24 research outputs found
Getting a kick out of the stellar disk(s) in the galactic center
Recent observations of the Galactic center revealed a nuclear disk of young
OB stars, in addition to many similar outlying stars with higher eccentricities
and/or high inclinations relative to the disk (some of them possibly belonging
to a second disk). Binaries in such nuclear disks, if they exist in
non-negligible fractions, could have a major role in the evolution of the disks
through binary heating of this stellar system. We suggest that interactions
with/in binaries may explain some (or all) of the observed outlying young stars
in the Galactic center. Such stars could have been formed in a disk, and later
on kicked out from it through binary related interactions, similar to ejection
of high velocity runaway OB stars in young clusters throughout the galaxy.Comment: 2 pages, 2 figs. To be published in the proceedings of the IAU 246
symposium on "Dynamical evolution of dense stellar systems
Merger of Black Holes in the Galactic Center
We present the results of three body simulations focused on understanding the
fates of intermediate mass black holes (IBH) that drift within the central 0.5
pc of the Galaxy. In particular, we modeled the interactions between pairs of
black holes as they orbit a central blac k hole of mass
. The simulations performed assume a
Schwarzschild geometry and account for Chandrasekhar dynamical friction as well
as acceleration resulting from energy lost due to gravitational radiation.
We found the branching ratio for one of the orbiting IBHs to merge with the
CBH was 0.95 and is independent of the inner IBH's initial eccentricity as well
as the rate of sinking. This, coupled with an infall rate of yrs
for an IBH to drift into the Galactic center, results in an IBH-CBH merger
every Myrs. Lastly we found that the IBH-IBH-CBH triple body
system ``resets'' itself, in the sense that a system with an inner I BH with an
initially circular orbit generally left behind an IBH with a large
eccentricity, whereas a system in which the inner IBH had a high eccentricity
() usually left a remnant with low eccentricity. Branching ratios
for different outcomes are also similar in the two cases.Comment: Official paper to appear in November 2008 issue of Ap
Fast coalescence of post-Newtonian Supermassive Black Hole Binaries in real galaxies
We present the results of theoretical modeling of supermassive black hole binary (SMBHB) mergers using direct 2 -body simulations with a Hermite integration scheme. The BH’s gravitational interaction is described based on the post-Newtonian (PN-terms) approximation up to the 3.5 PN-terms. We carry out a large set of runs using a parametric description of SMBHB orbits.Представлены результаты теоретического моделирования слияния двойных сверхмассивных чёрных дыр с помощью прямого 2-тельного моделирования с эрмитовской схемой интегрирования. Гравитационное взаимодействие черных дыр описывается постньютоновским приближением до 3.5 PN-терма. На основе параметрического описания орбит ДСМЧД получен большой набор моделей.Приводяться результати теоретичного моделювання злиття подвійних надмасивних чорних дір за допомогою прямого 2-тільного моделювання з ермітівською схемою інтегрування. Гравітаційна взаємодія чорних дір описується постньютонівським наближенням до 3.5 PN-терму. На основі параметричного опису орбіт ПНЧД отримано великий набір моделей
Recoil velocities from equal-mass binary black-hole mergers: a systematic investigation of spin-orbit aligned configurations
Binary black-hole systems with spins aligned with the orbital angular
momentum are of special interest, as studies indicate that this configuration
is preferred in nature. If the spins of the two bodies differ, there can be a
prominent beaming of the gravitational radiation during the late plunge,
causing a recoil of the final merged black hole. We perform an accurate and
systematic study of recoil velocities from a sequence of equal-mass black holes
whose spins are aligned with the orbital angular momentum, and whose individual
spins range from a = +0.584 to -0.584. In this way we extend and refine the
results of a previous study and arrive at a consistent maximum recoil of 448 +-
5 km/s for anti-aligned models as well as to a phenomenological expression for
the recoil velocity as a function of spin ratio. This relation highlights a
nonlinear behavior, not predicted by the PN estimates, and can be readily
employed in astrophysical studies on the evolution of binary black holes in
massive galaxies. An essential result of our analysis is the identification of
different stages in the waveform, including a transient due to lack of an
initial linear momentum in the initial data. Furthermore we are able to
identify a pair of terms which are largely responsible for the kick, indicating
that an accurate computation can be obtained from modes up to l=3. Finally, we
provide accurate measures of the radiated energy and angular momentum, finding
these to increase linearly with the spin ratio, and derive simple expressions
for the final spin and the radiated angular momentum which can be easily
implemented in N-body simulations of compact stellar systems. Our code is
calibrated with strict convergence tests and we verify the correctness of our
measurements by using multiple independent methods whenever possible.Comment: 24 pages, 15 figures, 5 table
Weighing Super-Massive Black Holes with Narrow Fe K Line
It has been suggested that the narrow cores of the Fe K emission
lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the
inner radius of which can be measured by observing the lag time between the
and band flux variations. In this paper we compare the virial products of
the infrared time lags and the narrow Fe K widths for 10 type 1 AGNs
