32 research outputs found
Supercomputer Simulations of Disk Galaxies
The time evolution of models for an isolated disk of highly flattened
galaxies of stars is investigated by direct integration of the Newtonian
equations of motion of N=30,000 identical stars over a time span of many
galactic rotations. Certain astronomical implications of the simulations to
actual disk-shaped (i.e. rapidly rotating) galaxies are explored as well.Comment: 3 pages, 2 figure Aat.sty, Aattable.sty, presented by E. Griv at the
JENAM 2000, S02, Moscow, Russia, 200
Kinematic segregation of nearby disk stars from the Hipparcos database
To better understand our Galaxy, we investigate the pertinency of describing
the sys tem of nearby disk stars in terms of a two-components Schwarzschild
velocity distributio n.Using the proper motion and parallax information of
Hipparcos database, we determine t he parameters characterizing the local
stellar velocity field of a sample of 22000 disk stars. The sample we use is
essentially the same as the one described by the criteria ad opted to study the
LSR and the stream motion of the nearby stellar populationComment: 19 page
Stability of disk galaxies in the modified dynamics
General analytic arguments lead us to expect that in the modified dynamics
(MOND) self-gravitating disks are more stable than their like in Newtonian
dynamics. We study this question numerically, using a particle-mesh code based
on a multi-grid solver for the (nonlinear) MOND field equation. We start with
equilibrium distribution functions for MOND disk models having a smoothly
truncated, exponential surface-density profiles and a constant Toomre
parameter. We find that, indeed, disks of a given ``temperature'' are locally
more stable in MOND than in Newtonian dynamics. As regards global instability
to bar formation, we find that as the mean acceleration in the disk is lowered,
the stability of the disk is increased as we cross from the Newtonian to the
MOND regime. The degree of stability levels off deep in the MOND regime, as
expected from scaling laws in MOND. For the disk model we use, this maximum
degree of stability is similar to the one imparted to a Newtonian disk by a
halo three times as massive at five disk scale lengths.Comment: 20 pages, Latex, 8 embedded figures, version to be published in The
Astrophys.
Bending Instability of Stellar Disks: The Stabilizing Effect of a Compact Bulge
The saturation conditions for bending modes in inhomogeneous thin stellar
disks that follow from an analysis of the dispersion relation are compared with
those derived from -body simulations. In the central regions of
inhomogeneous disks, the reserve of disk strength against the growth of bending
instability is smaller than that for a homogeneous layer. The spheroidal
component (a dark halo, a bulge) is shown to have a stabilizing effect. The
latter turns out to depend not only on the total mass of the spherical
component, but also on the degree of mass concentration toward the center. We
conclude that the presence of a compact (not necessarily massive) bulge in
spiral galaxies may prove to be enough to suppress the bending perturbations
that increase the disk thickness. This conclusion is corroborated by our
-body simulations in which we simulated the evolution of almost equilibrium,
but unstable finite-thickness disks in the presence of spheroidal components.
The final disk thickness at the same total mass of the spherical component
(dark halo + bulge) has been found to be much smaller than that in the
simulations where a concentrated bulge is present.Comment: 27 pages including 10 figures. To be published in Astronomy Letters
(v.31, No. 1, pp. 15-29 2005
The interaction of dark matter cusp with the baryon component in disk galaxies
In this paper we examine the effect of the formation and evolution of the
disk galaxy on the distribution of dark halo matter. We have made simulations
of isolated dark matter (DM) halo and two component (DM + baryons). N-body
technique was used for stellar and DM particles and TVD MUSCL scheme for
gas-dynamic simulations. The simulations include the processes of star
formation, stellar feedback, heating and cooling of the interstellar medium.
