324 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
A Magellanic Origin for the Warp of the Galaxy
We show that a Magellanic Cloud origin for the warp of the Milky Way can
explain most quantitative features of the outer HI layer recently identified by
Levine, Blitz & Heiles (2005). We construct a model similar to that of Weinberg
(1998) that produces distortions in the dark matter halo, and we calculate the
combined effect of these dark-halo distortions and the direct tidal forcing by
the Magellanic Clouds on the disk warp in the linear regime. The interaction of
the dark matter halo with the disk and resonances between the orbit of the
Clouds and the disk account for the large amplitudes observed for the vertical
m=0,1,2 harmonics. The observations lead to six constraints on warp forcing
mechanisms and our model reasonably approximates all six. The disk is shown to
be very dynamic, constantly changing its shape as the Clouds proceed along
their orbit. We discuss the challenges to MOND placed by the observations.Comment: 4 pages, 3 figures, submitted to ApJ Letters. Additional graphics, 3d
visualizations and movies available at
http://www.astro.umass.edu/~weinberg/lm
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
On the generation of asymmetric warps in disk galaxies
The warps in many spiral galaxies are now known to asymmetric. Recent
sensitive observations have revealed that asymmetry of warps may be the norm
rather than exception. However there exists no generic mechanism to generate
these asymmetries in warps. We have derived the dispersion relation in a
compact form for the S-shaped warps(described by the m=1 mode) and the
bowl-shaped distribution(described by the m=0 mode) in galactic disk embedded
in a dark matter halo. We then performed the numerical modal analysis and used
the linear and time-dependent superposition principle to generate asymmetric
warps in the disk. On doing the modal analysis we find the frequency of the
mode is much larger than that of the mode. The linear and
time-dependent superposition of these modes with their unmodulated
amplitudes(that is, the coefficients of superposition being unity) results in
an asymmetry in warps of ~ 20 - 40 %, whereas a smaller coefficient for the m=0
mode results in a smaller asymmetry. The resulting values agree well with the
recent observations. We study the dependence of the asymmetry index on the dark
matter halo parameters. This approach can also naturally produce U-shaped warps
and L-shaped warps. We show that a rich variety of possible asymmetries in the
z-distribution of the spiral galaxies can naturally arise due to a dynamical
wave interference between the first two bending modes(i.e. m=0 and m=1) in the
disk. This is a simple but general method for generating asymmetric warps that
is independent of how the individual modes arise in the disk.Comment: Accepted for publication in A &
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
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.
Invariant manifolds and the response of spiral arms in barred galaxies
The unstable invariant manifolds of the short-period family of periodic
orbits around the unstable Lagrangian points and of a barred galaxy
define loci in the configuration space which take the form of a trailing spiral
pattern. In the present paper we investigate this association in the case of
the self-consistent models of Kaufmann & Contopoulos (1996) which provide an
approximation of real barred-spiral galaxies. We also examine the relation of
`response' models of barred-spiral galaxies with the theory of the invariant
manifolds. Our main results are the following: The invariant manifolds yield
the correct form of the imposed spiral pattern provided that their calculation
is done with the spiral potential term turned on. We provide a theoretical
model explaining the form of the invariant manifolds that supports the spiral
structure. The azimuthal displacement of the Lagrangian points with respect to
the bar's major axis is a crucial parameter in this modeling. When this is
taken into account, the manifolds necessarily develop in a spiral-like domain
of the configuration space, delimited from below by the boundary of a
banana-like non-permitted domain, and from above either by rotational KAM tori
or by cantori forming a stickiness zone. We construct `spiral response' models
on the basis of the theory of the invariant manifolds and examine the
connection of the latter to the `response' models (Patsis 2006) used to fit
real barred-spiral galaxies, explaining how are the manifolds related to a
number of morphological features seen in such models.Comment: 16 Page
The puzzle of the polar structure in NGC 4650A
This work presents new surface photometry and two-dimensional modeling of the
light distribution of the Polar Ring Galaxy NGC 4650A, based on near-infrared
(NIR) observations and high resolution optical imaging acquired during the
Hubble Heritage program. The NIR and optical integrated colors of the S0 and
the polar ring, and their scale parameters, are compared with those for
standard galaxy morphological types. The polar structure appears to be a disk
of a very young age, while the colors and light distribution of the host galaxy
do not resemble that of a typical early-type system. We compare these
observational results with the predictions from different formation scenarios
for polar ring galaxies. The peculiarities of the central S0 galaxy, the polar
disk structure and stellar population ages suggest that the polar ring galaxy
NGC 4650A may be the result of a dissipative merger event, rather than of an
accretion process.Comment: 26 pages, 10 figures, accepted for publication to the Astronomical
Journa
Asymptotic Orbits in Barred Spiral Galaxies
We study the formation of the spiral structure of barred spiral galaxies,
using an -body model. The evolution of this -body model in the adiabatic
approximation maintains a strong spiral pattern for more than 10 bar rotations.
We find that this longevity of the spiral arms is mainly due to the phenomenon
of stickiness of chaotic orbits close to the unstable asymptotic manifolds
originated from the main unstable periodic orbits, both inside and outside
corotation. The stickiness along the manifolds corresponding to different
energy levels supports parts of the spiral structure. The loci of the disc
velocity minima (where the particles spend most of their time, in the
configuration space) reveal the density maxima and therefore the main
morphological structures of the system. We study the relation of these loci
with those of the apocentres and pericentres at different energy levels. The
diffusion of the sticky chaotic orbits outwards is slow and depends on the
initial conditions and the corresponding Jacobi constant.Comment: 17 pages, 24 figure
The Potential-Density Phase Shift Method for Determining the Corotation Radii in Spiral and Barred Galaxies
We have developed a new method for determining the corotation radii of
density waves in disk galaxies, which makes use of the radial distribution of
an azimuthal phase shift between the potential and density wave patterns. The
approach originated from improved theoretical understandings of the relation
between the morphology and kinematics of galaxies, and on the dynamical
interaction between density waves and the basic-state disk stars which results
in the secular evolution of disk galaxies. In this paper, we present the
rationales behind the method, and the first application of it to several
representative barred and grand-design spiral galaxies, using near-infrared
images to trace the mass distributions, as well as to calculate the potential
distributions used in the phase shift calculations. We compare our results with
those from other existing methods for locating the corotations, and show that
the new method both confirms the previously-established trends of bar-length
dependence on galaxy morphological types, as well as provides new insights into
the possible extent of bars in disk galaxies. Application of the method to a
larger sample and the preliminary analysis of which show that the phase shift
method is likely to be a generally-applicable, accurate, and essentially
model-independent method for determining the pattern speeds and corotation
radii of single or nested density wave patterns in galaxies. Other implications
of this work are: most of the nearby bright disk galaxies appear to possess
quasi-stationary spiral modes; that these density wave modes and the associated
basic state of the galactic disk slowly transform over time; and that
self-consistent N-particle systems contain physics not revealed by the passive
orbit analysis approaches.Comment: 48 pages, 16 figures. Accepted for publication in the Astronomical
Journa
- …