1,275 research outputs found
Channel-forming properties of cecropins and related model compounds incorporated into planar lipid membranes.
The Pattern Speed of the Galactic Bar
Most late-type stars in the solar neighborhood have velocities similar to the
local standard of rest (LSR), but there is a clearly separated secondary
component corresponding to a slower rotation and a mean outward motion.
Detailed simulations of the response of a stellar disk to a central bar show
that such a bi-modality is expected from outer-Lindblad resonant scattering.
When constraining the run of the rotation curve by the proper motion of Sgr A*
and the terminal gas velocities, the value observed for the rotation velocity
separating the two components results in a value of (53+/-3)km/s/kpc for the
pattern speed of the bar, only weakly dependent on the precise values for Ro
and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter
Kinematics of Spiral Arm Streaming in M51
We use CO and H alpha velocity fields to study the gas kinematics in the
spiral arms and interarms of M51 (NGC 5194), and fit the 2D velocity field to
estimate the radial and tangential velocity components as a function of spiral
phase (arm distance). We find large radial and tangential streaming velocities,
which are qualitatively consistent with the predictions of density wave theory
and support the existence of shocks. The streaming motions are complex, varying
significantly across the galaxy as well as along and between arms. Aberrations
in the velocity field indicate that the disk is not coplanar, perhaps as far in
as 20\arcsec\ (800 pc) from the center. Velocity profile fits from CO and H
alpha are typically similar, suggesting that most of the H alpha emission
originates from regions of recent star formation. We also explore vortensity
and mass conservation conditions. Vortensity conservation, which does not
require a steady state, is empirically verified. The velocity and density
profiles show large and varying mass fluxes, which are inconsistent with a
steady flow for a single dominant global spiral mode. We thus conclude that the
spiral arms cannot be in a quasi-steady state in any rotating frame, and/or
that out of plane motions may be significant.Comment: 50 pages, including 20 figures; Accepted for publication in ApJ. PDF
version with high resolution figures available at
http://www.astro.umd.edu/~shetty/Research
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
Dynamical Friction and the Distribution of Dark Matter in Barred Galaxies
We use fully self-consistent N-body simulations of barred galaxies to show
that dynamical friction from a dense dark matter halo dramatically slows the
rotation rate of bars. Our result supports previous theoretical predictions for
a bar rotating within a massive halo. On the other hand, low density halos,
such as those required for maximum disks, allow the bar to continue to rotate
at a high rate. There is somewhat meager observational evidence indicating that
bars in real galaxies do rotate rapidly and we use our result to argue that
dark matter halos must have a low central density in all high surface
brightness disk galaxies, including the Milky Way. Bars in galaxies that have
larger fractions of dark matter should rotate slowly, and we suggest that a
promising place to look for such candidate objects is among galaxies of
intermediate surface brightness.Comment: 6 pages, Latex, 3 figures, Accepted by Ap.J.L., revised copy,
includes an added paragrap
The properties of the Galactic bar implied by gas kinematics in the inner Milky Way
Longitude-velocity (l-V) diagrams of H I and CO gas in the inner Milky Way
have long been known to be inconsistent with circular motion in an axisymmetric
potential. Several lines of evidence suggest that the Galaxy is barred, and gas
flow in a barred potential could be consistent with the observed ``forbidden''
velocities and other features in the data. We compare the H I observations to
l-V diagrams synthesized from 2-D fluid dynamical simulations of gas flows in a
family of barred potentials. The gas flow pattern is very sensitive to the
parameters of the assumed potential, which allows us to discriminate among
models. We present a model that reproduces the outer contour of the H I l-V
diagram reasonably well; this model has a strong bar with a semimajor axis of
3.6 kpc, an axis ratio of approximately 3:1, an inner Lindblad resonance (ILR),
and a pattern speed of 42 km/s/kpc, and matches the data best when viewed from
34\deg to the bar major axis. The behavior of the models, combined with the
constraint that the shocks in the Milky Way bar should resemble those in
external barred galaxies, leads us to conclude that wide ranges of parameter
space are incompatible with the observations. In particular we suggest that the
bar must be fairly strong, must have an ILR, and cannot be too end-on, with the
bar major axis at 35\deg +/- 5\deg to the line of sight. The H I data exhibit
larger forbidden velocities over a wider longitude range than are seen in
molecular gas; this important difference is the reason our favored model
differs so significantly from other recently proposed models.Comment: 23 pages, 14 figures, 1 table, uses emulateapj and psfig, 640 kb.
