
single membership probabilities of GB and LGD stars, thus drawing a picture of
the spatial distribution of young stars in the vicinity of the Sun.Comment: 28 pages including 9 Postscript figures, one of them in color.
Accepted for publication in The Astronomical Journal, 30 January 200
The Potential-Density Phase Shift Method for Determining the Corotation Radii in Spiral and Barred Galaxies
We have developed a new method for determining the corotation radii of
density waves in disk galaxies, which makes use of the radial distribution of
an azimuthal phase shift between the potential and density wave patterns. The
approach originated from improved theoretical understandings of the relation
between the morphology and kinematics of galaxies, and on the dynamical
interaction between density waves and the basic-state disk stars which results
in the secular evolution of disk galaxies. In this paper, we present the
rationales behind the method, and the first application of it to several
representative barred and grand-design spiral galaxies, using near-infrared
images to trace the mass distributions, as well as to calculate the potential
distributions used in the phase shift calculations. We compare our results with
those from other existing methods for locating the corotations, and show that
the new method both confirms the previously-established trends of bar-length
dependence on galaxy morphological types, as well as provides new insights into
the possible extent of bars in disk galaxies. Application of the method to a
larger sample and the preliminary analysis of which show that the phase shift
method is likely to be a generally-applicable, accurate, and essentially
model-independent method for determining the pattern speeds and corotation
radii of single or nested density wave patterns in galaxies. Other implications
of this work are: most of the nearby bright disk galaxies appear to possess
quasi-stationary spiral modes; that these density wave modes and the associated
basic state of the galactic disk slowly transform over time; and that
self-consistent N-particle systems contain physics not revealed by the passive
orbit analysis approaches.Comment: 48 pages, 16 figures. Accepted for publication in the Astronomical
Journa
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
Dynamical Friction and the Distribution of Dark Matter in Barred Galaxies
We use fully self-consistent N-body simulations of barred galaxies to show
that dynamical friction from a dense dark matter halo dramatically slows the
rotation rate of bars. Our result supports previous theoretical predictions for
a bar rotating within a massive halo. On the other hand, low density halos,
such as those required for maximum disks, allow the bar to continue to rotate
at a high rate. There is somewhat meager observational evidence indicating that
bars in real galaxies do rotate rapidly and we use our result to argue that
dark matter halos must have a low central density in all high surface
brightness disk galaxies, including the Milky Way. Bars in galaxies that have
larger fractions of dark matter should rotate slowly, and we suggest that a
promising place to look for such candidate objects is among galaxies of
intermediate surface brightness.Comment: 6 pages, Latex, 3 figures, Accepted by Ap.J.L., revised copy,
includes an added paragrap
Giant Molecular Clouds are More Concentrated to Spiral Arms than Smaller Clouds
From our catalog of Milky Way molecular clouds, created using a temperature
thresholding algorithm on the Bell Laboratories 13CO Survey, we have extracted
two subsets:(1) Giant Molecular Clouds (GMCs), clouds that are definitely
larger than 10^5 solar masses, even if they are at their `near distance', and
(2) clouds that are definitely smaller than 10^5 solar masses, even if they are
at their `far distance'. The positions and velocities of these clouds are
compared to the loci of spiral arms in (l, v) space. The velocity separation of
each cloud from the nearest spiral arm is introduced as a `concentration
statistic'. Almost all of the GMCs are found near spiral arms. The density of
smaller clouds is enhanced near spiral arms, but some clouds (~10%) are
unassociated with any spiral arm. The median velocity separation between a GMC
and the nearest spiral arm is 3.4+-0.6 km/s, whereas the median separation
between smaller clouds and the nearest spiral arm is 5.5+-0.2 km/s.Comment: 11 pages, 3 figure
Geometric phase distributions for open quantum systems
In an open system, the geometric phase should be described by a distribution.
We show that a geometric phase distribution for open system dynamics is in
general ambiguous, but the imposition of reasonable physical constraints on the
environment and its coupling with the system yields a unique geometric phase
distribution that applies even for mixed states, non-unitary dynamics, and
non-cyclic evolutions.Comment: Some minor revisions, references update
Test Particle in a Quantum Gas
A master equation with a Lindblad structure is derived, which describes the
interaction of a test particle with a macroscopic system and is expressed in
terms of the operator valued dynamic structure factor of the system. In the
case of a free Fermi or Bose gas the result is evaluated in the Brownian limit,
thus obtaining a single generator master equation for the description of
quantum Brownian motion in which the correction due to quantum statistics is
explicitly calculated. The friction coefficients for Boltzmann and Bose or
Fermi statistics are compared.Comment: 9 pages, revtex, no figure
CO Distribution and Kinematics Along the Bar in the Strongly Barred Spiral NGC 7479
We report on the 2.5 arcsec (400 pc) resolution CO (J = 1 -> 0) observations
covering the whole length of the bar in the strongly barred late-type spiral
galaxy NGC 7479. CO emission is detected only along a dust lane that traverses
the whole length of the bar, including the nucleus. The emission is strongest
in the nucleus. The distribution of emission is clumpy along the bar outside
the nucleus, and consists of gas complexes that are unlikely to be
gravitationally bound. The CO kinematics within the bar consist of two separate
components. A kinematically distinct circumnuclear disk, < 500 pc in diameter,
is undergoing predominantly circular motion with a maximum rotational velocity
of 245 km/s at a radius of 1 arcsec (160 pc). The CO-emitting gas in the bar
outside the circumnuclear disk has substantial noncircular motions which are
consistent with a large radial velocity component, directed inwards. The CO
emission has a large velocity gradient across the bar dust lane, ranging from
0.5 to 1.9 km/s/pc after correcting for inclination, and the projected velocity
change across the dust lane is as high as 200 km/s. This sharp velocity
gradient is consistent with a shock front at the location of the bar dust lane.
A comparison of H-alpha and CO kinematics across the dust lane shows that
although the H-alpha emission is often observed both upstream and downstream
from the dust lane, the CO emission is observed only where the velocity
gradient is large. We also compare the observations with hydrodynamic models
and discuss star formation along the bar.Comment: 16 pages, including 10 figures. Accepted for publication in Ap
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