257 research outputs found
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
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 &
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
Regular Orbits and Periodic Loops in Multiply-Barred Galactic Potentials
We show that non-chaotic multiply-periodic particle orbits can exist in a
galaxy-like potential where a small fast-tumbling nuclear bar is nested inside
a main bar which has a slower pattern speed. We introduce the concept of a
loop: a one-dimensional curve such that particles distributed along it at some
initial instant return to the same curve (as viewed in the rotating frame of
one of the bars) after the bars return to the same relative position. Just as
particles following regular orbits in a simply-barred potential are trapped
around closed periodic orbits, so regular orbits in a doubly-barred potential
oscillate about stable loops. We find both loops which remain aligned with the
inner bar and loops aligned with the outer bar: particles trapped around these
loops could form the building blocks for a long-lived, self-consistent,
doubly-barred galaxy. In a realistic doubly-barred galaxy potential, we
demonstrate the existence of stable loops which support the structure of the
inner bar. We use the epicyclic approximation to preview the pattern of gas
flow in a doubly-barred potential.Comment: 6 pages, 1 figure, submitted to ApJ Letter
Double Bars, Inner Disks, and Nuclear Rings in Early-Type Disk Galaxies
We present results from an imaging survey of an unbiased sample of
thirty-eight early-type (S0--Sa), low-inclination, optically barred galaxies in
the field. Our goal was to find and characterize central stellar and gaseous
structures: secondary bars, inner disks, and nuclear rings. Bars inside bars
are surprisingly common: at least one quarter of the sample galaxies (possibly
as many as 40%) are double-barred, with no preference for Hubble type or the
strength of the primary bar. A typical secondary bar is ~12% of the size of its
primary bar and 240--750 pc in radius. We see no significant effect of
secondary bars on nuclear activity. We also find kiloparsec-scale inner disks
in at least 20% of our sample, almost exclusively in S0 galaxies. These disks
are on average 20% the size of their host bar, and show a wider range of
relative sizes than do secondary bars. Nuclear rings are present in about a
third of our sample. Most are dusty, sites of current or recent star formation,
or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies
(15% of the S0's) appear to have purely stellar nuclear rings, with no evidence
for dust or recent star formation. The fact that these central stellar
structures are so common indicates that the inner regions of early-type barred
galaxies typically contain dynamically cool and disklike structures. This is
especially true for S0 galaxies, where secondary bars, inner disks, and/or
stellar nuclear rings are present at least two thirds of the time. (abridged)Comment: LaTeX, 15 pages, 7 EPS figures; to appear in The Astronomical Journal
(July 2002
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
Neutral Hydrogen and Optical Observations of Edge-on Galaxies: Hunting for Warps
We present 21-cm HI line and optical R-band observations for a sample of 26
edge-on galaxies. The HI observations were obtained with the Westerbork
Synthesis Radio Telescope, and are part of the WHISP database (Westerbork HI
Survey of Spiral and Irregular Galaxies). We present HI maps, optical images,
and radial HI density profiles. We have also derived the rotation curves and
studied the warping and lopsidedness of the HI disks.
20 out of the 26 galaxies of our sample are warped, confirming that warping
of the HI disks is a very common phenomenon in disk galaxies. Indeed, we find
that all galaxies that have an extended HI disk with respect to the optical are
warped.
The warping usually starts around the edge of the optical disk. The degree of
warping varies considerably from galaxy to galaxy. Furthermore, many warps are
asymmetric, as they show up in only one side of the disk or exhibit large
differences in amplitude in the approaching and receding sides of the galaxy.
