1,292 research outputs found
Equilibrium Sequences and Gravitational Instability of Rotating Isothermal Rings
Nuclear rings at centers of barred galaxies exhibit strong star formation
activities. They are thought to undergo gravitational instability when
sufficiently massive. We approximate them as rigidly-rotating isothermal
objects and investigate their gravitational instability. Using a
self-consistent field method, we first construct their equilibrium sequences
specified by two parameters: alpha corresponding to the thermal energy relative
to gravitational potential energy, and R_B measuring the ellipticity or ring
thickness. Unlike in the incompressible case, not all values of R_B yield an
isothermal equilibrium, and the range of R_B for such equilibria shrinks with
decreasing alpha. The density distributions in the meridional plane are steeper
for smaller alpha, and well approximated by those of infinite cylinders for
slender rings. We also calculate the dispersion relations of nonaxisymmetric
modes in rigidly-rotating slender rings with angular frequency Omega_0 and
central density rho_max. Rings with smaller alpha are found more unstable with
a larger unstable range of the azimuthal mode number. The instability is
completely suppressed by rotation when Omega_0 exceeds the critical value. The
critical angular frequency is found to be almost constant at ~ 0.7
sqrt(G*rho_c) for alpha > 0.01 and increases rapidly for smaller alpha. We
apply our results to a sample of observed star-forming rings and confirm that
rings without a noticeable azimuthal age gradient of young star clusters are
indeed gravitationally unstable.Comment: 17 figures and 2 tables; Accepted for publication in the Ap
Hydrodynamical Simulations of Nuclear Rings in Barred Galaxies
Dust lanes, nuclear rings, and nuclear spirals are typical gas structures in
the inner region of barred galaxies. Their shapes and properties are linked to
the physical parameters of the host galaxy. We use high-resolution
hydrodynamical simulations to study 2D gas flows in simple barred galaxy
models. The nuclear rings formed in our simulations can be divided into two
groups: one group is nearly round and the other is highly elongated. We find
that roundish rings may not form when the bar pattern speed is too high or the
bulge central density is too low. We also study the periodic orbits in our
galaxy models, and find that the concept of inner Lindblad resonance (ILR) may
be generalized by the extent of orbits. All roundish nuclear rings in our
simulations settle in the range of orbits (or ILRs). However, knowing the
resonances is insufficient to pin down the exact location of these nuclear
rings. We suggest that the backbone of round nuclear rings is the orbital
family, i.e. round nuclear rings are allowed only in the radial range of
orbits. A round nuclear ring forms exactly at the radius where the residual
angular momentum of infalling gas balances the centrifugal force, which can be
described by a parameter measured from the rotation curve. The
gravitational torque on gas in high pattern speed models is larger, leading to
a smaller ring size than in the low pattern speed models. Our result may have
important implications for using nuclear rings to measure the parameters of
real barred galaxies with 2D gas kinematics.Comment: ApJ accepted version; we expanded the discussion of the limitations
of this work in Section 4.7, and included a new subsection (Section 4.8) to
demonstrate the convergence test for the resolution effects; 15 pages;
emulateapj format. A movie showing the gas evolution in the canonical model
is available on the ApJ website and at
http://hubble.shao.ac.cn/~shen/nuclear_rings/canonicalmodel2.gi
Three Dimensional Hydrodynamic Simulations of Multiphase Galactic Disks with Star Formation Feedback: II. Synthetic HI 21 cm Line Observations
We use three-dimensional numerical hydrodynamic simulations of the turbulent,
multiphase atomic interstellar medium (ISM) to construct and analyze synthetic
HI 21 cm emission and absorption lines. Our analysis provides detailed tests of
21 cm observables as physical diagnostics of the atomic ISM. In particular, we
construct (1) the "observed" spin temperature, , and its optical-depth weighted mean
T_s,obs; (2) the absorption-corrected "observed" column density,
; and (3) the "observed"
fraction of cold neutral medium (CNM), for T_c
the CNM temperature; we compare each observed parameter with true values
obtained from line-of-sight (LOS) averages in the simulation. Within individual
velocity channels, T_s,obs(v_ch) is within a factor 1.5 of the true value up to
. As a consequence, N_H,obs and T_s,obs are
respectively within 5% and 12% of the true values for 90% and 99% of LOSs. The
optically thin approximation significantly underestimates N_H for .
Provided that T_c is constrained, an accurate observational estimate of the CNM
mass fraction can be obtained down to 20%. We show that T_s,obs cannot be used
to distinguish the relative proportions of warm and thermally-unstable atomic
gas, although the presence of thermally-unstable gas can be discerned from 21
cm lines with 200K<<1000K. Our mock observations
successfully reproduce and explain the observed distribution of the brightness
temperature, optical depth, and spin temperature in Roy et al. (2013a). The
threshold column density for CNM seen in observations is also reproduced by our
mock observations. We explain this observed threshold behavior in terms of
vertical equilibrium in the local Milky Way's ISM disk.Comment: 34 pages, 12 figures. Accepted for publication in ApJ. For Paper I,
see http://arxiv.org/abs/1308.323
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