44,070 research outputs found
Shapes of Stellar Systems and Dark Halos from Simulations of Galaxy Major Mergers
Using a sample of 89 snapshots from 58 hydrodynamic binary galaxy major
merger simulations, we find that stellar remnants are mostly oblate while dark
matter halos are mostly prolate or triaxial. The stellar minor axis and the
halo major axis are almost always nearly perpendicular. This can be understood
by considering the influence of angular momentum and dissipation during the
merger. If binary mergers of spiral galaxies are responsible for the formation
of elliptical galaxies or some subpopulation thereof, these galaxies can be
expected to be oblate and inhabit their halos with the predicted shapes and
orientations. These predictions are potentially relevant to observational
studies of weak gravitational lensing, where one must stack many optically
aligned galaxies in order to determine the shape of the resulting stacked mass
distribution. The simple relationship between the dark and luminous matter
presented here can be used to guide the stacking of galaxies to minimize the
information lost.Comment: 4 pages, 5 figures. Minor changes to match published versio
Gravitational waves from eccentric intermediate-mass black hole binaries
If binary intermediate-mass black holes (IMBHs; with masses between 100 and
10^4 \Msun) form in dense stellar clusters, their inspiral will be detectable
with the planned Laser Interferometer Space Antenna (LISA) out to several Gpc.
Here we present a study of the dynamical evolution of such binaries using a
combination of direct -body techniques (when the binaries are well
separated) and three-body relativistic scattering experiments (when the
binaries are tight enough that interactions with stars occur one at a time). We
find that for reasonable IMBH masses there is only a mild effect on the
structure of the surrounding cluster even though the binary binding energy can
exceed the binding energy of the cluster. We demonstrate that, contrary to
standard assumptions, the eccentricity in the LISA band can be in {\em some}
cases as large as and that it induces a measurable phase
difference from circular binaries in the last year before merger. We also show
that, even though energy input from the binary decreases the density of the
core and slows down interactions, the total time to coalescence is short enough
(typically less than a hundred million years) that such mergers will be unique
snapshots of clustered star formation.Comment: Accepted for publication by ApJ Lett
COCOA Code for Creating Mock Observations of Star Cluster Models
We introduce and present results from the COCOA (Cluster simulatiOn
Comparison with ObservAtions) code that has been developed to create idealized
mock photometric observations using results from numerical simulations of star
cluster evolution. COCOA is able to present the output of realistic numerical
simulations of star clusters carried out using Monte Carlo or \textit{N}-body
codes in a way that is useful for direct comparison with photometric
observations. In this paper, we describe the COCOA code and demonstrate its
different applications by utilizing globular cluster (GC) models simulated with
the MOCCA (MOnte Carlo Cluster simulAtor) code. COCOA is used to synthetically
observe these different GC models with optical telescopes, perform PSF
photometry and subsequently produce observed colour magnitude diagrams. We also
use COCOA to compare the results from synthetic observations of a cluster model
that has the same age and metallicity as the Galactic GC NGC 2808 with
observations of the same cluster carried out with a 2.2 meter optical
telescope. We find that COCOA can effectively simulate realistic observations
and recover photometric data. COCOA has numerous scientific applications that
maybe be helpful for both theoreticians and observers that work on star
clusters. Plans for further improving and developing the code are also
discussed in this paper.Comment: 18 pages, 12 figures, accepted for publication in MNRAS. Revised
manuscript has a new title, better quality figures and many other
improvements. COCOA can be downloaded from: https://github.com/abs2k12/COCOA
(comments are welcome
The kinematic signature of the inspiral phase of massive binary black holes
Supermassive black holes are expected to pair as a result of galaxy mergers,
and form a bound binary at parsec or sub-parsec scales. These scales are
unresolved even in nearby galaxies, and thus detection of non-active black hole
binaries must rely on stellar dynamics. Here we show that these systems could
be indirectly detected through the trail that the black holes leave as they
spiral inwards. We analyze two numerical simulations of inspiralling black
holes (equal masses and 10:1 mass ratio) in the stellar environment of a
galactic centre. We studied the effect of the binary on the structure of the
stellar population, with particular emphasis on projected kinematics and
directly measurable moments of the velocity distribution. We present those
moments as high-resolution 2D maps. As shown in past scattering experiments, a
torus of stars counter-rotating with respect to the black holes exists in
scales ~ 5 to 10 times larger than the binary separation. While this is seen in
the average velocity map in the unequal mass case, it is obscured by a more
strongly co-rotating outer region in the equal mass case; however, the inner
counter-rotation could still be detected by studying the higher moments of the
velocity distribution. Additionally, the maps reveal a dip in velocity
dispersion in the inner region, as well as more pronounced signatures in the
higher distribution moments. These maps could serve as templates for integral
field spectroscopy observations of nearby galactic centres. The discovery of
such signatures may help census the population of supermassive black hole
binaries and refine signal rate predictions for future space-based low
frequency gravitational wave detectors.Comment: Accepted for publication in MNRAS; 9 pages, 7 figure
Meeting in a Polygon by Anonymous Oblivious Robots
The Meeting problem for searchers in a polygon (possibly with
holes) consists in making the searchers move within , according to a
distributed algorithm, in such a way that at least two of them eventually come
to see each other, regardless of their initial positions. The polygon is
initially unknown to the searchers, and its edges obstruct both movement and
vision. Depending on the shape of , we minimize the number of searchers
for which the Meeting problem is solvable. Specifically, if has a
rotational symmetry of order (where corresponds to no
rotational symmetry), we prove that searchers are sufficient, and
the bound is tight. Furthermore, we give an improved algorithm that optimally
solves the Meeting problem with searchers in all polygons whose
barycenter is not in a hole (which includes the polygons with no holes). Our
algorithms can be implemented in a variety of standard models of mobile robots
operating in Look-Compute-Move cycles. For instance, if the searchers have
memory but are anonymous, asynchronous, and have no agreement on a coordinate
system or a notion of clockwise direction, then our algorithms work even if the
initial memory contents of the searchers are arbitrary and possibly misleading.
Moreover, oblivious searchers can execute our algorithms as well, encoding
information by carefully positioning themselves within the polygon. This code
is computable with basic arithmetic operations, and each searcher can
geometrically construct its own destination point at each cycle using only a
compass. We stress that such memoryless searchers may be located anywhere in
the polygon when the execution begins, and hence the information they initially
encode is arbitrary. Our algorithms use a self-stabilizing map construction
subroutine which is of independent interest.Comment: 37 pages, 9 figure
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