The results of numerical experiments with high spatial resolution let us to
conclude in two main findings. First, accounting of star formation and
supernova feedback resolves the so-called problem of cusp in distribution of
dark matter predicted by cosmological simulations. Second, the interaction of
dark matter with dynamic substructures of stellar and gaseous galactic disk
(e.g., spiral waves, bar) has an impact on the shape of the dark halo. In
particular, the in-plane distribution of dark matter is more symmetric in runs,
where the baryonic component was taken into account.Comment: 7 pages, 6 figure
Intermediate-mass black holes and ultraluminous X-ray sources in the Cartwheel ring galaxy
Chandra and XMM-Newton observations of the Cartwheel galaxy show ~17 bright
X-ray sources (>~5x10^38 erg s^-1), all within the gas-rich outer ring. We
explore the hypothesis that these X-ray sources are powered by
intermediate-mass black holes (IMBHs) accreting gas or undergoing mass transfer
from a stellar companion. To this purpose, we run N-body/SPH simulations of the
galaxy interaction which might have led to the formation of Cartwheel, tracking
the dynamical evolution of two different IMBH populations: halo and disc IMBHs.
Halo IMBHs cannot account for the observed X-ray sources, as only a few of them
cross the outer ring. Instead, more than half of the disc IMBHs are pulled in
the outer ring as a consequence of the galaxy collision. However, also in the
case of disc IMBHs, accretion from surrounding gas clouds cannot account for
the high luminosities of the observed sources. Finally, more than 500 disc
IMBHs are required to produce <~15 X-ray sources via mass transfer from very
young stellar companions. Such number of IMBHs is very large and implies
extreme assumptions. Thus, the hypothesis that all the observed X-ray sources
in Cartwheel are associated with IMBHs is hardly consistent with our
simulations, even if it is still possible that IMBHs account for the few
(<~1-5) brightest ultraluminous X-ray sources (ULXs).Comment: 16 pages, 12 figures, MNRAS, in press, higher resolution version at
http://www-theorie.physik.unizh.ch/~mapelli/astroph/cartwheel_ULX2.p
Spatial Structure and Coherent Motion in Dense Planetary Rings Induced by Self-Gravitational Instability
We investigate the formation of spatial structure in dense, self-gravitating
particle systems such as Saturn's B-ring through local -body simulations to
clarify the intrinsic physics based on individual particle motion. In such a
system, Salo (1995) showed that the formation of spatial structure such as
wake-like structure and particle grouping (clump) arises spontaneously due to
gravitational instability and the radial velocity dispersion increases as the
formation of the wake structure. However, intrinsic physics of the phenomena
has not been clarified. We performed local -body simulations including
mutual gravitational forces between ring particles as well as direct
(inelastic) collisions with identical (up to ) particles. In the
wake structure particles no longer move randomly but coherently. We found that
particle motion was similar to Keplerian motion even in the wake structure and
that the coherent motion was produced since the particles in a clump had
similar eccentricity and longitude of perihelion. This coherent motion causes
the increase and oscillation in the radial velocity dispersion. The mean
velocity dispersion is rather larger in a more dissipative case with a smaller
restitution coefficient and/or a larger surface density since the coherence is
stronger in the more dissipative case. Our simulations showed that the
wavelength of the wake structure was approximately given by the longest
wavelength \hs{\lambda}{cr} = 4\pi^2 G\Sigma/\kappa^2 in the linear theory of
axisymmetric gravitational instability in a thin disk, where , , and
are the gravitational constant, surface density, and a epicyclic
frequency.Comment: Accepted by Earth, Planets, and Space. 39 pages, 20 figures.
