8,984 research outputs found
3D MHD Modeling of the Gaseous Structure of the Galaxy: Synthetic Observations
We generated synthetic observations from the four-arm model presented in
Gomez & Cox (2004) for the Galactic ISM in the presence of a spiral
gravitational perturbation. We found that velocity crowding and diffusion have
a strong effect in the l-v diagram. The v-b diagram presents structures at the
expected spiral arm velocities, that can be explained by the off-the-plane
structure of the arms presented in previous papers of this series. Such
structures are observed in the Leiden/Dwingeloo HI survey. The rotation curve,
as measured from the inside of the modeled galaxy, shows similarities with the
observed one for the Milky Way Galaxy, although it has large deviations from
the smooth circular rotation corresponding to the background potential. The
magnetic field inferred from a synthetic synchrotron map shows a largely
circular structure, but with interesting deviations in the midplane due to
distortion of the field from circularity in the interarm regions.Comment: Accepted for publication in ApJ. Better quality figures in
http://www.astro.umd.edu/~gomez/publica/3d_galaxy-3.pd
Dynamical Friction of a Circular-Orbit Perturber in a Gaseous Medium
We investigate the gravitational wake due to, and dynamical friction on, a
perturber moving on a circular orbit in a uniform gaseous medium using a
semi-analytic method. This work is a straightforward extension of Ostriker
(1999) who studied the case of a straight-line trajectory. The circular orbit
causes the bending of the wake in the background medium along the orbit,
forming a long trailing tail. The wake distribution is thus asymmetric, giving
rise to the drag forces in both opposite (azimuthal) and lateral (radial)
directions to the motion of the perturber, although the latter does not
contribute to orbital decay much. For subsonic motion, the density wake with a
weak tail is simply a curved version of that in Ostriker and does not exhibit
the front-back symmetry. The resulting drag force in the opposite direction is
remarkably similar to the finite-time, linear-trajectory counterpart. On the
other hand, a supersonic perturber is able to overtake its own wake, possibly
multiple times, and develops a very pronounced tail. The supersonic tail
surrounds the perturber in a trailing spiral fashion, enhancing the perturbed
density at the back as well as far front of the perturber. We provide the
fitting formulae for the drag forces as functions of the Mach number, whose
azimuthal part is surprisingly in good agreement with the Ostriker's formula,
provided Vp t=2 Rp, where Vp and Rp are the velocity and orbital radius of the
perturber, respectively.Comment: 28 pages, 9 figures, accepted for publication in Astrophysical
Journa
Online discussion compensates for suboptimal timing of supportive information presentation in a digitally supported learning environment
This study used a sequential set-up to investigate the consecutive effects of timing of supportive information presentation (information before vs. information during the learning task clusters) in interactive digital learning materials (IDLMs) and type of collaboration (personal discussion vs. online discussion) in computer-supported collaborative learning (CSCL) on student knowledge construction. Students (N = 87) were first randomly assigned to the two information presentation conditions to work individually on a case-based assignment in IDLM. Students who received information during learning task clusters tended to show better results on knowledge construction than those who received information only before each cluster. The students within the two separate information presentation conditions were then randomly assigned to pairs to discuss the outcomes of their assignments under either the personal discussion or online discussion condition in CSCL. When supportive information had been presented before each learning task cluster, online discussion led to better results than personal discussion. When supportive information had been presented during the learning task clusters, however, the online and personal discussion conditions had no differential effect on knowledge construction. Online discussion in CSCL appeared to compensate for suboptimal timing of presentation of supportive information before the learning task clusters in IDLM
On the size and shape of excluded volume polymers confined between parallel plates
A number of recent experiments have provided detailed observations of the
configurations of long DNA strands under nano-to-micrometer sized confinement.
We therefore revisit the problem of an excluded volume polymer chain confined
between two parallel plates with varying plate separation. We show that the
non-monotonic behavior of the overall size of the chain as a function of
plate-separation, seen in computer simulations and reproduced by earlier
theories, can already be predicted on the basis of scaling arguments. However,
the behavior of the size in a plane parallel to the plates, a quantity observed
in recent experiments, is predicted to be monotonic, in contrast to the
experimental findings. We analyze this problem in depth with a mean-field
approach that maps the confined polymer onto an anisotropic Gaussian chain,
which allows the size of the polymer to be determined separately in the
confined and unconfined directions. The theory allows the analytical
construction of a smooth cross-over between the small plate-separation de
Gennes regime and the large plate-separation Flory regime. The results show
good agreement with Langevin dynamics simulations, and confirm the scaling
predictions.Comment: 15 pages, 3 figure
Trading interactions for topology in scale-free networks
Scale-free networks with topology-dependent interactions are studied. It is
shown that the universality classes of critical behavior, which conventionally
depend only on topology, can also be explored by tuning the interactions. A
mapping, , describes how a shift of the
standard exponent of the degree distribution can absorb the
effect of degree-dependent pair interactions .
