147 research outputs found
Heating and Turbulence Driving by Galaxy Motions in Galaxy Clusters
Using three-dimensional hydrodynamic simulations, we investigate heating and
turbulence driving in an intracluster medium (ICM) by orbital motions of
galaxies in a galaxy cluster. We consider Ng member galaxies on isothermal and
isotropic orbits through an ICM typical of rich clusters. An introduction of
the galaxies immediately produces gravitational wakes, providing perturbations
that can potentially grow via resonant interaction with the background gas.
When Ng^{1/2}Mg_11 < 100, where Mg_11 is each galaxy mass in units of 10^{11}
Msun, the perturbations are in the linear regime and the resonant excitation of
gravity waves is efficient to generate kinetic energy in the ICM, resulting in
the velocity dispersion sigma_v ~ 2.2 Ng^{1/2}Mg_11 km/s. When Ng^{1/2}Mg_11 >
100, on the other hand, nonlinear fluctuations of the background ICM destroy
galaxy wakes and thus render resonant excitation weak or absent. In this case,
the kinetic energy saturates at the level corresponding to sigma_v ~ 220 km/s.
The angle-averaged velocity power spectra of turbulence driven in our models
have slopes in the range of -3.7 to -4.3. With the nonlinear saturation of
resonant excitation, none of the cooling models considered are able to halt
cooling catastrophe, suggesting that the galaxy motions alone are unlikely to
solve the cooling flow problem.Comment: 12 pages including 3 figures, To appear in ApJ
Physical Properties of Tidal Features in Interacting Disk Galaxies
We explore tidal interactions of a galactic disk with Toomre parameter Q ~ 2
embedded in rigid halo/bulge with a point mass companion moving in a prescribed
parabolic orbit. Tidal interactions produce well-defined spiral arms and
extended tidal features such as bridge and tail that are all transient, but
distinct in nature. In the extended disks, strong tidal force is able to lock
the perturbed epicycle phases of the near-side particles to the perturber,
shaping them into a tidal bridge that corotates with the perturber. A tidal
tail develops at the opposite side as strongly-perturbed, near-side particles
overtake mildly-perturbed, far-side particles. The tail is essentially a narrow
material arm with a roughly logarithmic shape, dissolving with time because of
large velocity dispersions. Inside the disks where tidal force is relatively
weak, on the other hand, a two-armed logarithmic spiral pattern emerges due to
the kinematic alignment of perturbed particle orbits. While self-gravity makes
the spiral arms a bit stronger, the arms never become fully self-gravitating,
wind up progressively with time, and decay after the peak almost exponentially
in a time scale of ~ 1 Gyr. The arm pattern speed varying with both radius and
time converges to Omega-kappa/2 at late time, suggesting that the pattern speed
of tidally-driven arms may depend on radius in real galaxies. We present the
parametric dependences of various properties of tidal features on the tidal
strength, and discuss our findings in application to tidal spiral arms in
grand-design spiral galaxies. (Abridged)Comment: 49 pages, 17 figures, 1 table. Accepted for publication in
Astrophysical Journal. PDF version with higher resolution figures is
available at
http://astro.snu.ac.kr/~shoh/research/publications/astroph/Tidally_Induced_Spiral_Structure.pd
Formation and Fragmentation of Gaseous Spurs in Spiral Galaxies
Intermediate-scale spurs are common in spiral galaxies, but perhaps most
distinctively evident in a recent HST image of M51 (Scoville & Rector 2001). We
investigate, using time-dependent numerical MHD simulations, how such spurs
could form (and subsequently fragment) from the interaction of a gaseous ISM
with a stellar spiral arm. We model the gaseous medium as a self-gravitating,
magnetized, differentially-rotating, razor-thin disk. The basic flow shocks and
compresses as it passes through a local segment of a tightly-wound, trailing
stellar spiral arm, modeled as a rigidly-rotating gravitational potential. We
first construct 1D profiles for flows with spiral shocks. When the post-shock
Toomre parameter Q_sp is sufficiently small, self-gravity is too large for
one-dimensional steady solutions to exist. The critical values of Q_sp are 0.8,
0.5, and 0.4 for our models with zero, sub-equipartition, and equipartition
magnetic fields, respectively. We then study the growth of self-gravitating
perturbations in fully 2D flows, and find that spur-like structures rapidly
emerge in our magnetized models. We associate this gravitational instability
with the magneto-Jeans mechanism, in which magnetic tension forces oppose the
Coriolis forces. The shearing and expanding velocity field shapes the condensed
material into spurs as it flows downstream from the arms. Although we find
swing amplification can help form spurs when the arm-interarm contrast is
moderate, unmagnetized systems that are quasi-axisymmetrically stable are
generally also stable to nonaxisymmetric perturbations, suggesting that
magnetic effects are essential. In nonlinear stages, the spurs in our models
undergo fragmentation to form 4\times 10^6 solar mass clumps, which we suggest
could evolve into bright arm/interarm HII regions as seen in spiral galaxies.Comment: 32 pages, 14 figures, Accepted for publication in ApJ; better
postscript figures available from http://www.astro.umd.edu/~kimwt/FIGURE2/ ;
for associated Animated GIF movies, see
http://www.astro.umd.edu/~kimwt/MOVIES
Magnetorotationally-Driven Galactic Turbulence and the Formation of Giant Molecular Clouds
Using local 3D MHD simulations, we investigate ways in which galactic
turbulence associated with the magnetorotational instability (MRI) may
influence the formation and properties of GMCs. Our disk models are vertically
stratified and subject to uniform shear. Initial magnetic fields are weak and
purely vertical. For simplicity, we adopt an isothermal equation of state. We
find that MRI-driven turbulence develops rapidly, with the saturated-state
Shakura & Sunyaev parameter alpha~(0.15-0.3) dominated by Maxwell stresses.
