1,943 research outputs found
Halo Geometry and Dark Matter Annihilation Signal
We study the impact of the halo shape and geometry on the expected weakly
interacting massive particle (WIMP) dark matter annihilation signal from the
galactic center. As the halo profile in the innermost region is still poorly
constrained, we consider different density behaviors like flat cores, cusps and
spikes, as well as geometrical distortions. We show that asphericity has a
strong impact on the annihilation signal when the halo profile near the
galactic center is flat, but becomes gradually less significant for cuspy
profiles, and negligible in the presence of a central spike. However, the
astrophysical factor is strongly dependent on the WIMP mass and annihilation
cross-section in the latter case.Comment: 5 pages, 4 figures, PR
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
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 Planetary Nebulae Spectrograph: the green light for Galaxy Kinematics
Planetary nebulae are now well established as probes of galaxy dynamics and
as standard candles in distance determinations. Motivated by the need to
improve the efficiency of planetary nebulae searches and the speed with which
their radial velocities are determined, a dedicated instrument - the Planetary
Nebulae Spectrograph or PN.S - has been designed and commissioned at the 4.2m
William Herschel Telescope. The high optical efficiency of the spectrograph
results in the detection of typically ~ 150 PN in galaxies at the distance of
the Virgo cluster in one night of observations. In the same observation the
radial velocities are obtained with an accuracy of ~ 20 km/sComment: Accepted by PASP, to appear November 2002; the figures have been
degraded for archival purpose
The Kinematically Measured Pattern Speeds of NGC 2523 and NGC 4245
We have applied the Tremaine-Weinberg continuity equation method to derive
the bar pattern speed in the SB(r)b galaxy NGC 2523 and the SB(r)0/a galaxy NGC
4245 using the Calcium Triplet absorption lines. These galaxies were selected
because they have strong inner rings which can be used as independent tracers
of the pattern speed. The pattern speed of NGC 2523 is 26.4 6.1 km
s kpc, assuming an inclination of 49.7 and a distance
of 51.0 Mpc. The pattern speed of NGC 4245 is 75.5 31.3 km s
kpc, assuming an inclination of 35.4 and a distance of 12.6
Mpc. The ratio of the corotation radius to the bar radius of NGC 2523 and NGC
4245 is 1.4 0.3 and 1.1 0.5, respectively. These values place the
bright inner rings near and slightly inside the corotation radius, as predicted
by barred galaxy theory. Within the uncertainties, both galaxies are found to
have fast bars that likely indicate dark halos of low central concentration.
The photometric properties, bar strengths, and disk stabilities of both
galaxies are also discussed.Comment: Accepted for publication in The Astronomical Journal, 11 figures, 2
table
Study of arc-jet propulsion devices Final report, 20 Nov. 1964 - 19 Dec. 1965
Energy transfer mechanisms in radiation, water, and regeneratively cooled, and MPD arc jet propulsion device
Integral Field Spectroscopy of 23 Spiral Bulges
We have obtained Integral Field Spectroscopy for 23 spiral bulges using
INTEGRAL on the William Herschel Telescope and SPIRAL on the Anglo-Australian
Telescope. This is the first 2D survey directed solely at the bulges of spiral
galaxies. Eleven galaxies of the sample do not have previous measurements of
the stellar velocity dispersion (sigma*). These data are designed to complement
our Space Telescope Imaging Spectrograph program for estimating black hole
masses in the range 10^6-10^8M_sun using gas kinematics from nucleated disks.
