1,402 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
The Visibility of Galactic Bars and Spiral Structure At High Redshifts
We investigate the visibility of galactic bars and spiral structure in the
distant Universe by artificially redshifting 101 B-band CCD images of local
spiral galaxies from the Ohio State University Bright Spiral Galaxy Survey. Our
artificially redshifted images correspond to Hubble Space Telescope I-band
observations of the local galaxy sample seen at z=0.7, with integration times
matching those of both the very deep Northern Hubble Deep Field data, and the
much shallower Flanking Field observations. The expected visibility of galactic
bars is probed in two ways: (1) using traditional visual classification, and
(2) by charting the changing shape of the galaxy distribution in "Hubble
space", a quantitative two-parameter description of galactic structure that
maps closely on to Hubble's original tuning fork. Both analyses suggest that
over 2/3 of strongly barred luminous local spirals i.e. objects classified as
SB in the Third Reference Catalog) would still be classified as strongly barred
at z=0.7 in the Hubble Deep Field data. Under the same conditions, most weakly
barred spirals (classified SAB in the Third Reference Catalog) would be
classified as regular spirals. The corresponding visibility of spiral structure
is assessed visually, by comparing luminosity classifications for the
artificially redshifted sample with the corresponding luminosity
classifications from the Revised Shapley Ames Catalog. We find that for
exposures times similar to that of the Hubble Deep Field spiral structure
should be detectable in most luminous low-inclination spiral galaxies at z=0.7
in which it is present. [ABRIDGED]Comment: Accepted for publication in The Astronomical Journa
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Sixteen years of bathymetry and waves at San Diego beaches.
Sustained, quantitative observations of nearshore waves and sand levels are essential for testing beach evolution models, but comprehensive datasets are relatively rare. We document beach profiles and concurrent waves monitored at three southern California beaches during 2001-2016. The beaches include offshore reefs, lagoon mouths, hard substrates, and cobble and sandy (medium-grained) sediments. The data span two energetic El Niño winters and four beach nourishments. Quarterly surveys of 165 total cross-shore transects (all sites) at 100âm alongshore spacing were made from the backbeach to 8âm depth. Monthly surveys of the subaerial beach were obtained at alongshore-oriented transects. The resulting dataset consists of (1) raw sand elevation data, (2) gridded elevations, (3) interpolated elevation maps with error estimates, (4) beach widths, subaerial and total sand volumes, (5) locations of hard substrate and beach nourishments, (6) water levels from a NOAA tide gauge (7) wave conditions from a buoy-driven regional wave model, and (8) time periods and reaches with alongshore uniform bathymetry, suitable for testing 1-dimensional beach profile change models
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
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
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 link between the masses and central stellar populations of S0 galaxies
Using high signal-to-noise ratio VLT/FORS2 long-slit spectroscopy, we have
studied the properties of the central stellar populations and dynamics of a
sample of S0 galaxies in the Fornax Cluster. The central absorption-line
indices in these galaxies correlate well with the central velocity dispersions
(Sigma0) in accordance with what previous studies found for elliptical
galaxies. However, contrary to what it is usually assumed for cluster
ellipticals, the observed correlations seem to be driven by systematic age and
alpha-element abundance variations, and not changes in overall metallicity. We
also found that the observed scatter in the Index-Sigma0 relations can be
partially explained by the rotationally-supported nature of these systems.
Indeed, even tighter correlations exist between the line indices and the
maximum circular velocity of the galaxies. This study suggests that the
dynamical mass is the physical property driving these correlations, and for S0
galaxies such masses have to be estimated assuming a large degree of rotational
support. The observed trends imply that the most massive S0s have the shortest
star-formation timescales and the oldest stellar populations.Comment: 21 pages, 17 figures, 9 tables, accepted for publication in MNRA
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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