660 research outputs found
Secular Evolution in Disk Galaxies
Disk galaxies evolve over time through processes that may rearrange both the
radial mass profile and the metallicity distribution within the disk. This
review of such slow changes is largely, though not entirely, restricted to
internally-driven processes that can be distinguished from evolution driven by
galaxy interactions. It both describes our current understanding of disk
evolution, and identifies areas where more work is needed. Stellar disks are
heated through spiral scattering, which increases random motion components in
the plane, while molecular clouds redirect some fraction of the random energy
into vertical motion. The recently discovered process of radial migration at
the corotation resonance of a transient spiral mode does not alter the
underlying structure of the disk, since it neither heats the disk nor causes it
to spread, but it does have a profound effect on the expected distribution of
metallicities among the disk stars. Bars in disks are believed to be major
drivers of secular evolution through interactions with the outer disk and with
the halo. Once the material that makes up galaxy disks is converted into stars,
their overall angular momentum distribution cannot change by much, but that of
the gas is generally far more liable to rearrangement, allowing rings and
pseudo-bulges to form. While simulations are powerful tools from which we have
learned a great deal, those of disks may suffer from collisional relaxation
that requires some results to be interpreted with caution.Comment: Minor revisions to become as it will appear in the journa
Relaxation in N-body simulations of spherical systems
I present empirical measurements of the rate of relaxation in N-body
simulations of stable spherical systems and distinguish two separate types of
relaxation: energy diffusion that is largely independent of particle mass, and
energy exchange between particles of differing masses. While diffusion is
generally regarded as a Fokker-Planck process, it can equivalently be viewed as
the consequence of collective oscillations that are driven by shot noise.
Empirical diffusion rates scale as N^{-1} in inhomogeneous models, in agreement
with Fokker-Planck predictions, but collective effects cause relaxation to
scale more nearly as N^{-1/2} in the special case of a uniform sphere. I use
four different methods to compute the gravitational field, and a 100-fold range
in the numbers of particles in each case. I find the rate at which energy is
exchanged between particles of differing masses does not depend at all on the
force determination method, but I do find the energy diffusion rate is
marginally lower when a field method is used. The relaxation rate in 3D is
virtually independent of the method used because it is dominated by distant
encounters; any method to estimate the gravitational field that correctly
captures the contributions from distant particles must also capture their
statistical fluctuations and the collective modes they drive.Comment: 7 pages, 4 figures, accepted to appear in MNRAS. Very minor changes
in proof
Bar instability in disk-halo systems
We show that the exponential growth rate of a bar in a stellar disk is
substantially greater when the disk is embedded in a live halo than in a rigid
one having the same mass distribution. We also find that the vigor of the
instability in disk-halo systems varies with the shape of the halo velocity
ellipsoid. Disks in rigid halos that are massive enough to be stable by the
usual criteria, quickly form bars in isotropic halos and much greater halo mass
is needed to avoid a strong bar; thus stability criteria derived for disks in
rigid halos do not apply when the halo is responsive. The study presented here
is of an idealized family of models with near uniform central rotation and that
lack an extended halo; we present more realistic models with extended halos in
a companion paper. The puzzle presented by the absence of strong bars in some
galaxies having gently rising inner rotation curves is compounded by the
results presented here.Comment: 8 pages, 4 figures. Revised version submitted to Ap
Relaxation in N-body simulations of disk galaxies
I use N-body simulations with two mass species of particles to demonstrate
that disk galaxy simulations are subject to collisional relaxation at a higher
rate than is widely assumed. Relaxation affects the vertical thickness of the
disk most strongly, and drives the velocity ellipsoid to a moderately flattened
shape similar to that observed for disk stars in the solar neighborhood. The
velocity ellipsoid in simulations with small numbers of particles quickly
approaches this shape, but shot noise also dominates the in-plane behavior.
