495 research outputs found
The Destruction of Bars by Central Mass Concentrations
More than two thirds of disk galaxies are barred to some degree. Many today
harbor massive concentrations of gas in their centers, and some are known to
possess supermassive black holes (SMBHs) and their associated stellar cusps.
Previous theoretical work has suggested that a bar in a galaxy could be
dissolved by the formation of a mass concentration in the center, although the
precise mass and degree of central concentration required is not
well-established. We report an extensive study of the effects of central masses
on bars in high-quality N-body simulations of galaxies. We have varied the
growth rate of the central mass, its final mass and degree of concentration to
examine how these factors affect the evolution of the bar. Our main conclusions
are: (1) Bars are more robust than previously thought. The central mass has to
be as large as several percent of the disk mass to completely destroy the bar
on a short timescale. (2) For a given mass, dense objects cause the greatest
reduction in bar amplitude, while significantly more diffuse objects have a
lesser effect. (3) The bar amplitude always decreases as the central mass is
grown, and continues to decay thereafter on a cosmological time-scale. (4) The
first phase of bar-weakening is due to the destruction by the CMC of
lower-energy, bar-supporting orbits, while the second phase is a consequence of
secular changes to the global potential which further diminish the number of
bar-supporting orbits. We provide detailed phase-space and orbit analysis to
support this suggestion. Thus current masses of SMBHs are probably too small,
even when dressed with a stellar cusp, to affect the bar in their host
galaxies. The molecular gas concentrations found in some barred galaxies are
also too diffuse to affect the amplitude of the bar significantly.Comment: AASTeX v5.0 preprint; 44 pages, including 1 table and 16 figures. To
appear in ApJ. High resolution version can be found at
http://www.physics.rutgers.edu/~shen/bar_destruct/paper_high_res.pd
Unstable Disk Galaxies. II. the Origin of Growing and Stationary Modes
I decompose the unstable growing modes of stellar disks to their Fourier
components and present the physical mechanism of instabilities in the context
of resonances. When the equilibrium distribution function is a non-uniform
function of the orbital angular momentum, the capture of stars into the
corotation resonance imbalances the disk angular momentum and triggers growing
bar and spiral modes. The stellar disk can then recover its angular momentum
balance through the response of non-resonant stars. I carry out a complete
analysis of orbital structure corresponding to each Fourier component in the
radial angle, and present a mathematical condition for the occurrence of van
Kampen modes, which constitute a continuous family. I discuss on the
discreteness and allowable pattern speeds of unstable modes and argue that the
mode growth is saturated due to the resonance overlapping mechanism. An
individually growing mode can also be suppressed if the corotation and inner
Lindblad resonances coexist and compete to capture a group of stars. Based on
this mechanism, I show that self-consistent scale-free disks with a sufficient
distribution of non-circular orbits should be stable under perturbations of
angular wavenumber . I also derive a criterion for the stability of
stellar disks against non-axisymmetric excitations.Comment: 15 Pages (emulateapj), 7 Figures, Accepted for Publication in The
Astrophysical Journa
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
Bar-Halo Friction in Galaxies II: Metastability
It is well-established that strong bars rotating in dense halos generally
slow down as they lose angular momentum to the halo through dynamical friction.
Angular momentum exchanges between the bar and halo particles take place at
resonances. While some particles gain and others lose, friction arises when
there is an excess of gainers over losers. This imbalance results from the
generally decreasing numbers of particles with increasing angular momentum, and
friction can therefore be avoided if there is no gradient in the density of
particles across the major resonances. Here we show that anomalously weak
friction can occur for this reason if the pattern speed of the bar fluctuates
upwards. After such an event, the density of resonant halo particles has a
local inflexion created by the earlier exchanges, and bar slowdown can be
delayed for a long period; we describe this as a metastable state. We show that
this behavior in purely collisionless N-body simulations is far more likely to
occur in methods with adaptive resolution. We also show that the phenomenon
could arise in nature, since bar-driven gas inflow could easily raise the bar
pattern speed enough to reach the metastable state. Finally, we demonstrate
that mild external, or internal, perturbations quickly restore the usual
frictional drag, and it is unlikely therefore that a strong bar in a galaxy
having a dense halo could rotate for a long period without friction.Comment: 13 pages, 11 figures, to appear in Ap
THE OPTIMAL N-BODY METHOD FOR STABILITY STUDIES OF GALAXIES
The stability of a galaxy model is most easily assessed through N-body
simulation. Particle-mesh codes have been widely used for this purpose, since
they enable the largest numbers of particles to be employed. We show that the
functional expansion technique, originally proposed by Clutton-Brock for other
simulation problems, is in fact superior for stability work. For simulations of
linear evolution it is not much slower than grid methods using the same number
of particles, and reproduces analytical results with much greater accuracy.
This success rests on its ability to represent global modes with a modest
number of basis functions; grid methods may be more effective for other
applications, however. Our conclusions are based on implementations of
functional expansion and grid algorithms for disk galaxies.Comment: Accepted for publication in The Astrophysical Journal, to appear
October 1, 1995; 16 pages including 4 figures, self-unpacking uuencoded
gzipped postscript, also available by email from [email protected]
Long-Lived Double-Barred Galaxies From Pseudo-Bulges
A large fraction of barred galaxies host secondary bars that are embedded in
their large-scale primary counterparts. These are common also in gas poor
early-type barred galaxies. The evolution of such double-barred galaxies is
still not well understood, partly because of a lack of realistic -body
models with which to study them. Here we report a new mechanism for generating
such systems, namely the presence of rotating pseudo-bulges. We demonstate with
high mass and force resolution collisionless -body simulations that
long-lived secondary bars can form spontaneously without requiring gas,
contrary to previous claims. We find that secondary bars rotate faster than
primary ones. The rotation is not, however, rigid: the secondary bars pulsate,
with their amplitude and pattern speed oscillating as they rotate through the
primary bars. This self-consistent study supports previous work based on
orbital analysis in the potential of two rigidly rotating bars. The pulsating
nature of secondary bars may have important implications for understanding the
central region of double-barred galaxies.Comment: Paper submitted to ApJ
Physical Properties of Tidal Features in Interacting Disk Galaxies
We explore tidal interactions of a galactic disk with Toomre parameter Q ~ 2
embedded in rigid halo/bulge with a point mass companion moving in a prescribed
parabolic orbit. Tidal interactions produce well-defined spiral arms and
extended tidal features such as bridge and tail that are all transient, but
distinct in nature. In the extended disks, strong tidal force is able to lock
the perturbed epicycle phases of the near-side particles to the perturber,
shaping them into a tidal bridge that corotates with the perturber. A tidal
tail develops at the opposite side as strongly-perturbed, near-side particles
overtake mildly-perturbed, far-side particles. The tail is essentially a narrow
material arm with a roughly logarithmic shape, dissolving with time because of
large velocity dispersions. Inside the disks where tidal force is relatively
weak, on the other hand, a two-armed logarithmic spiral pattern emerges due to
the kinematic alignment of perturbed particle orbits. While self-gravity makes
the spiral arms a bit stronger, the arms never become fully self-gravitating,
wind up progressively with time, and decay after the peak almost exponentially
in a time scale of ~ 1 Gyr. The arm pattern speed varying with both radius and
time converges to Omega-kappa/2 at late time, suggesting that the pattern speed
of tidally-driven arms may depend on radius in real galaxies. We present the
parametric dependences of various properties of tidal features on the tidal
strength, and discuss our findings in application to tidal spiral arms in
grand-design spiral galaxies. (Abridged)Comment: 49 pages, 17 figures, 1 table. Accepted for publication in
Astrophysical Journal. PDF version with higher resolution figures is
available at
http://astro.snu.ac.kr/~shoh/research/publications/astroph/Tidally_Induced_Spiral_Structure.pd
Bars and Dark Matter Halo Cores
Self-consistent bars that form in galaxies embedded within cuspy halos are
unable to flatten the cusp. Short bars form in models with quasi-flat rotation
curves. They lose angular momentum to the halo through dynamical friction, but
the continuous concentration of mass within the disk as the bar grows actually
compresses the halo further, overwhelming any density reduction due to the
modest angular momentum transfer to the halo. Thus the Weinberg-Katz proposed
solution to the non-existence of the predicted cuspy halos from CDM simulations
would seem to be unworkable. I also find that the concerns over the performance
of N-body codes raised by these authors do not apply to the methods used here.Comment: Latex 11 pages (uses emulateapj.sty), 8 figures, revised version to
appear ApJ, very minor change
Bar-halo Friction in Galaxies I: Scaling Laws
It has been known for some time that rotating bars in galaxies slow due to
dynamical friction against the halo. However, recent attempts to use this
process to place constraints on the dark matter density in galaxies and
possibly also to drive dark matter out of the center have been challenged. This
paper uses simplified numerical experiments to clarify several aspects of the
friction mechanism. I explicitly demonstrate the Chandrasekhar scaling of the
friction force with bar mass, halo density, and halo velocity dispersion. I
present direct evidence that exchanges between the bar and halo orbits at major
resonances are responsible for friction and study both individual orbits and
the net changes at these resonances. I also show that friction alters the phase
space density of particles in the vicinity of a major resonance, which is the
reason the magnitude of the friction force depends on the prior evolution. I
demonstrate that bar slow down can be captured correctly in simulations having
modest spatial resolution and practicable numbers of particles. Subsequent
papers in this series delineate the dark matter density that can be tolerated
in halos of different density profiles.Comment: 15 pages, 13 figures, to appear in ApJ - major revisions from version
Structure and Star Formation in NGC 925
We present the results from an optical study of the stellar & star formation
properties of NGC 925 using the WIYN 3.5m telescope. Images in B,V,R, & H-alpha
reveal a galaxy that is fraught with asymmetries. From isophote fits we
discover that the bar center is not coincident with the center of the outer
isophotes nor with the dynamical center (from Pisano et al. 1998). Cuts across
the spiral arms reveal that the northern arms are distinctly different from the
southern arm. The southern arm not only appears more coherent, but the peaks in
stellar and H-alpha emission are found to be coincident with those of the HI
distribution, while no such consistency is present in the northern disk. We
also examine the gas surface density criterion for massive star formation in
NGC 925, and find that its behavior is more consistent with that for irregular
galaxies, than with late-type spirals. In particular, star formation persists
beyond the radius at which the gas surface density falls below the predicted
critical value for star formation for late-type spirals. Such properties are
characteristic of Magellanic spirals, but are present at a less dramatic level
in NGC 925, a late-type spiral.Comment: accepted for publication in the August 2000 Astronomical Journal 12
pages, 3 tables, 14 figure
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