These asymmetries are more pronounced in rich environments, which may indicate
that tidal interactions are a source of warp asymmetry. A rich environment
tends to produce larger warps as well. The presence of lopsidedness seems to be
related to the presence of nearby companions.Comment: To appear in Astronomy & Astrophysic
The Shape and Figure Rotation of NGC 2915's Dark Halo
NGC 2915 is a blue compact dwarf galaxy with a very extended HI disk showing
a short central bar and extended spiral arms, both reaching far beyond the
optical component. We use Tremaine & Weinberg (1984) method to measure the
pattern speed of the bar from HI radio synthesis data. Our measurements yield a
pattern speed of 0.21+/-0.06 km/s/arcsec (8.0+/-2.4 km/s/kpc for D=5.3 Mpc), in
disagreement with the general view that corotation in barred disks lies just
outside the end of the bar, but consistent with recent models of barred
galaxies with dense dark matter halos. Our adopted bar semi-length puts
corotation at more than 1.7 bar radii. The existence of the pattern is also
problematic. Because NGC 2915 is isolated, interactions cannot account for the
structure observed in the HI disk. We also demonstrate that the low observed
disk surface density and the location of the pseudo-rings make it unlikely that
swing amplification or bar-driven spiral arms could explain the bar and spiral
pattern.
Based on the similarity of the dark matter and HI surface density profiles,
we discuss the possibility of dark matter distributed in a disk and following
closely the HI distribution. The disk then becomes unstable and can naturally
form a bar and spiral pattern. However, this explanation is hard to reconcile
with some properties of NGC 2915. We also consider the effect of a massive and
extended triaxial dark matter halo with a rotating figure. The existence of
such halos is supported by CDM simulations showing strongly triaxial dark halos
with slow figure rotation. The observed structure of the HI disk can then arise
through forcing by the rotating triaxial figure. We associate the measured
pattern speed in NGC 2915 with the figure rotation of its dark halo.Comment: 37 pages, including 8 figures and 2 tables (AASTeX, aaspp4.sty).
Fig.1 and 2 available as jpg. Accepted for publication in The Astronomical
Journal. Online manuscript with PostScript figures available at:
http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
A Physical Model of Warped Galaxy Disks
Warped H I gas layers in the outer regions of spiral galaxies usually display
a noticeably twisted structure. This structure almost certainly arises
primarily as a result of differential precession in the H I disk as it settles
toward a preferred orientation in an underlying dark halo potential well that
is not spherically symmetric. In an attempt to better understand the structure
and evolution of these twisted, warped disk structures, we have adopted the
"twist-equation" formalism originally developed by Petterson (1977) to study
accretion onto compact objects. Utilizing more recent treatments of this
formalism, we have generalized the twist-equation to allow for the treatment of
non-Keplerian disks and from it have derived a steady-state structure of
twisted disks that develops from free precession in a nonspherical, logarithmic
halo potential. We have used this steady-state solution to produce H I maps of
five galaxies (M83, NGC 300, NGC 2841, NGC 5033, NGC5055), which match the
general features of the observed maps of these galaxies quite well. In
addition, the model provides an avenue through which the kinematical viscosity
of the H I disk and the quadrupole distortion of the dark halo in each galaxy
can be quantified. This generalized equation can also be used to examine the
time-evolutionary behavior of warped galaxy disks.Comment: 22 pages, 5 figures, to be published in The Astrophysical Journa
Radio Continuum Emission in Polar-Ring Galaxies
We have used the Very Large Array aperture synthesis telescope to conduct a
radio continuum survey of polar-ring galaxies, at 20cm and 6cm. Forty objects
were observed at 20cm with 5" resolution. Twenty (50%) of the program sources
were detected at 20cm, down to our 5-sigma limit of 0.5 mJy/beam. This
detection rate is similar to those in surveys with comparable sensitivity for
early-type galaxies without polar rings. Sixteen of the objects we detected at
20cm were also observed at 6cm. We show radio continuum maps for the five
objects in our sample that have noticeably extended emission. Our spatial
resolution was sufficient to distinguish between emission originating in the
host galaxy from that in the polar ring. The radio morphology of the extended
sources, as well as the radio to far-infrared flux ratio and the radio spectral
indices of our detected sources, indicate that star formation, not nuclear
activity, is the dominant source of the radio continuum emission in polar-ring
galaxies. However, the implied star-formation rates are modest, and only one of
our sample galaxies will consume its supply of cool gas within 500 Myr.Comment: 21 pages, 7 figures. Produced with AASTeX 5.2 To appear in November
2004 Astronomical Journa
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