979 research outputs found
Shape, spin and baryon fraction of clusters in the MareNostrum Universe
The MareNostrum Universe is one of the largest cosmological
SPH simulation done so far. It consists of dark and
gas particles in a box of 500 Mpc on a side. Here we study
the shapes and spins of the dark matter and gas components of the 10,000 most
massive objects extracted from the simulation as well as the gas fraction in
those objects. We find that the shapes of objects tend to be prolate both in
the dark matter and gas. There is a clear dependence of shape on halo mass, the
more massive ones being less spherical than the less massive objects. The gas
distribution is nevertheless much more spherical than the dark matter, although
the triaxiality parameters of gas and dark matter differ only by a few percent
and it increases with cluster mass. The spin parameters of gas and dark matter
can be well fitted by a lognormal distribution function. On average, the spin
of gas is 1.4 larger than the spin of dark matter. We find a similar behavior
for the spins at higher redshifts, with a slightly decrease of the spin ratios
to 1.16 at The cosmic normalized baryon fraction in the entire cluster
sample ranges from , at to at . At both
redshifts we find a slightly, but statistically significant decrease of
with cluster mass.Comment: 7 pages, 6 figures. Accepted for publication in The Astrophysical
Journa
The Ellipticity of the Disks of Spiral Galaxies
The disks of spiral galaxies are generally elliptical rather than circular.
The distribution of ellipticities can be fit with a log-normal distribution.
For a sample of 12,764 galaxies from the Sloan Digital Sky Survey Data Release
1 (SDSS DR1), the distribution of apparent axis ratios in the i band is best
fit by a log-normal distribution of intrinsic ellipticities with ln epsilon =
-1.85 +/- 0.89. For a sample of nearly face-on spiral galaxies, analyzed by
Andersen and Bershady using both photometric and spectroscopic data, the best
fitting distribution of ellipticities has ln epsilon = -2.29 +/- 1.04. Given
the small size of the Andersen-Bershady sample, the two distribution are not
necessarily inconsistent. If the ellipticity of the potential were equal to
that of the light distribution of the SDSS DR1 galaxies, it would produce 1.0
magnitudes of scatter in the Tully-Fisher relation, greater than is observed.
The Andersen-Bershady results, however, are consistent with a scatter as small
as 0.25 magnitudes in the Tully-Fisher relation.Comment: 19 pages, 5 figures; ApJ, accepte
Galactic Spiral Structure
We describe the structure and composition of six major stellar streams in a
population of 20 574 local stars in the New Hipparcos Reduction with known
radial velocities. We find that, once fast moving stars are excluded, almost
all stars belong to one of these streams. The results of our investigation have
lead us to re-examine the hydrogen maps of the Milky Way, from which we
identify the possibility of a symmetric two-armed spiral with half the
conventionally accepted pitch angle. We describe a model of spiral arm motions
which matches the observed velocities and composition of the six major streams,
as well as the observed velocities of the Hyades and Praesepe clusters at the
extreme of the Hyades stream. We model stellar orbits as perturbed ellipses
aligned at a focus in coordinates rotating at the rate of precession of
apocentre. Stars join a spiral arm just before apocentre, follow the arm for
more than half an orbit, and leave the arm soon after pericentre. Spiral
pattern speed equals the mean rate of precession of apocentre. Spiral arms are
shown to be stable configurations of stellar orbits, up to the formation of a
bar and/or ring. Pitch angle is directly related to the distribution of orbital
eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve
to form bars and rings. We show that orbits of gas clouds are stable only in
bisymmetric spirals. We conclude that spiral galaxies evolve toward grand
design two-armed spirals. We infer from the velocity distributions that the
Milky Way evolved into this form about 9 Gyrs ago.Comment: Published in Proc Roy Soc A. A high resolution version of this file
can be downloaded from http://papers.rqgravity.net/SpiralStructure.pdf. A
simplified account with animations begins at
http://rqgravity.net/SpiralStructur
AGN effect on cooling flow dynamics
We analyzed the feedback of AGN jets on cooling flow clusters using
three-dimensional AMR hydrodynamic simulations. We studied the interaction of
the jet with the intracluster medium and creation of low X-ray emission
cavities (Bubbles) in cluster plasma. The distribution of energy input by the
