54,177 research outputs found
A Chandra View of the Multiple Merger In Abell 2744
We present a Chandra observation of the merging cluster of galaxies Abell
2744. The cluster shows strong evidence for an ongoing major merger which we
believe to be responsible for the radio halo. X-ray emission and temperature
maps of the cluster, combined with the spatial and redshift distribution of the
galaxies, indicate a roughly north-south axis for the merger, with a
significant velocity component along the line of sight. The merger is occurring
at a very large velocity, with M = 2-3. In addition, there is a small merging
subcluster toward the northwest, unrelated to the major merger, which shows
evidence of a bow shock. A hydrodynamical analysis of the subcluster indicates
a merger velocity corresponding to a Mach number of ~1.2, consistent with a
simple infall model. This infalling subcluster may also be re-exciting
electrons in the radio halo. Its small Mach number lends support to turbulent
reacceleration models for radio halo formation.Comment: 8 pages, 6 figures (5 color). Submitted to MNRA
Time-frequency detection of Gravitational Waves
We present a time-frequency method to detect gravitational wave signals in
interferometric data. This robust method can detect signals from poorly modeled
and unmodeled sources. We evaluate the method on simulated data containing
noise and signal components. The noise component approximates initial LIGO
interferometer noise. The signal components have the time and frequency
characteristics postulated by Flanagan and Hughes for binary black hole
coalescence. The signals correspond to binaries with total masses between to and with (optimal filter) signal-to-noise ratios of 7
to 12. The method is implementable in real time, and achieves a coincident
false alarm rate for two detectors 1 per 475 years. At this false
alarm rate, the single detector false dismissal rate for our signal model is as
low as 5.3% at an SNR of 10. We expect to obtain similar or better detection
rates with this method for any signal of similar power that satisfies certain
adiabaticity criteria. Because optimal filtering requires knowledge of the
signal waveform to high precision, we argue that this method is likely to
detect signals that are undetectable by optimal filtering, which is at present
the best developed detection method for transient sources of gravitational
waves.Comment: 24 pages, 5 figures, uses REVTE
CO-0.30-0.07: A Peculiar Molecular Clump with an Extremely Broad Velocity Width
The high velocity dispersion compact cloud CO-0.30-0.07 is a peculiar
molecular clump discovered in the central moleculr zone of the Milky Way, which
is characterized by its extremely broad velocity emissions () despite the absence of internal energy sources. We present new
interferometric maps of the cloud in multiple molecular lines in frequency
ranges of 265--269 GHz and 276--280 GHz obtained using the Sumbmillimeter
Array, along with the single-dish images previously obtained with the ASTE 10-m
telescope. The data show that the characteristic broad velocity emissions are
predominantly confined in two parallel ridges running through the cloud center.
The central ridges are tightly anti-correlated with each other in both space
and velocity, thereby sharply dividing the entire cloud into two distinct
velocity components (+15 km s and +55 km s). This morphology is
consistent with a model in which the two velocity components collide with a
relative velocity of 40 at the interface defined by the
central ridges, although an alternative explanation with a highly inclined
expanding-ring model is yet to be fully invalidated. We have also unexpectedly
detected several compact clumps (pc in radius) likely formed by
shock compression. The clumps have several features in common with typical
star-forming clouds: high densities (), rich
abundances of hot-core-type molecular species, and relatively narrow velocity
widths apparently decoupled from the furious turbulence dominating the cloud.
The cloud CO-0.30-0.07 is possibly at an early phase of star formation activity
triggered by the shock impact.Comment: 29 pages, 10 figures, accepted for publication in Ap
A topographic mechanism for arcing of dryland vegetation bands
Banded patterns consisting of alternating bare soil and dense vegetation have
been observed in water-limited ecosystems across the globe, often appearing
along gently sloped terrain with the stripes aligned transverse to the
elevation gradient. In many cases these vegetation bands are arced, with field
observations suggesting a link between the orientation of arcing relative to
the grade and the curvature of the underlying terrain. We modify the water
transport in the Klausmeier model of water-biomass interactions, originally
posed on a uniform hillslope, to qualitatively capture the influence of terrain
curvature on the vegetation patterns. Numerical simulations of this modified
model indicate that the vegetation bands change arcing-direction from
convex-downslope when growing on top of a ridge to convex-upslope when growing
in a valley. This behavior is consistent with observations from remote sensing
data that we present here. Model simulations show further that whether bands
grow on ridges, valleys, or both depends on the precipitation level. A survey
of three banded vegetation sites, each with a different aridity level,
indicates qualitatively similar behavior.Comment: 26 pages, 13 figures, 2 table
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