with the black hole masses from other techniques. We find the narrow Fe
K line width is in average 2.6 times broader than
expected assuming an isotropic velocity distribution of the torus at the
distance measured by the infrared lags. We propose the thick disk model of the
torus could explain the observed larger line width. Another possibility is the
contamination by emission from the broad line region or the outer accretion
disk. Alternatively, the narrow iron line might originate from the inner most
part of the obscuring torus within the sublimation radius, while the infrared
emission from outer cooler part. We note the correlation between the black hole
masses based on this new technique and those based on other known techniques is
statistically insignificant. We argue that this could be attributed to the
small sample size and the very large uncertainties in the measurements of iron
K line widths. The next generation of X-ray observatories could help verify the
origin of the narrow iron K line and the reliability of this new
technique.Comment: 12 pages, 2 figures, 2 tables, Science China G, in pres
Computation of the Fourier parameters of RR Lyrae stars by template fitting
Due to the importance of accurate Fourier parameters, we devise a method that is more appropriate for deriving these parameters on low-quality data than the traditional Fourier fitting. Based on the accurate light curves of 248 fundamental mode RR Lyrae stars, we test the power of a full-fetched implementation of the template method in the computation of the Fourier decomposition. The applicability of the method is demonstrated also on datasets of filter passbands different from that of the template set. We examine in more detail the question of the estimation of Fourier- based iron abundance [Fe/H] and average brightness. We get, for example, for light curves sampled randomly in 30 data points with sigma=0.03 mag observational noise that optimized direct Fourier fits yield sigma_[Fe/H]=0.33, whereas the template fits result in sigma_[Fe/H]=0.18. Tests made on the RR Lyrae database of the Large Magellanic Cloud (LMC) of the Optical Gravitational Lensing Experiment (OGLE) support the applicability of the method on real photometric time series. These tests also show that the dominant part of error in estimating the average brightness comes from other sources, most probably from crowding effects, even for under-sampled light curves
Parallelization, Special Hardware and Post-Newtonian Dynamics in Direct N - Body Simulations
The formation and evolution of supermassive black hole (SMBH) binaries during and after galaxy mergers is an important ingredient for our understanding of galaxy formation and evolution in a cosmological context, e.g. for predictions of cosmic star formation histories or of SMBH demographics (to predict events that emit gravitational waves). If galaxies merge in the course of their evolution, there should be either many binary or even multiple black holes, or we have to find out what happens to black hole multiples in galactic nuclei, e.g. whether they come sufficiently close to merge resulting from emission of gravitational waves, or whether they eject each other in gravitational slingshot interactions
Galactic rotation curves in brane world models
In the braneworld scenario the four dimensional effective Einstein equation
has extra source terms, which arise from the embedding of the 3-brane in the
bulk. These non-local effects, generated by the free gravitational field of the
bulk, may provide an explanation for the dynamics of the neutral hydrogen
clouds at large distances from the galactic center, which is usually explained
by postulating the existence of the dark matter. In the present paper we
consider the asymptotic behavior of the galactic rotation curves in the brane
world models, and we compare the theoretical results with observations of both
High Surface Brightness and Low Surface Brightness galaxies. For the chosen
sample of galaxies we determine first the baryonic parameters by fitting the
photometric data to the adopted galaxy model; then we test the hypothesis of
the Weyl fluid acting as dark matter on the chosen sample of spiral galaxies by
fitting the tangential velocity equation of the combined baryonic-Weyl model to
the rotation curves. We give an analytical expression for the rotational
velocity of a test particle on a stable circular orbit in the exterior region
to a galaxy, with Weyl fluid contributions included. The model parameter ranges
for which the test provides agreement (within 1 confidence
level) with observations on the velocity fields of the chosen galaxy sample are
then determined. There is a good agreement between the theoretical predictions
and observations, showing that extra-dimensional models can be effectively used
as a viable alternative to the standard dark matter paradigm.Comment: to be published in MNRAS, 17 pages, 31 figures, version including
corrections on the proo
Dynamical Processes in Globular Clusters
Globular clusters are among the most congested stellar systems in the
Universe. Internal dynamical evolution drives them toward states of high
central density, while simultaneously concentrating the most massive stars and
binary systems in their cores. As a result, these clusters are expected to be
sites of frequent close encounters and physical collisions between stars and
binaries, making them efficient factories for the production of interesting and
observable astrophysical exotica. I describe some elements of the competition
among stellar dynamics, stellar evolution, and other processes that control
globular cluster dynamics, with particular emphasis on pathways that may lead
to the formation of blue stragglers.Comment: Chapter 10, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe