381 research outputs found
The Stellar and Gas Kinematics of Several Irregular Galaxies
We present long-slit spectra of three irregular galaxies from which we
determinethe stellar kinematics in two of the galaxies (NGC 1156 and NGC 4449)
and ionized-gas kinematics in all three (including NGC 2366). We compare this
to the optical morphology and to the HI kinematics of the galaxies. In the
ionized gas, we see a linear velocity gradient in all three galaxies. In NGC
1156 we also detect a weak linear velocity gradient in the stars of (5+/-1/sin
i) km/s/kpc to a radius of 1.6 kpc. The stars and gas are rotating about the
same axis, but this is different from the major axis of the stellar bar which
dominates the optical light of the galaxy. In NGC 4449 we do not detect
organized rotation of the stars and place an upper limit of (3/sin i) km/s/kpc
to a radius of 1.2 kpc. For NGC 4449, which has signs of a past interaction
with another galaxy, we develop a model to fit the observed kinematics of the
stars and gas. In this model the stellar component is in a rotating disk seen
nearly face-on while the gas is in a tilted disk with orbits whose planes
precess in the gravitational potential. This model reproduces the apparent
counter-rotation of the inner gas of the galaxy. The peculiar orbits of the gas
are presumed due to acquisition of gas in the past interaction.Comment: To be published in ApJ, November 20, 200
Circumstellar and Circumbinary Disks in Eccentric Stellar Binaries
We explore test particle orbits in the orbital plane of eccentric stellar
binary systems, searching for ``invariant loops'': closed curves that change
shape periodically as a function of binary orbital phase as the test particles
in them move under the stars' gravity. Stable invariant loops play the same
role in this periodically-varying potential as stable periodic orbits do in
stationary potentials; in particular, when dissipation is weak, gas will most
likely follow the non-intersecting loops, while nearby particle orbits librate
around them. We use this method to set bounds on the sizes of disks around the
stars, and on the gap between those and the inner edge of a possible
circumbinary disk. Gas dynamics may impose further restrictions, but our study
sets upper bounds for the size of circumstellar disks, and a lower bound for
the inner radius of a circumbinary disk. We find that circumstellar disks are
sharply reduced as the binary's eccentricity grows. Disks change in size and
shape only marginally with the binary phase, with no strong preference to
increase or decrease at any particular phase. The circumstellar disks in
particular can be quite asymmetric. We compare our results with other numerical
and theoretical results and with observations of the Centauri and
L1551 systems, finding very good agreement. The calculated changes in the
shapes and crowding of the circumstellar orbits can be used to predict how the
disk luminosity and mass inflow should vary with binary phase.Comment: 11 pages, submitted to MNRA
Geometrical and physical properties of circumbinary discs in eccentric stellar binaries
In a previous work, we studied stable configurations for circumstellar discs in eccentric binary systems. We searched for âinvariant loops': closed curves (analogous to stable periodic orbits in time-independent potentials) that change shape with the binary orbital phase, as test particles in them move under the influence of the binary potential. This approach allows us to identify stable configurations when pressure forces are unimportant, and dissipation acts only to prevent gas clouds from colliding with one another. We now extend this work to study the main geometrical properties of circumbinary discs. We have studied more than 100 cases with a range in eccentricity 0 â€e†0.9 and mass ratio 0.1 â€q†0.9. Although gas dynamics may impose further restrictions, our study sets lower stable bounds for the size of the central hole in a simple and computationally cheap way, with a relation that depends on the eccentricity and mass ratio of the central binary. We extend our previous studies and focus on an important component of these systems: circumbinary discs. The radii for stable orbits that can host gas in circumbinary discs are sharply constrained as a function of the binary's eccentricity. The circumbinary disc configurations are almost circular, with eccentricity ed < 0.15, but if the mass ratio is unequal the disc is offset from the centre of mass of the system. We compare our results with other models, and with observations of specific systems like GG Tauri A, UY Aurigae, HD 98800 B, and Fomalhaut, restricting the plausible parameters for the binar
Kinematically detected polar rings/disks in blue compact dwarf galaxies
Polar ring galaxies are systems with nearly orthogonally rotated components.
We have found the gas on polar (or strongly inclined) orbits in two BCD
galaxies using ionized gas velocity fields taken with a Fabry-Perot
interferometer of the SAO RAS 6-m telescope. Our analysis shows that all
ionized gas in Mrk 33 is concentrated in a compact disk (3 kpc in diameter)
which rotates in the polar plane relative to the main stellar body. The gaseous
disk in Mrk 370 has a more complex structure with a heavily warped innermost
part. The presence of polar gaseous structures supports an idea that current
the burst of star formation in these galaxies is due to the external gas
accretion or merging. A possible fraction of polar structures among BCD
galaxies seems to be very large (up to 10-15%)Comment: to appear in the proceedings of the conference "A Universe of dwarf
galaxies" (Lyon, June 14-18, 2010
Self-gravitating warped discs around supermassive black holes
We consider warped equilibrium configurations for stellar and gaseous disks
in the Keplerian force-field of a supermassive black hole, assuming that the
self-gravity of the disk provides the only acting torques. Modeling the disk as
a collection of concentric circular rings, and computing the torques in the
non-linear regime, we show that stable, strongly warped precessing equilibria
are possible. These solutions exist for a wide range of disk-to-black hole mass
ratios , can span large warp angles of up to ,
have inner and outer boundaries, and extend over a radial range of a factor of
typically two to four. These equilibrium configurations obey a scaling relation
such that in good approximation \phidot/\Omega\propto M_d/M_{bh} where
\phidot is the (retrograde) precession frequency and is a
characteristic orbital frequency in the disk. Stability was determined using
linear perturbation theory and, in a few cases, confirmed by numerical
integration of the equations of motion. Most of the precessing equilibria are
found to be stable, but some are unstable. The main result of this study is
that highly warped disks near black holes can persist for long times without
any persistent forcing other than by their self-gravity. The possible relevance
of this to galactic nuclei is briefly discussed.Comment: 13 pages, 21 figures, published in MNRA
Evidence for coupling between the Sagittarius dwarf galaxy and the Milky Way warp
Using recent determinations of the mass and orbit of Sagittarius, I calculate
its orbital angular momentum. From the latest observational data, I also
calculate the angular momentum of the Milky Way's warp. I find that both
angular momenta are directed toward l=270, b=0, and have magnitude 2-8x10^12
M_Sun kpc km s^-1, where the range in both cases reflects uncertainty in the
mass. The coincidence of the angular momenta is suggestive of a coupling
between these systems. Direct gravitational torque of Sgr on the disk is ruled
out as the coupling mechanism. Gravitational torque due to a wake in the halo
and the impulsive deposition of momentum by a passage of Sgr through the disk
are still both viable mechanisms pending better simulations to test their
predictions on the observed Sgr-MW system.Comment: 11 pages, to appear in the February 1 issue of ApJ
The stellar velocity dispersion in the inner 1.3 disk scale-lengths of the irregular galaxy NGC 4449
We present measurements of the stellar velocity dispersion in the inner 1
arcmin radius (1.3 disk scale-lengths) of the irregular galaxy NGC 4449
determined from long-slit absorption-line spectra. The average observed
dispersion is 29 +/-2 km/s, the same as predicted from NGC 4449's luminosity.
No significant rotation in the stars is detected. If we assume a maximum
rotation speed of the stars from the model determined from the gas kinematics
of Hunter et al. (2002), the ratio V_max/sigma_z measured globally is 3. This
ratio is comparable to values measured in spiral galaxies, and implies that the
stellar disk in NGC 4449 is kinematically relatively cold. The intrinsic
minor-to-major axis ratio (b/a)_0 is predicted to be in the range 0.3-0.6,
similar to values derived from the distribution of observed b/a of Im galaxies.