Many of the dimensionless characteristics of the turbulence (e.g. the ratio of
the Maxwell to Reynolds stresses) are similar to results from previous MRI
studies of accretion disks, hence insensitive to the degree of vertical disk
compression, shear rate, and the presence of self-gravity. The density-weighted
velocity dispersions in non- or weakly self-gravitating disks are sigma_x ~
sigma_y ~ (0.4-0.6)c_s and sigma_z~(0.2-0.3)c_s, suggesting that MRI can
contribute significantly to the observed level of galactic turbulence. The
saturated-state magnetic field strength B ~ 2 \mu G is similar to typical
galactic values. When self-gravity is strong enough, MRI-driven high-amplitude
density perturbations are swing-amplified to form Jeans-mass (~10^7 Msun) bound
clouds. Compared to previous unmagnetized or strongly-magnetized disk models,
the threshold for nonlinear instability in the present models occurs for
surface densities at least 50% lower, corresponding to the Toomre parameter
Q_th~1.6. We present evidence that self-gravitating clouds like GMCs formed
under conditions similar to our models can lose much of their original spin
angular momenta by magnetic braking, preferentially via fields threading
near-perpendicularly to their spin axes.Comment: 39 pages, 8 figures, to appear in ApJ, vol. 599, Dec. 20, 2003; For
better postscript figures and mpeg animations, see
http://cfa-www.harvard.edu/~wkim/FIGURE4
OGLE-2016-BLG-1227L: A Wide-separation Planet from a Very Short-timescale Microlensing Event
We present the analysis of the microlensing event OGLE-2016-BLG-1227. The light curve of this short-duration event appears to be a single-lens event affected by severe finite-source effects. Analysis of the light curve based on single-lens single-source (1L1S) modeling yields very small values of the event timescale, t_E ⌠3.5 days, and the angular Einstein radius, Ξ_E ⌠0.009 mas, making the lens a candidate of a free-floating planet. Close inspection reveals that the 1L1S solution leaves small residuals with amplitude ÎI âČ 0.03 mag. We find that the residuals are explained by the existence of an additional widely-separated heavier lens component, indicating that the lens is a wide-separation planetary system rather than a free-floating planet. From Bayesian analysis, it is estimated that the planet has a mass of _p = 0.79^(+1.30)_(â0.39) M_J and it is orbiting a low-mass host star with a mass of M_(host) = 0.10+0.17â0.05 M_â located with a projected separation of a_ = 3.4^(+2.1)_(â1.0) au. The planetary system is located in the Galactic bulge with a line-of-sight separation from the source star of D_(LS) = 1.21^(+0.96)_(â0.63) kpc. The event shows that there are a range of deviations in the signatures of host stars for apparently isolated planetary lensing events and that it is possible to identify a host even when a deviation is subtle
Three-Dimensional Simulations of Parker, Magneto-Jeans, and Swing Instabilities in Shearing Galactic Gas Disks
We use 3D MHD simulations to investigate nonlinear development of the Parker,
magneto-Jeans (MJI), and swing mechanisms in galactic disks. The model disks
are local, isothermal, and begin from a vertically-stratified equilibrium. We
first construct axisymmetric equilibria and examine their stability. Finite
disk thickness reduces the critical Toomre Q parameter below unity; we find Q_c
\~ 0.75, 0.72, and 0.57 for \beta=\infty, 10, and 1 cases, respectively. We
then pursue fully 3D models. In non-self-gravitating cases, the peak density
enhancement from the `pure' Parker instability is less a factor of two. The
dominant growing modes have radial wavelengths comparable to the disk scale
height H, much shorter than the azimuthal wavelength (~10-20 H). Shearing
disks, being more favorable to midplane-symmetric modes, have somewhat
different late-time magnetic field profiles from nonshearing disks, but
otherwise saturated states are similar. Late-time velocity fluctuations at 10%
of the sound speed persist, but no characteristic structural signatures of
Parker modes remain in the new equilibria. In self-gravitating cases, the
development of density structure is qualitatively similar to our previous
results from thin-disk simulations. The Parker instability, although it may
help seed structure or tip the balance under marginal conditions, appears to
play a secondary role. In shearing disks with Q less than a threshold level ~
1, swing amplification can produce bound clouds of a few times the local Jeans
mass. The most powerful cloud-condensing mechanism appears to be the MJI. Our
simulations show that condensations of a local Jeans mass (~3\times 10^7 M_sun)
grow very rapidly, supporting the idea that MJI is at least partly responsible
for the formation of bound cloud complexes in spiral galaxies.Comment: 36 pages, 15 figures, Accepted for publication in ApJ; better
postscript figures available from http://www.astro.umd.edu/~kimwt/FIGURE3/ ;
for associated Animated GIF movies, see
http://www.astro.umd.edu/~kimwt/MOVIES/3DLoca
Gravitational Runaway and Turbulence Driving in Star-Gas Galactic Disks
