39 research outputs found
The vertical structure and kinematics of grand design spirals
We use an N-body simulation to study the 3D density distribution of spirals and the resulting stellar vertical velocities. Relative to the disc's rotation, the phase of the spiral's peak density away from the mid-plane trails that at the mid-plane. In addition, at fixed radius the density distribution is azimuthally skewed, having a shallower slope on the trailing side inside corotation and switching to shallower on the leading side beyond corotation. The spirals induce non-zero average vertical velocities, 〈Vz〉, as large as 〈Vz〉 ∼ 10–20 km s−1, consistent with recent observations in the Milky Way. The vertical motions are compressive (towards the mid-plane) as stars enter the spiral, and expanding (away from the mid-plane) as they leave it. Since stars enter the spiral on the leading side outside corotation and on the trailing side within corotation, the relative phase of the expanding and compressive motions switches sides at corotation. Moreover, because stars always enter the spiral on the shallow density gradient side and exit on the steeper side, the expanding motions are larger than the compressing motion
Constraints from Dynamical Friction on the Dark Matter Content of Barred Galaxies
We show that bars in galaxy models having halos of moderate density and a
variety of velocity distributions all experience a strong drag from dynamical
friction unless the halo has large angular momentum in the same sense as the
disk. The frictional drag decreases the bar pattern speed, driving the
corotation point out to distances well in excess of those estimated in barred
galaxies. The halo angular momentum required to avoid strong braking is
unrealistically large, even when rotation is confined to the inner halo only.
We conclude, therefore, that bars are able to maintain their observed high
pattern speeds only if the halo has a central density low enough for the disk
to provide most of the central attraction in the inner galaxy. We present
evidence that this conclusion holds for all bright galaxies.Comment: 46 pages, 18 figures, to appear in ApJ. Uses aaspp4.st
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 counter-streaming instability in dwarf ellipticals with off-center nuclei
n many nucleated dwarf elliptical galaxies (dE,N's), the nucleus is offset by
a significant fraction of the scale radius with respect to the center of the
outer isophotes. Using a high-resolution N-body simulation, we demonstrate that
the nucleus can be driven off-center by the m=1 counterstreaming instability,
which is strong in flattened stellar systems with zero rotation. The model
develops a nuclear offset on the order of 30% of the exponential scale length.
We compare our numerical results with the photometry and kinematics of FCC 046,
a Fornax Cluster dE,N with a nucleus offset by 1.2" we find good agreement
between the model and FCC 046. We also discuss mechanisms that may cause
counterrotation in dE,N's and conclude that the destruction of box orbits in an
initially triaxial galaxy is the most promising.Comment: 5 pages, 4 figure
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
Warped Galaxies From Misaligned Angular Momenta
A galaxy disk embedded in a rotating halo experiences a dynamical friction
force which causes it to warp when the angular momentum axes of the disk and
halo are misaligned. Our fully self-consistent simulations of this process
induce long-lived warps in the disk which mimic Briggs's rules of warp
behavior. They also demonstrate that random motion within the disk adds
significantly to its stiffness. Moreover, warps generated in this way have no
winding problem and are more pronounced in the extended \h1 disk. As emphasized
by Binney and his co-workers, angular momentum misalignments, which are
expected in hierarchical models of galaxy formation, can account for the high
fraction of warped galaxies. Our simulations exemplify the role of misaligned
spins in warp formation even when the halo density is not significantly
flattened.Comment: 6 pages, 5 figures. Accepted for publication in Ap.J.
A unified framework for the orbital structure of bars and triaxial ellipsoids
We examine a large random sample of orbits in two self-consistent simulations of N-body bars. Orbits in these bars are classified both visually and with a new automated orbit classification method based on frequency analysis. The well-known prograde x1 orbit family originates from the same parent orbit as the box orbits in stationary and rotating triaxial ellipsoids. However, only a small fraction of bar orbits (~4%) have predominately prograde motion like their periodic parent orbit. Most bar orbits arising from the x1 orbit have little net angular momentum in the bar frame, making them equivalent to box orbits in rotating triaxial potentials. In these simulations a small fraction of bar orbits (~7%) are long-axis tubes that behave exactly like those in triaxial ellipsoids: they are tipped about the intermediate axis owing to the Coriolis force, with the sense of tipping determined by the sign of their angular momentum about the long axis. No orbits parented by prograde periodic x2 orbits are found in the pure bar model, but a tiny population (~2%) of short-axis tube orbits parented by retrograde x4 orbits are found. When a central point mass representing a supermassive black hole (SMBH) is grown adiabatically at the center of the bar, those orbits that lie in the immediate vicinity of the SMBH are transformed into precessing Keplerian orbits that belong to the same major families (short-axis tubes, long-axis tubes and boxes) occupying the bar at larger radii. During the growth of an SMBH, the inflow of mass and outward transport of angular momentum transform some x1 and long-axis tube orbits into prograde short-axis tubes. This study has important implications for future attempts to constrain the masses of SMBHs in barred galaxies using orbit-based methods like the Schwarzschild orbit superposition scheme and for understanding the observed features in barred galaxies
Radial Migration in Disk Galaxies I: Transient Spiral Structure and Dynamics
We seek to understand the origin of radial migration in spiral galaxies by
analyzing in detail the structure and evolution of an idealized, isolated
galactic disk. To understand the redistribution of stars, we characterize the
time-evolution of properties of spirals that spontaneously form in the disk.
Our models unambiguously show that in such disks, single spirals are unlikely,
but that a number of transient patterns may coexist in the disk. However, we
also show that while spirals are transient in amplitude, at any given time the
disk favors patterns of certain pattern speeds. Using several runs with
different numerical parameters we show that the properties of spirals that
occur spontaneously in the disk do not sensitively depend on resolution. The
existence of multiple transient patterns has large implications for the orbits
of stars in the disk, and we therefore examine the resonant scattering
mechanisms that profoundly alter angular momenta of individual stars. We
confirm that the corotation scattering mechanism described by Sellwood & Binney
(2002) is responsible for the largest angular momentum changes in our
simulations.Comment: accepted to MNRAS; substantial additions to Section 4 dealing with
the radial mixing proces