198 research outputs found
Modeling the X-rays Resulting from High Velocity Clouds
With the goal of understanding why X-rays have been reported near some high
velocity clouds, we perform detailed 3 dimensional hydrodynamic and
magnetohydrodynamic simulations of clouds interacting with environmental gas
like that in the Galaxy's thick disk/halo or the Magellanic Stream. We examine
2 scenarios. In the first, clouds travel fast enough to shock-heat warm
environmental gas. In this scenario, the X-ray productivity depends strongly on
the speed of the cloud and the radiative cooling rate. In order to shock-heat
environmental gas to temperatures of > or = 10^6 K, cloud speeds of > or = 300
km/s are required. If cooling is quenched, then the shock-heated ambient gas is
X-ray emissive, producing bright X-rays in the 1/4 keV band and some X-rays in
the 3/4 keV band due to O VII and other ions. If, in contrast, the radiative
cooling rate is similar to that of collisional ionizational equilibrium plasma
with solar abundances, then the shocked gas is only mildly bright and for only
about 1 Myr. The predicted count rates for the non-radiative case are bright
enough to explain the count rate observed with XMM-Newton toward a Magellanic
Stream cloud and some enhancement in the ROSAT 1/4 keV count rate toward
Complex C, while the predicted count rates for the fully radiative case are
not. In the second scenario, the clouds travel through and mix with hot ambient
gas. The mixed zone can contain hot gas, but the hot portion of the mixed gas
is not as bright as those from the shock-heating scenario.Comment: 15 pages, 9 figures, 1 table. Accepted for publication in the
Astrophysical Journa
Very Extended X-ray and H-alpha Emission in M82: Implications for the Superwind Phenomenon
We discuss the properties and implications of a 3.7x0.9 kpc region of
spatially-coincident X-ray and H-alpha emission about 11.6 kpc to the north of
the galaxy M82 previously discussed by Devine and Bally (1999). The PSPC X-ray
spectrum is fit by thermal plasma (kT=0.80+-0.17 keV) absorbed by only the
Galactic foreground column density. We evaluate the relationship of the
X-ray/H-alpha ridge to the M82 superwind. The main properties of the X-ray
emission can all be explained as being due to shock-heating driven as the
superwind encounters a massive ionized cloud in the halo of M82. This encounter
drives a slow shock into the cloud, which contributes to the excitation of the
observed H-alpha emission. At the same time, a fast bow-shock develops in the
superwind just upstream of the cloud, and this produces the observed X-ray
emission. This interpretation would imply that the superwind has an outflow
speed of roughly 800 km/s, consistent with indirect estimates based on its
general X-ray properties and the kinematics of the inner kpc-scale region of
H-alpha filaments. The gas in the M82 ridge is roughly two orders-of-magnitude
hotter than the minimum "escape temperature" at this radius, so this gas will
not be retained by M82.
(abridged)Comment: 24 pages (latex), 3 figures (2 gif files and one postscript),
accepted for publication in Part 1 of The Astrophysical Journa
Triassic Leech Cocoon From Antarctica Contains Fossil Bell Animal
This is the publisher's version of this article, which has been shared with permission. The article is also available from: http://dx.doi.org/10.1073/pnas.1218879109Our understanding of the evolution of life on Earth is limited by
the imperfection of the fossil record. One reason for this imperfect
record is that organisms without hard parts, such as bones, shells,
and wood, have a very low potential to enter the fossil record.