jet into the system was quantified in its different forms, i.e. internal,
kinetic and potential. We find that the energy associated with the bubbles, (pV
+ gamma pV/(gamma-1)), accounts for less than 10 percent of the jet energy.Comment: "Accepted for publication in Astrophysics & Space Science
Triaxial orbit based galaxy models with an application to the (apparent) decoupled core galaxy NGC 4365
We present a flexible and efficient method to construct triaxial dynamical
models of galaxies with a central black hole, using Schwarzschild's orbital
superposition approach. Our method is general and can deal with realistic
luminosity distributions, which project to surface brightness distributions
that may show position angle twists and ellipticity variations. The models are
fit to measurements of the full line-of-sight velocity distribution (wherever
available). We verify that our method is able to reproduce theoretical
predictions of a three-integral triaxial Abel model. In a companion paper (van
de Ven, de Zeeuw & van den Bosch), we demonstrate that the method recovers the
phase-space distribution function. We apply our method to two-dimensional
observations of the E3 galaxy NGC 4365, obtained with the integral-field
spectrograph SAURON, and study its internal structure, showing that the
observed kinematically decoupled core is not physically distinct from the main
body and the inner region is close to oblate axisymmetric.Comment: 21 Pages, 14 (Colour) Figures, Companion paper is arXiv:0712.0309
Accepted to MNRAS. Full resolution version at
http://www.strw.leidenuniv.nl/~bosch/papers/RvdBosch_triaxmethod.pd
Self-Consistent Response of a Galactic Disk to an Elliptical Perturbation Halo Potential
We calculate the self-consistent response of an axisymmetric galactic disk
perturbed by an elliptical halo potential of harmonic number m = 2, and obtain
the net disk ellipticity. Such a potential is commonly expected to arise due to
a galactic tidal encounter and also during the galaxy formation process. The
self-gravitational potential corresponding to the self-consistent,
non-axisymmetric density response of the disk is obtained by inversion of
Poisson equation for a thin disk. This response potential is shown to oppose
the perturbation potential, because physically the disk self-gravity resists
the imposed potential. This results in a reduction in the net ellipticity of
the perturbation halo potential in the disk plane. The reduction factor
denoting this decrease is independent of the strength of the perturbation
potential, and has a typical minimum value of 0.75 - 0.9 for a wide range of
galaxy parameters. The reduction is negligible at all radii for higher
harmonics (m > or = 3) of the halo potential. (abridged).Comment: 26 pages (LaTex- aastex style), 3 .eps figures. To appear in the
Astrophysical Journal, Vol. 542, Oct. 20, 200
Milky Way Mass Models and MOND
Using the Tuorla-Heidelberg model for the mass distribution of the Milky Way,
I determine the rotation curve predicted by MOND. The result is in good
agreement with the observed terminal velocities interior to the solar radius
and with estimates of the Galaxy's rotation curve exterior thereto. There are
no fit parameters: given the mass distribution, MOND provides a good match to
the rotation curve. The Tuorla-Heidelberg model does allow for a variety of
exponential scale lengths; MOND prefers short scale lengths in the range 2.0 to
2.5 kpc. The favored value of scale length depends somewhat on the choice of
interpolation function. There is some preference for the `simple' interpolation
function as found by Famaey & Binney. I introduce an interpolation function
that shares the advantages of the simple function on galaxy scales while having
a much smaller impact in the solar system. I also solve the inverse problem,
inferring the surface mass density distribution of the Milky Way from the
terminal velocities. The result is a Galaxy with `bumps and wiggles' in both
its luminosity profile and rotation curve that are reminiscent of those
frequently observed in external galaxies.Comment: Accepted for publication in the Astrophysical Journal. 31 pages
including 8 figures and 3 table
SPH Simulations of Galactic Gaseous Disk with Bar: Distribution and Kinematic Structure of Molecular Clouds toward the Galactic Center
We have performed Smoothed Particle Hydrodynamic (SPH) simulations to study
the response of molecular clouds in the Galactic disk to a rotating bar and
their subsequent evolution in the Galactic Center (GC) region. The Galactic
potential in our models is contributed by three axisymmetric components
(massive halo, exponential disk, compact bulge) and a non-axisymmetric bar.