Submitted to Ap
A burst search for gravitational waves from binary black holes
Compact binary coalescence (CBC) is one of the most promising sources of
gravitational waves. These sources are usually searched for with matched
filters which require accurate calculation of the GW waveforms and generation
of large template banks. We present a complementary search technique based on
algorithms used in un-modeled searches. Initially designed for detection of
un-modeled bursts, which can span a very large set of waveform morphologies,
the search algorithm presented here is constrained for targeted detection of
the smaller subset of CBC signals. The constraint is based on the assumption of
elliptical polarisation for signals received at the detector. We expect that
the algorithm is sensitive to CBC signals in a wide range of masses, mass
ratios, and spin parameters. In preparation for the analysis of data from the
fifth LIGO-Virgo science run (S5), we performed preliminary studies of the
algorithm on test data. We present the sensitivity of the search to different
types of simulated CBC waveforms. Also, we discuss how to extend the results of
the test run into a search over all of the current LIGO-Virgo data set.Comment: 12 pages, 4 figures, 2 tables, submitted for publication in CQG in
the special issue for the conference proceedings of GWDAW13; corrected some
typos, addressed some minor reviewer comments one section restructured and
references updated and correcte
An Ongoing Shift in Pacific Ocean Sea Level
Based on the satellite altimeter data, sea level off the west coast of the United States has increased over the past 5 years, while sea level in the western tropical Pacific has declined. Understanding whether this is a shortâterm shift or the beginning of a longerâterm change in sea level has important implications for coastal planning efforts in the coming decades. Here, we identify and quantify the recent shift in Pacific Ocean sea level, and also seek to describe the variability in a manner consistent with recent descriptions of El NinoâSouthern Oscillation (ENSO) and particularly the Pacific Decadal Oscillation (PDO). More specifically, we extract two dominant modes of sea level variability, one related to the biennial oscillation associated with ENSO and the other representative of lowerâfrequency variability with a strong signal in the northern Pacific. We rely on cyclostationary empirical orthogonal function (CSEOF) analysis along with sea level reconstructions to describe these modes and provide historical context for the recent sea level changes observed in the Pacific. As a result, we find that a shift in sea level has occurred in the Pacific Ocean over the past few years that will likely persist in the coming years, leading to substantially higher sea level off the west coast of the United States and lower sea level in the western tropical Pacific. Sea level in the Pacific has undergone a shift in the past 5 years, with sea level in the eastern (western) Pacific rising (falling) Sea level variability in the Pacific Ocean has been separated into a biennial oscillation mode and a decadal mode This shift appears to result from a change of phase of a lowâfrequency climate signal, that could continue on for the next several year
Molecular Gas Kinematics in Barred Spiral Galaxies
To quantify the effect that bar driven mass inflow can have on the evolution
of a galaxy requires an understanding of the dynamics of the inflowing gas. In
this paper we study the kinematics of the dense molecular gas in a set of seven
barred spiral galaxies to determine which dynamical effects dominate. The
kinematics are derived from observations of the CO J=(1-0) line made with the
Berkeley-Illinois-Maryland Association (BIMA) millimeter array. We compare the
observed kinematics to those predicted by ideal gas hydrodynamic and ballistic
cloud-based models of gas flow in a barred potential. The hydrodynamic model is
in good qualitative agreement with both the current observations of the dense
gas and previous observations of the kinematics of the ionized gas. The
observed kinematics indicate that the gas abruptly changes direction upon
entering the dust lanes to flow directly down the dust lanes along the leading
edge of the bar until the dust lanes approach the nuclear ring. Near the
location where the dust lanes intersect the nuclear ring, we see two velocity
components: a low velocity component, corresponding to gas on circular orbits,
and a higher velocity component, which can be attributed to the fraction of gas
flowing down the bar dust lane which sprays past the contact point toward the
other half of the bar. The ballistic cloud-based model of the ISM is not
consistent with the observed kinematics. The kinematics in the dust lanes
require large velocity gradients which cannot be reproduced by an ISM composed
of ballistic clouds with long mean-free-paths. Therefore, even the dense ISM
responds to hydrodynamic forces.Comment: To be published in the Astrophysical Journal, Nov. 20, 199
Maximum Disk Mass Models for Spiral Galaxies
We present axisymmetric maximum disk mass models for a sample of 74 spiral
galaxies taken from the southern sky Fabry-Perot Tully-Fisher survey (Schommer
et al. 1993). The sample contains galaxies spanning a large range of
morphologies and having rotation widths from 180 km/sec to 680 km/secs. For
each galaxy we have an I-band image and a 2-d H_alpha velocity field. The
distribution of mass is modeled as a sum of disk and bulge components with
distinct, constant mass-to-light ratios. No dark matter halo is included in the
fits. The models reproduce the overall structure of the rotation curves in the
majority of galaxies, providing good fits to galaxies which exhibit pronounced
structural differences in their surface brightness profiles. 75% of galaxies
for which the rotation curve is measured to R_23.5 or beyond are well fit by a
mass-traces-light model for the entire region within R_23.5. The models for
about 20% of the galaxies do not fit well; the failure of most of these models
is traced directly to non-axisymmetric structures, primarily bars but also
strong spiral arms. The median I-band M/L of the disk plus bulge is 2.4+/-0.9
h_75 in solar units, consistent with normal stellar populations. These results
require either that the mass of dark matter within the optical disk of spiral
galaxies is small, or that its distribution is very precisely coupled to the
distribution of luminous matter.Comment: accepted by AJ, 29 pages, 8 figures, uses emulateapj.st
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