Galactic disks consist of both stars and gas. The gas is more dynamically responsive than the stars, and strongly nonlinear structures and velocities can develop in the ISM even while stellar surface density perturbations remain fractionally small. We use 2D numerical simulations to explore formation of bound clouds and turbulence generation in the gas of two-component galactic disks. We represent the stars with collisionless particles and follow their orbits using a PM method, and treat the gas as an isothermal, unmagnetized fluid. The two components interact through a combined gravity. Using stellar parameters typical of mid-disk conditions, we find that models with gaseous Toomre parameter Q_g 1-2. The bound gaseous clouds that form have mass 6x10^7 Msun each; these represent superclouds that would subsequently fragment into GMCs. Self-gravity and sheared rotation also interact to drive turbulence in the gas when Q_g > Q_c. This turbulence is anisotropic, with more power in sheared than compressive motions. The gaseous velocity dispersion is ~ 0.6 times the thermal speed when Q_g ~ Q_c. This suggests that gravity is important in driving ISM turbulence in many spiral galaxies, since the low efficiency of star formation naturally leads to a state of marginal instability
Spectroscopic Mass and Host-star Metallicity Measurements for Newly Discovered Microlensing Planet OGLE-2018-BLG-0740Lb
We report the discovery of the microlensing planet OGLE-2018-BLG-0740Lb. The
planet is detected with a very strong signal of , but
the interpretation of the signal suffers from two types of degeneracies. One
type is caused by the previously known close/wide degeneracy, and the other is
caused by an ambiguity between two solutions, in which one solution requires to
incorporate finite-source effects, while the other solution is consistent with
a point-source interpretation. Although difficult to be firmly resolved based
on only the photometric data, the degeneracy is resolved in strong favor of the
point-source solution with the additional external information obtained from
astrometric and spectroscopic observations. The small astrometric offset
between the source and baseline object supports that the blend is the lens and
this interpretation is further secured by the consistency of the spectroscopic
distance estimate of the blend with the lensing parameters of the point-source
solution. The estimated mass of the host is and the mass
of the planet is (close solution) or (wide solution) and the lens is located at a distance of ~kpc.
The bright nature of the lens, with (), combined with
its dominance of the observed flux suggest that radial-velocity (RV) follow-up
observations of the lens can be done using high-resolution spectrometers
mounted on large telescopes, e.g., VLT/ESPRESSO, and this can potentially not
only measure the period and eccentricity of the planet but also probe for
close-in planets. We estimate that the expected RV amplitude would be .Comment: 12 pages, 11 figures, 4 table
Developing indicators of pattern identification in patients with stroke using traditional Korean medicine
Abstract Background The traditional Korean medical diagnoses employ pattern identification (PI), a diagnostic system that entails the comprehensive analysis of symptoms and signs. The PI needs to be standardized due to its ambiguity. Therefore, this study was performed to establish standard indicators of the PI for stroke through the traditional Korean medical literature, expert consensus and a clinical field test. Methods We sorted out stroke patterns with an expert committee organized by the Korean Institute of Oriental Medicine. The expert committee composed a document for a standardized pattern of identification for stroke based on the traditional Korean medical literature, and we evaluated the clinical significance of the document through a field test. Results We established five stroke patterns from the traditional Korean medical literature and extracted 117 indicators required for diagnosis. The indicators were evaluated by a field test and verified by the expert committee. Conclusions This study sought to develop indicators of PI based on the traditional Korean medical literature. This process contributed to the standardization of traditional Korean medical diagnoses.</p
OGLE-2018-BLG-0022: First Prediction of an Astrometric Microlensing Signal from a Photometric Microlensing Event
In this work, we present the analysis of the binary microlensing event
OGLE-2018-BLG-0022 that is detected toward the Galactic bulge field. The dense
and continuous coverage with the high-quality photometry data from ground-based
observations combined with the space-based {\it Spitzer} observations of this
long time-scale event enables us to uniquely determine the masses and of the individual lens components.
Because the lens-source relative parallax and the vector lens-source relative
proper motion are unambiguously determined, we can likewise unambiguously
predict the astrometric offset between the light centroid of the magnified
images (as observed by the {\it Gaia} satellite) and the true position of the
source. This prediction can be tested when the individual-epoch {\it Gaia}
astrometric measurements are released.Comment: 10 pages, 10 figures, 4 table
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