PostScript files also available from
http://www.geo.titech.ac.jp/nakazawalab/hdaisaka/works
Revisiting the Transit Timing Variations in the TrES-3 and Qatar-1 Systems with TESS Data
We present and analyze 58 transit light curves of TrES-3b and 98 transit light curves of Qatar-1b, observed by the Transiting Exoplanet Survey Satellite, plus two transit light curves of Qatar-1b, observed by us, using a ground-based 1.23 m telescope. These light curves are combined with the best-quality light curves taken from the Exoplanet Transit Database and the literature. The precisely determined midtransit times from these light curves enable us to obtain the refined orbital ephemerides, with improved precision, for both hot Jupiters. From the timing analysis, we find indications of the presence of transit timing variations (TTVs) in both systems. Since the observed TTVs are unlikely to be short-term and periodic, the possibility of additional planets in orbits close to TrES-3b and Qatar-1b is ruled out. The possible causes of long-term TTVs, such as orbital decay, apsidal precession, the Applegate mechanism, and line-of-sight acceleration, are also examined. However, none of these possibilities are found to explain the observed TTV of TrES-3b. In contrast to this, line-of-sight acceleration appears to be a plausible explanation for the observed TTV of Qatar-1b. In order to confirm these findings, further high-precision transit and radial velocity observations of both systems would be worthwhile
The Planetary Nebula System of M33
We report the results of a photometric and spectroscopic survey for planetary
nebulae (PNe) in the Local Group spiral galaxy M33. We use our sample of 152
PNe to derive an [O III] planetary nebula luminosity function (PNLF) distance
of (m-M)_0 = 24.86^+0.07-0.11 (0.94^+0.03-0.05 Mpc). Although this value is ~
15% larger than the galaxy's Cepheid distance, the discrepancy likely arises
from differing assumptions about the system's internal extinction. Our
photometry (which extends >3 mag down the PNLF), also reveals that the
faint-end of M33's PN luminosity function is non-monotonic, with an inflection
point ~2 mag below the PNLF cutoff. We argue that this feature is due to the
galaxy's large population of high core-mass planetaries, and that its amplitude
may eventually be useful as a diagnostic for studies of stellar populations.
Fiber-coupled spectroscopy of 140 of the PN candidates confirms that M33's PN
population rotates along with the old disk, with a small asymmetric drift of \~
10km/s. Remarkably, the population's line-of-sight velocity dispersion varies
little over ~4 optical disk scale lengths, with sigma_{rad}~20km/s. We show
that this is due to a combination of factors, including a decline in the radial
component of the velocity ellipsoid at small galactocentric radii, and a
gradient in the ratio of the vertical to radial velocity dispersion. We use our
data to show that the mass scale length of M33's disk is ~2.3 times larger than
that of the system's IR luminosity and that the disk's V-band mass-to-light
ratio changes from M/L_V ~0.3 in the galaxy's inner regions to M/L_V ~2.0 at ~9
kpc. Models in which the dark matter is distributed in the plane of the galaxy
are excluded by our data. (abridged)Comment: 45 pages, including 12 figures (some with reduced resolution);
accepted for publication in the Astrophysical Journa
Minimum Velocity Dispersion in Stable Stellar Disks. Numerical Simulations
N-body dynamical simulations are used to analyze the conditions for the
gravitational stability of a three-dimensional stellar disk in the
gravitational field of two rigid spherical components--a bulge and a halo whose
central concentrations and relative masses vary over wide ranges. The number of
point masses N in the simulations varies from 40 to 500 thousands and the
evolution of the simulated models is followed over 10--20 rotation periods of
the outer edge of the disk. The initially unstable disks are heated and, as a
rule, reach a quasi-stationary equilibrium with a steady-state radial-velocity
dispersion over five to eight periods of rotation. The radial behavior of
the Toomre stability parameter for the final state of the disk is
estimated. Numerical models are used to analyze the dependence of the
gravitational stability of the disk on the relative masses of the spherical
components, disk thickness, degree of differential rotation, and initial state
of the disk. Formal application of existing, analytical, local criteria for
marginal stability of the disk can lead to errors in radial velocity dispersion
of more than a factor of 1.5. It is suggested that the approximate
constancy of for (where L
is the radial scale of disk surface density), valid for a wide range of models,
can be used to estimate upper limits for the mass and density of a disk based
on the observed distributions of the rotational velocity of the gaseous
component and of the stellar velocity dispersion.Comment: 33 pages, 8 Figs. Published in Astronomy Reports,2003,v.47,p.357 The
paper may also be found at http://neptun.sai.msu.su/~zasov/articles/k_z.zi