However, V/sigma_z measured locally is 0.5-1.1, and so the circular velocity of
NGC 4449 is comparable or less than the velocity of the stars within the
central 1.3 disk scale-lengths of the galaxy.Comment: To be published in ApJ, Nov 200
First Interferometric Observations of Molecular Gas in a Polar Ring: The Helix Galaxy NGC 2685
We have detected four Giant Molecular cloud Associations (GMAs) (sizes <
6.6'' ~ 430 pc) in the western and eastern region of the polar ring in NGC2685
(the Helix galaxy) using the Owens Valley Radio Observatory (OVRO) millimeter
interferometer. Emission from molecular gas is found close to the brightest
Halpha and HI peaks in the polar ring and is confirmed by new IRAM 30m single
dish observations. The CO and HI line velocities are very similar, providing
additional kinematic confirmation that the CO emission emerges from the polar
ring. For the first time, the total molecular mass within a polar ring is
determined (M_H2~(8-11)x10^6 M_sol, using the standard Galactic conversion
factor). We detect about M_H2~4.4x10^6 M_sol in the nuclear region with the
single dish. Our upper limit derived from the interferometric data is lower
(M_H2<0.7x10^6 M_sol) suggesting that the molecular gas is distributed in an
extended (< 1.3 kpc) diffuse disk. These new values are an order of magnitude
lower than in previous reports. The total amount of molecular gas and the
atomic gas content of the polar ring are consistent with formation due to
accretion of a small gas-rich object, such as a dwarf irregular. The properties
of the NGC2685 system suggest that the polar ring and the host galaxy have been
in a stable configuration for a considerable time (few Gyr). The second (outer)
HI ring within the disk of NGC2685 is very likely at the outer Lindblad
resonance (OLR) of the ~ 11 kpc long stellar bar.Comment: 8 pages, 4 figures, accepted by ApJ Letter
The Destruction of Bars by Central Mass Concentrations
More than two thirds of disk galaxies are barred to some degree. Many today
harbor massive concentrations of gas in their centers, and some are known to
possess supermassive black holes (SMBHs) and their associated stellar cusps.
Previous theoretical work has suggested that a bar in a galaxy could be
dissolved by the formation of a mass concentration in the center, although the
precise mass and degree of central concentration required is not
well-established. We report an extensive study of the effects of central masses
on bars in high-quality N-body simulations of galaxies. We have varied the
growth rate of the central mass, its final mass and degree of concentration to
examine how these factors affect the evolution of the bar. Our main conclusions
are: (1) Bars are more robust than previously thought. The central mass has to
be as large as several percent of the disk mass to completely destroy the bar
on a short timescale. (2) For a given mass, dense objects cause the greatest
reduction in bar amplitude, while significantly more diffuse objects have a
lesser effect. (3) The bar amplitude always decreases as the central mass is
grown, and continues to decay thereafter on a cosmological time-scale. (4) The
first phase of bar-weakening is due to the destruction by the CMC of
lower-energy, bar-supporting orbits, while the second phase is a consequence of
secular changes to the global potential which further diminish the number of
bar-supporting orbits. We provide detailed phase-space and orbit analysis to
support this suggestion. Thus current masses of SMBHs are probably too small,
even when dressed with a stellar cusp, to affect the bar in their host
galaxies. The molecular gas concentrations found in some barred galaxies are
also too diffuse to affect the amplitude of the bar significantly.Comment: AASTeX v5.0 preprint; 44 pages, including 1 table and 16 figures. To
appear in ApJ. High resolution version can be found at
http://www.physics.rutgers.edu/~shen/bar_destruct/paper_high_res.pd
Warped Galaxies From Misaligned Angular Momenta
A galaxy disk embedded in a rotating halo experiences a dynamical friction
force which causes it to warp when the angular momentum axes of the disk and
halo are misaligned. Our fully self-consistent simulations of this process
induce long-lived warps in the disk which mimic Briggs's rules of warp
behavior. They also demonstrate that random motion within the disk adds
significantly to its stiffness. Moreover, warps generated in this way have no
winding problem and are more pronounced in the extended \h1 disk. As emphasized
by Binney and his co-workers, angular momentum misalignments, which are
expected in hierarchical models of galaxy formation, can account for the high
fraction of warped galaxies. Our simulations exemplify the role of misaligned
spins in warp formation even when the halo density is not significantly
flattened.Comment: 6 pages, 5 figures. Accepted for publication in Ap.J.
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