These observations will serve to derive the stellar dynamical bulge properties
using the traditional Mgb and CaII triplets. We use both Cross Correlation and
Maximum Penalized Likelihood to determine projected sigma* in these systems and
present radial velocity fields, major axis rotation curves, curves of growth
and sigma* fields. Using the Cross Correlation to extract the low order 2D
stellar dynamics we generally see coherent radial rotation and irregular
velocity dispersion fields suggesting that sigma* is a non-trivial parameter to
estimate.Comment: 11 pages, 30 figures, accepted for publication in ApJ
The scaling properties of dissipation in incompressible isotropic three-dimensional magnetohydrodynamic turbulence
The statistical properties of the dissipation process constrain the analysis
of large scale numerical simulations of three dimensional incompressible
magnetohydrodynamic (MHD) turbulence, such as those of Biskamp and Muller
[Phys. Plasmas 7, 4889 (2000)]. The structure functions of the turbulent flow
are expected to display statistical self-similarity, but the relatively low
Reynolds numbers attainable by direct numerical simulation, combined with the
finite size of the system, make this difficult to measure directly. However, it
is known that extended self-similarity, which constrains the ratio of scaling
exponents of structure functions of different orders, is well satisfied. This
implies the extension of physical scaling arguments beyond the inertial range
into the dissipation range. The present work focuses on the scaling properties
of the dissipation process itself. This provides an important consistency check
in that we find that the ratio of dissipation structure function exponents is
that predicted by the She and Leveque [Phys. Rev. Lett 72, 336 (1994)] theory
proposed by Biskamp and Muller. This supplies further evidence that the cascade
mechanism in three dimensional MHD turbulence is non-linear random eddy
scrambling, with the level of intermittency determined by dissipation through
the formation of current sheets.Comment: 9 pages, 6 figures. Figures embedded in text. Typos corrected in text
and references. Published in Physics of Plasmas. Abstract can be found
at:http://link.aip.org/link/?php/12/02230
Why Buckling Stellar Bars Weaken in Disk Galaxies
Young stellar bars in disk galaxies experience a vertical buckling
instability which terminates their growth and thickens them, resulting in a
characteristic peanut/boxy shape when viewed edge on. Using N-body simulations
of galactic disks embedded in live halos, we have analyzed the bar structure
throughout this instability and found that the outer third of the bar dissolves
completely while the inner part (within the vertical inner Lindblad resonance)
becomes less oval. The bar acquires the frequently observed peanut/boxy-shaped
isophotes. We also find that the bar buckling is responsible for a mass
injection above the plane, which is subsequently trapped by specific 3-D
families of periodic orbits of particular shapes explaining the observed
isophotes, in line with previous work. Using a 3-D orbit analysis and surfaces
of sections, we infer that the outer part of the bar is dissolved by a rapidly
widening stochastic region around its corotation radius -- a process related to
the bar growth. This leads to a dramatic decrease in the bar size, decrease in
the overall bar strength and a mild increase in its pattern speed, but is not
expected to lead to a complete bar dissolution. The buckling instability
appears primarily responsible for shortening the secular diffusion timescale to
a dynamical one when building the boxy isophotes. The sufficiently long
timescale of described evolution, ~1 Gyr, can affect the observed bar fraction
in local universe and at higher redshifts, both through reduced bar strength
and the absence of dust offset lanes in the bar.Comment: 7 pages, 4 figures, ApJ Letters, in pres
The Distribution of Bar and Spiral Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamental property
of the galaxy population that has only begun to be explored. We have applied
the bar/spiral separation method of Buta, Block, and Knapen to derive the
distribution of maximum relative gravitational bar torques, Q_b, for 147 spiral
galaxies in the statistically well-defined Ohio State University Bright Galaxy
Survey (OSUBGS) sample. Our goal is to examine the properties of bars as
independently as possible of their associated spirals. We find that the
distribution of bar strength declines smoothly with increasing Q_b, with more
than 40% of the sample having Q_b <= 0.1. In the context of recurrent bar
formation, this suggests that strongly-barred states are relatively short-lived
compared to weakly-barred or non-barred states. We do not find compelling
evidence for a bimodal distribution of bar strengths. Instead, the distribution
is fairly smooth in the range 0.0 <= Q_b < 0.8. Our analysis also provides a
first look at spiral strengths Q_s in the OSU sample, based on the same torque
indicator. We are able to verify a possible weak correlation between Q_s and
Q_b, in the sense that galaxies with the strongest bars tend also to have
strong spirals.Comment: Accepted for publication in the Astronomical Journal, August 2005
issue (LaTex, 23 pages + 11 figures, uses aastex.cls
- …