Occasionally, however, exceptional fossil deposits that preserve
soft-bodied organisms provide a rare glimpse of the true biodiversity
during past periods of Earth history. We here present an extraordinary
find of a fossil ciliate that is encased inside the wall
layer of a more than 200 Ma leech cocoon from Antarctica. The
microfossil consists of a helically contractile stalk that attaches to
a main body with a peristomial feeding apparatus and a large Cshaped
macronucleus. It agrees in every aspect with the living bell
animals, such as Vorticella. Vorticellids and similar peritrichs are
vital constituents of aquatic ecosystems worldwide, but so far have
lacked any fossil record. This discovery offers a glimpse of ancient
soft-bodied protozoan biotas, and also highlights the potential of
clitellate cocoons as microscopic âconservation trapsâ comparable
to amber
Systematics and Paleoecology of a New Peltaspermalean Seed Fern From the Triassic Polar Vegetation of Gondwana
This is the publisher's version, which has been shared with permission. The original version may be found at: http://dx.doi.org/10.1017/S0954102011000241A new Triassic seed fern is described on the basis of a large collection of well-preserved plant compressions
from the Upper Triassic of Mt. Falla, Queen Alexandra Range, central Transantarctic Mountains. The foliage
is simple entire-margined to pinnatifid to partly pinnate and is assigned to Dejerseya lobata (Jones et de Jersey)
Herbst emend. nov. Associated with these leaves occur two new reproductive structures. The ovulate organ
Matatiella dejerseyi sp. nov. is a lax, conelike structure with primarily shield-shaped megasporophylls that are
dissected into three to four narrow lobes, each bearing a single recurved, naked ovule. The pollen organ
Townrovia polaris sp. nov. consists of a slender axis arising from a covered reproductive bud, bearing pinnately
arranged, stalked, elongate receptacles each with ;20 unilocular, clavate pollen sacs; the pollen is bisaccate
nontaeniate with an irregular longitudinal sulcus and coarse saccus endoreticulations, corresponding to the
dispersed pollen Falcisporites australis. Similar pollen grains occur in the micropylar canals of seed cuticles of
M. dejerseyi. Co-occurrence data indicate that the three taxa are probably different organs from one plant
species. This new seed fern is assigned to the Matatiellaceae, which we place into the Peltaspermales because of
structural similarities with vegetative and reproductive organs of other peltaspermalean seed ferns. It appears
that several Triassic Gondwanan plant fossil taxa of currently uncertain affinitiesâsuch as Pachydermophyllum,
Linguifolium, Carpolithus mackayi, and Andersoniaâmay belong to the Matatiellaceae as well. We
suggest that the matatiellacean peltasperms were opportunistic, early successional plants that were particularly
successful in colonizing stressed wetland environments in polar latitudes during the Triassic
2D-1D Wavelet Reconstruction As A Tool For Source Finding In Spectroscopic Imaging Surveys
Today, image denoising by thresholding of wavelet coefficients is a commonly
used tool for 2D image enhancement. Since the data product of spectroscopic
imaging surveys has two spatial and one spectral dimension, the techniques for
denoising have to be adapted to this change in dimensionality. In this paper we
will review the basic method of denoising data by thresholding wavelet
coefficients and implement a 2D-1D wavelet decomposition to obtain an efficient
way of denoising spectroscopic data cubes. We conduct different simulations to
evaluate the usefulness of the algorithm as part of a source finding pipeline.Comment: 8 pages, 7 figures, 1 table, accepted for publication in PASA Special
Issue on Source Finding and Visualizatio
A Multiband Approach to AGN: Radioscopy & Radio Astronomy
Only in the radio-loud population of active galactic nuclei (AGN) does the
production, collimation, and acceleration of powerful relativistic jets take
place. We introduce here a concept of combined VLBI- and X-ray spectroscopic
observations of sources with relativistic, broad iron lines. This approach has
enormous potential to yield deep insights into the accretion/jet-production
process in AGN. Better knowledge of the milliarcsecond-resolution radio
structure of the nuclear radio cores in so-called ``radio-quiet''
broad-iron-line Seyfert galaxies is essential for future combined radio/X-ray
studies of the different modes of radio-jet production in accreting black hole
systems.Comment: 4 pages, 1 figure, needes mem.cls, Proceedings contribution to the
workshop "Multiband approach to AGN" held in Bonn (September 30 - October 2,
2004
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