278 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
Supersonic Collisions between Two Gas Streams
A star around a massive black hole can be disrupted tidally by the gravity of
the black hole. Then, its debris may form a precessing stream which may even
collide with itself. In order to understand the dynamical effects of the
stream-stream collision on the eventual accretion of the stellar debris onto
the black hole, we have studied how gas flow behaves when the outgoing stream
collides supersonically with the incoming stream. We have investigated the
problem analytically with one-dimensional plane-parallel streams and
numerically with more realistic three-dimensional streams. A shock formed
around the contact surface converts the bulk of the orbital streaming kinetic
energy into thermal energy. In three-dimensional simulations, the accumulated
hot post-shock gas then expands adiabatically and drives another shock into the
low density ambient region. Through this expansion, thermal energy is converted
back to the kinetic energy associated with the expanding motion. Thus, in the
end, only a small fraction of the orbital kinetic energy is actually converted
to the thermal energy, while most of it is transferred to the kinetic energy of
the expanding gas. Nevertheless the collision is effective in circularizing the
debris orbit, because the shock efficiently transforms the ordered motion of
the streams into the expanding motion in directions perpendicular to the
streams. The circularization efficiency decreases, if two colliding streams
have a large ratio of cross sections and a large density contrast. But even in
such cases, the main shock extends beyond the overlapping contact surface and
the high pressure region behind the shock keeps the stream of the larger cross
section from passing freely. Thus the stream-stream collisions are still
expected to circularize the stellar debris rather efficiently, unless the ratioComment: ApJ accepted, 14 pages with 9 figures, uuencoded, gzipped, tarred
postscript files, or available upon request to [email protected]
P-Cygni Type Lya from Starburst Galaxies
P-Cygni type Lya profiles exhibited in nearly half of starburst galaxies,
both nearby and high-z, are believed to be formed by an expanding supershell
surrounding a star-forming region. We apply the Monte Carlo code which was
developed previously for static and plane-parallel medium to calculate the Lya
line transfer in a supershell of neutral hydrogen which are expanding radially
in a spherical bulk flow. We consider typical cases that the supershell has the
Lya line-centre optical depth of , a radial expansion
velocity of \tau_0$ and V_exp of the
supershell. We discuss the effects of dust extinction and the implication of
our works in relation to recent spectroscopic observations of starburst
galaxies.Comment: 15 pages, 6 figures, submitted to MNRA
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