These components are assumed to be invariant in time in the frame corotating
with the bar. Some noticeable features such as an elliptical outer ring, spiral
arms, a gas-depletion region, and a central concentration have been developed
due to the influence of the bar. The rotating bar induces non-circular motions
of the SPH particles, but hydrodynamic collisions tend to suppress the random
components of the velocity. The velocity field of the SPH particles is
consistent with the kinematics of molecular clouds observed in HCN (1-0)
transition; these clouds are thought to be very dense clouds. However, the l-v
diagram of the clouds traced by CO is quite different from that of our SPH
simulation, being more similar to that obtained from simulations using
collisionless particles. The diagram of a mixture of collisional and
collisionless particles gives better reproduction of the kinematic structures
of the GC clouds observed in the CO line. The fact that the kinematics of HCN
clouds can be reproduced by the SPH particles suggests that the dense clouds in
the GC are formed via cloud collisions induced by rotating bar.Comment: 31 pages, 10 pigures, accepted for publication in Ap
A key-formula to compute the gravitational potential of inhomogeneous discs in cylindrical coordinates
We have established the exact expression for the gravitational potential of a
homogeneous polar cell - an elementary pattern used in hydrodynamical
simulations of gravitating discs. This formula, which is a closed-form, works
for any opening angle and radial extension of the cell. It is valid at any
point in space, i.e. in the plane of the distribution (inside and outside) as
well as off-plane, thereby generalizing the results reported by Durand (1953)
for the circular disc. The three components of the gravitational acceleration
are given. The mathematical demonstration proceeds from the "incomplete version
of Durand's formula" for the potential (based on complete elliptic integrals).
We determine first the potential due to the circular sector (i.e. a pie-slice
sheet), and then deduce that of the polar cell (from convenient radial scaling
and subtraction). As a by-product, we generate an integral theorem stating that
"the angular average of the potential of any circular sector along its tangent
circle is 2/PI times the value at the corner". A few examples are presented.
For numerical resolutions and cell shapes commonly used in disc simulations, we
quantify the importance of curvature effects by performing a direct comparison
between the potential of the polar cell and that of the Cartesian (i.e.
rectangular) cell having the same mass. Edge values are found to deviate
roughly like 2E-3 x N/256 in relative (N is the number of grid points in the
radial direction), while the agreement is typically four orders of magnitude
better for values at the cell's center. We also produce a reliable
approximation for the potential, valid in the cell's plane, inside and close to
the cell. Its remarkable accuracy, about 5E-4 x N/256 in relative, is
sufficient to estimate the cell's self-acceleration.Comment: Accepted for publication in Celestial Mechanics and Dynamical
Astronom
Cooling flows and quasars: II. Detailed models of feedback modulated accretion flows
Most elliptical galaxies contain central black holes (BHs), and most also
contain significant amounts of hot gas capable of accreting on to the central
BH due to cooling times short compared to the Hubble time. Why therefore do we
not see AGNs at the center of most elliptical galaxies rather than in only (at
most) a few per cent of them? We propose here the simple idea that feedback
from accretion events heats the ambient gas retarding subsequent infall. In
this context, we present a class of 1D hydrodynamical evolutionary sequences
for the gas flows in elliptical galaxies with a massive central BH. The
resulting evolution is characterized by strong oscillations, in which very fast
and energetic bursts of the BH are followed by longer periods in which the
X-ray galaxy emission comes from the coronal gas. We also allow for departures
from spherical symmetry by examining scenarios in which the central engine is
either an ADAF or a more conventional accretion disk that is optically thick
except for a polar region. In all cases the duty cycle (fraction of the time
that the system will be seen as an AGN) is quite small and in the range 10^{-4}
- 10^{-3}. Thus, for any reasonable value of the efficiency, the presence of a
massive BH at the center of a galaxy seems to be incompatible with the presence
of a long-lived cooling flow.Comment: 43 pages, 10 figures. Main additions concern observed Compton
temperatures and few extra numerical models. Conclusions unchanged. 1 new
table and 3 new figures. Accepted for publication on ApJ (main journal
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