22,290 research outputs found
Nuclear Spirals as Signatures of Supermassive Black Holes
Recent high resolution images of spiral galaxies show wide varieties of
features including nuclear spirals in the central parts. Some of the galaxies
show grand-design nuclear spirals. The morphology of grand-design spirals can
be further divided by the openness of the arms: tightly wound ones with winding
angle of around 3 radian and open spirals with winding angle of around
radian. Based on hydrodynamical simulations, we have investigated the
mechanism responsible for the openness of nuclear spirals. Since the gas flow
in the nuclear region is mainly governed by the central mass concentration near
the nuclei and the sound speed of the gas, we have examined various models with
different mass concentration represented by the mass of the central black hole
and different sound speeds. We found that the tightly wound spirals can be
formed when the mass of the black hole is large enough to remove the
inner-inner Lindblad resonances and sound speeds lie between 15 - 20 km/sec.
Thus, the presence of the tightly wound nuclear spiral could imply the presence
of relatively massive black hole in the center.Comment: 6 pages, 4 figure
Effect of Central Mass Concentration on the Formation of Nuclear Spirals in Barred Galaxies
We have performed smoothed particle hydrodynamics (SPH) simulations to study
the response of the central kiloparsec region of a gaseous disk to the
imposition of nonaxisymmetric bar potentials. The model galaxies are composed
of the three axisymmetric components (halo, disk, and bulge) and a
non-axisymmetric bar. These components are assumed to be invariant in time in
the frame corotating with the bar. The potential of spherical -models
of Dehnen is adopted for the bulge component whose density varies as
near the center and at larger radiiand hence, possesses
a central density core for and cusps for . Since the
central mass concentration of the model galaxies increases with the cusp
parameter , we have examined here the effect of the central mass
concentration by varying the cusp parameter on the mechanism
responsible for the formation of the symmetric two-armed nuclear spirals in
barred galaxies. Our simulations show that the symmetric two-armed nuclear
spirals are formed by hydrodynamic spiral shocks driven by the gravitational
torque of the bar for the models with and 0.5. On the other hand,
the symmetric two-armed nuclear spirals in the models with and 1.5
are explained by gas density waves. Thus, we conclude that the mechanism
responsible for the formation of the symmetric two-armed nuclear spirals in
barred galaxies changes from the hydrodynamic shocks to the gas density waves
when the central mass concentration increases from to 1.5.Comment: 29 pages, 5 figures (Color Figures 3-5), Accepted for Publication in
Astrophysical Journal (ApJ
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
Screen-Printed Flexible Circular and Rectangular Silver Spirals for Planar Electrodynamic Loudspeakers: A Comparative Study of Pressure Frequency Response
We present the fabrication and characterization of flexible planar electrodynamic loudspeakers. Conductive spirals are fabricated on a flexible and transparent polyethylene terephthalate substrate via screen printing. Different geometries (circular and rectangular) and sizes of the conductive spirals are investigated to understand their impact on the
performance. The optimized circular spiral allows achieving an average sound pressure level of 63 dB at 1m distance in 2kHz-20kHz band, proving the suitability of these devices as high-frequency loudspeaker drivers
Oscillations of the Inner Regions of Viscous Accretion Disks
Although quasi-periodic oscillations (QPOs) have been discovered in different
X-ray sources, their origin is still a matter of debate. Analytical studies of
hydrodynamic accretion disks have shown three types of trapped global modes
with properties that appear to agree with the observations. However, these
studies take only linear effects into account and do not address the issues of
mode excitation and decay. Moreover, observations suggest that resonances
between modes play a crucial role. A systematic, numerical study of this
problem is therefore needed. In this paper, we use a pseudo-spectral algorithm
to perform a parameter study of the inner regions of hydrodynamic disks. By
assuming alpha-viscosity, we show that steady state solutions rarely exist. The
inner edges of the disks oscillate and excite axisymmetric waves. In addition,
the flows inside the inner edges are sometimes unstable to non-axisymmetric
perturbations. One-armed, or even two-armed, spirals are developed, which
provides a plausible explanation for the high-frequency QPOs observed from
accreting black holes. When the Reynolds numbers are above certain critical
values, the inner disks go through some transient turbulent states
characterized by strong trailing spirals; while large-scale leading spirals
developed in the outer disks. We compared our numerical results with standard
thin disk oscillation models. Although the non-axisymmetric features have their
analytical counterparts, more careful study is needed to explain the
axisymmetric oscillations.Comment: 12 pages including 7 figures, submitted to ApJ, comments welcome,
movies are available at http://www.cfa.harvard.edu/~ckchan/astrophysics/inne
Formation of Nuclear Spirals in Barred Galaxies
We have performed smoothed particle hydrodynamics (SPH) simulations for the
response of the gaseous disk to the imposed moderately strong non-axisymmetric
potentials. The model galaxies are composed of the three stellar components
(disk, bulge and bar) and two dark ones (supermassive black hole and halo)
whose gravitational potentials are assumed to be invariant in time in the frame
corotating with the bar. We found that the torques alone generated by the
moderately strong bar that gives the maximum of tangential-to-radial force
ratio as are not sufficient to drive the gas
particles close to the center due to the barrier imposed by the inner Lindblad
resonances (ILRs). In order to transport the gas particles towards the nucleus
( pc), a central supermassive black hole (SMBH) and high sound speed of
the gas are required to be present. The former is required to remove the inner
inner Lindblad resonance (IILR) that prevents gas inflow close to the nucleus,
while the latter provides favourable conditions for the gas particles to lose
their angular momentum and to spiral in. Our models that have no IILR show the
trailing nuclear spirals whose innermost parts reach close to the center in a
curling way when the gas sound speed is km s. They
resemble the symmetric two-armed nuclear spirals observed in the central
kiloparsec of spiral galaxies. We found that the symmetric two-armed nuclear
spirals are formed by the hydrodynamic spiral shocks caused by the
gravitational torque of the bar in the presence of a central SMBH that can
remove IILR when the sound speed of gas is high enough to drive a large amount
of gas inflow deep inside the ILR. However, the detailed morphology of nuclear
spirals depends on the sound speed of gas.Comment: 38 pages, 10 figures, accepted for publication in Ap
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