Simulations with higher, but reachable, numbers of particles relax slowly
enough to be considered collisionless, allowing the in-plane dispersions to
rise due to spiral activity without heating the vertical motions. Relaxation
may have affected many previously published simulations of the formation and
evolution of galaxy disks.Comment: Accepted to ApJ Letters April 23, uses emulateapj.st
Disk Mass from Large-scale Dynamics
The radial distribution of mass in a disk galaxy is strongly constrained by
its rotation curve. The separate contributions from the individual stellar
populations and dark matter (DM) are not easily disentangled, however,
especially since there is generally no feature to indicate where the component
dominating the central attraction switches from luminous to dark matter. Here I
summarize three recent thesis projects at Rutgers University which all suggest
that DM has a low density in the inner parts of bright galaxies, and that most
of the mass therefore resides in the disk. In addition, I present some
preliminary work on the Milky Way.Comment: 3 pages including 1 figure, LaTeX with jd.sty file included, to
appear in "Highlights of Astronomy, v11" ed J Anderse
Dynamical Constraints on Disk Masses
While the total interior mass of a galaxy is reasonably well determined by a
good rotation curve, the relative contributions from disk, bulge and halo are
only weakly constrained by one-dimensional data. Barred galaxies are
intrinsically more complicated, but provide much tighter constraints on the
disk masses and support the idea that most of the mass in the inner parts of
bright galaxies is in their stars. There appears to be no systematic difference
in dark matter content between barred and unbarred galaxies, consistent with
the theoretical result that the global stability of galaxies with dense centers
does not depend on their halo fraction. The rotation curve shapes of lower
luminosity and low-surface-brightness galaxies, on the other hand, indicate
significant mass in the DM halo even near their centers. We find that most DM
halos appear to have large cores, inconsistent with the predictions from
cosmological simulations. We also show that such large-core halos can result
from compression by disk infall of physically reasonable initial halos. Maximum
disks, while apparently required by the data, do seem to present some puzzles;
most notably they re-open the old disk-halo ``conspiracy'' issue and
incorrectly predict that surface brightness should be a second parameter in the
Tully-Fisher relation.Comment: To Appear in "Galaxy Dynamics" eds. Merritt, Sellwood & Valluri, 14
pages, Latex needs paspconf.sty, 6 figure
Spiral Structure as a Recurrent Instability
A long-standing controversy in studies of spiral structure has concerned the
lifetimes of individual spiral patterns. Much theoretical work has sought
quasi-stationary spiral modes while N-body simulations have consistently
displayed recurrent, short-lived patterns. The simulations manifest a recurrent
cycle of true instabilities related to small-scale features in the angular
momentum distribution of particles, with the decay of each instability seeding
the growth of the next. Data from the recent Hipparcos mission seem to offer
support for the recurrent transient picture.Comment: To appear in "Astrophysical Dynamics -- in Commemoration of F. D.
Kahn", eds. D. Berry, et al. (Dordrecht: Kluwer) - 5 figures, 13 pages, LaTeX
uses paspconf.st
Galaxy Formation, Bars and QSOs
A model that accounts for the brief flaring of QSOs in the early stages of
galaxy formation is proposed. I argue that a bar must develop early in the life
of nearly every galaxy and that gas to create and fuel the QSO is driven into
the center of the galaxy by the bar. The QSO lifetime, and the mass of its
central engine, are also controlled by large-scale dynamics, since the fuel
supply is shut off after a short period by the development of an inner Lindblad
resonance. This resonance causes the gas inflow along the bar to stall at a
distance of a few hundred parsecs from the center. The ILR develops as a result
of previous inflow, making quasar activity self-limiting. The bars are weakened
and can be destroyed by the central mass concentration formed in this way.Comment: 4 pages, no figures, to appear in "Galaxy Dynamics", eds Merritt,
Sellwood & Vallur
Spiral Instabilities in N-body Simulations I: Emergence from Noise
The origin of spiral patterns in galaxies is still not fully understood.
Similar features also develop readily in N-body simulations of isolated cool,
collisionless disks, yet even here the mechanism has yet to be explained. In
this series of papers, I present a detailed study of the origin of spiral
activity in simulations in the hope that the mechanism that causes the patterns
is also responsible for some of these features galaxies. In this first paper, I
use a suite of highly idealized simulations of a linearly stable disk that
employ increasing numbers of particles. While the amplitudes of initial
non-axisymmetric features scale as the inverse square-root of the number of
particles employed, the final amplitude of the patterns is independent of the
particle number. I find that the amplitudes of non-axisymmetric disturbances
grow in two distinct phases: slow growth occurs when the relative overdensity
is below ~2%, but above this level the amplitude rises more rapidly. I show
that all features, even of very low amplitude, scatter particles at the inner
Lindblad resonance, changing the distribution of particles in the disk in such
a way as to foster continued growth. Stronger scattering by larger amplitude
waves provokes a vigorous instability that is a true linear mode of the
modified disk.Comment: 11 pages, 10 figures, revised after referee's report, accepted to
appear ApJ, minor textual revision
Stability and Evolution of Galactic Discs
In this review, I discuss just three aspects of the stability and evolution
of galactic discs. (1) I first review our understanding of the bar instability
and how it can be controlled. Disc galaxies in which the orbital speed does not
decrease much towards the centre have no difficulty avoiding bars, even when
dark matter makes an insignificant contribution to the inner part of the
rotation curve. (2) I then briefly discuss interactions between disturbances in
the discs of galaxies and the spherical components, which generally exert a
damping effect through dynamical friction. The fact that bars in real galaxies
appear to rotate quite rapidly, seems to require dark matter halos to have
large, low-density cores. (3) In the remainder of the article, I consider the
theory of spiral structure. The new development here is that the distribution
function for stars in the Solar neighbourhood, as measured by HIPPARCOS, is far
less smooth than most theoretical work had previously supposed. The strong
variations in the values of the \DF over small ranges in angular momentum have
the appearance of having been caused by scattering at Lindblad resonances with
spiral patterns. This result, if confirmed when the radial velocity data become
available, supports the picture of spiral patterns as dynamical instabilities
driven by substructure in the \DF. The details of how decaying patterns might
seed conditions for a new instability remain unclear, and deserve fresh
attention.Comment: To Appear in "Astrophysical Discs" eds. Sellwood & Goodman 15 pages,
5 figures, Latex needs paspconf.st
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