Replica technique, cavity method and Monte Carlo simulation support the
physical picture suggested by Landau theory for the critical exponents and by
the Bethe-Peierls approximation for the critical temperature. The equivalence
of topology and interaction holds for equilibrium and non-equilibrium systems,
and is illustrated with interdisciplinary applications.Comment: 4 pages, 5 figure
Optimization of a charge-state analyzer for ECRIS beams
A detailed experimental and simulation study of the extraction of a 24 keV
He-ion beam from an ECR ion source and the subsequent beam transport through an
analyzing magnet is presented. We find that such a slow ion beam is very
sensitive to space-charge forces, but also that the neutralization of the
beam's space charge by secondary electrons is virtually complete for beam
currents up to at least 0.5 mA. The beam emittance directly behind the
extraction system is 65 pi mm mrad and is determined by the fact that the ion
beam is extracted in the strong magnetic fringe field of the ion source. The
relatively large emittance of the beam and its non-paraxiality lead, in
combination with a relatively small magnet gap, to significant beam losses and
a five-fold increase of the effective beam emittance during its transport
through the analyzing magnet. The calculated beam profile and phase-space
distributions in the image plane of the analyzing magnet agree well with
measurements. The kinematic and magnet aberrations have been studied using the
calculated second-order transfer map of the analyzing magnet, with which we can
reproduce the phase-space distributions of the ion beam behind the analyzing
magnet. Using the transfer map and trajectory calculations we have worked out
an aberration compensation scheme based on the addition of compensating
hexapole components to the main dipole field by modifying the shape of the
poles. The simulations predict that by compensating the kinematic and geometric
aberrations in this way and enlarging the pole gap the overall beam transport
efficiency can be increased from 16 to 45%
Density Waves Inside Inner Lindblad Resonance: Nuclear Spirals in Disk Galaxies
We analyze formation of grand-design two-arm spiral structure in the nuclear
regions of disk galaxies. Such morphology has been recently detected in a
number of objects using high-resolution near-infrared observations. Motivated
by the observed (1) continuity between the nuclear and kpc-scale spiral
structures, and by (2) low arm-interarm contrast, we apply the density wave
theory to explain the basic properties of the spiral nuclear morphology. In
particular, we address the mechanism for the formation, maintenance and the
detailed shape of nuclear spirals. We find, that the latter depends mostly on
the shape of the underlying gravitational potential and the sound speed in the
gas. Detection of nuclear spiral arms provides diagnostics of mass distribution
within the central kpc of disk galaxies. Our results are supported by 2D
numerical simulations of gas response to the background gravitational potential
of a barred stellar disk. We investigate the parameter space allowed for the
formation of nuclear spirals using a new method for constructing a
gravitational potential in a barred galaxy, where positions of resonances are
prescribed.Comment: 18 pages, 9 figures, higher resolution available at
http://www.pa.uky.edu/~ppe/papers/nucsp.ps.g
Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)
M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging
Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m
telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along
with spectra from the International Ultraviolet Explorer and Lick 1-m
telescopes. The wide-field UIT image shows FUV emission from (a) an elongated
nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c)
a bright inner ring of H II regions at the perimeter of the inner disk (R = 48
arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the
ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc).
The HST/FOC image resolves the NUV emission from the nuclear region into a
bright core and a faint 20 arcsec. long ``mini-bar'' at a position angle of 30
deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk
indicates an approximately 10^7 or 10^8 yr-old stellar population from
low-level starbirth activity blended with some LINER activity. Analysis of the
H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers
of stars and gas in M94 indicates that most of the star formation is being
orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring,
oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at
intermediate radius, in particular, argue for bar-mediated resonances as the
primary drivers of evolution in M94 at the present epoch. Similar processes may
be governing the evolution of the ``core-dominated'' galaxies that have been
observed at high redshift. The gravitationally-lensed ``Pretzel Galaxy''
(0024+1654) at a redshift of approximately 1.5 provides an important precedent
in this regard.Comment: revised figure 1 (corrected coordinate labels on declination axis);
19 pages of text + 19 figures (jpg files); accepted for publication in A
Constraining the Collisional Nature of the Dark Matter Through Observations of Gravitational Wakes
We propose to use gravitational wakes as a direct observational probe of the
collisional nature of the dark matter. We calculate analytically the structure
of a wake generated by the motion of a galaxy in the core of an X-ray cluster
for dark matter in the highly-collisional and collisionless limits. We show
that the difference between these limits can be recovered from detailed X-ray
or weak lensing observations. We also discuss the sizes of sub-halos in these
limits. Preliminary X-ray data on the motion of NGC 1404 through the Fornax
group disfavors fluid-like dark matter but does not exclude scenarios in which
the dark matter is weakly collisional.Comment: 29 pages, 3 figures, submitted to Ap
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