4,652 research outputs found
Ionization, Kinematics, and Extent of the Diffuse Ionized Gas Halo of NGC 5775
We present key results from deep spectra of the Diffuse Ionized Gas (DIG)
halo of the edge-on galaxy NGC 5775. [NII]6583 has been detected up to about
z=13 kpc above the plane in one of two vertically oriented long slits -- making
this the spiral galaxy with the greatest spectroscopically detected halo extent
in emission. Key diagnostic line ratios have been measured up to about z=8 kpc,
allowing the source of ionization and physical state to be probed. Ionization
by a dilute radiation field from massive stars in the disk can explain some of
the line ratio behavior, but departures from this picture are clearly
indicated, most strongly by the rise of [OIII]/Halpha with z. Velocities of the
gas in both slits approach the systemic velocity of the galaxy at several kpc
above the plane. We interpret this trend as a decrease in rotation velocity
with z, with essentially no rotation at heights of several kpc. Such a trend
was observed in the edge-on galaxy NGC 891, but here much more dramatically.
This falloff is presumably due to the gravitational potential changing with z,
but will also depend on the hydrodynamic nature of the disk-halo cycling of gas
and projection effects. More detailed modeling of the ionization and kinematics
of this and other edge-ons will be presented in future papers.Comment: figures 1, 2a-d and 3 included. ApJ Letters, in pres
Apparent competition drives community-wide parasitism rates and changes in host abundance across ecosystem boundaries
Species have strong indirect effects on others, and predicting these effects is a central challenge in ecology. Prey species sharing an enemy (predator or parasitoid) can be linked by apparent competition, but it is unknown whether this process is strong enough to be a community-wide structuring mechanism that could be used to predict future states of diverse food webs. Whether species abundances are spatially coupled by enemy movement across different habitats is also untested. Here, using a field experiment, we show that predicted apparent competitive effects between species, mediated via shared parasitoids, can significantly explain future parasitism rates and herbivore abundances. These predictions are successful even across edges between natural and managed forests, following experimental reduction of herbivore densities by aerial spraying over 20ha. This result shows that trophic indirect effects propagate across networks and habitats in important, predictable ways, with implications for landscape planning, invasion biology and biological control
Fluctuations and oscillations in a simple epidemic model
We show that the simplest stochastic epidemiological models with spatial
correlations exhibit two types of oscillatory behaviour in the endemic phase.
In a large parameter range, the oscillations are due to resonant amplification
of stochastic fluctuations, a general mechanism first reported for
predator-prey dynamics. In a narrow range of parameters that includes many
infectious diseases which confer long lasting immunity the oscillations persist
for infinite populations. This effect is apparent in simulations of the
stochastic process in systems of variable size, and can be understood from the
phase diagram of the deterministic pair approximation equations. The two
mechanisms combined play a central role in explaining the ubiquity of
oscillatory behaviour in real data and in simulation results of epidemic and
other related models.Comment: acknowledgments added; a typo in the discussion that follows Eq. (3)
is corrected
Ionization Sources and Physical Conditions in the Diffuse Ionized Gas Halos of Four Edge-On Galaxies
Deep long-slit spectra of the diffuse ionized gas halos of the edge-on spiral
galaxies NGC 4302 and UGC 10288 are presented. These data, along with
previously presented data for NGC 5775 and NGC 891, are used to address the
issue of how DIG halos are energized. Composite photo-ionization/shock models
are generally better at explaining runs of line ratios in these galaxies than
photo-ionization models alone. Models of line ratios in NGC 5775 require a
greater contribution from shocks for filamentary regions than for
non-filamentary regions to explain the run of [OIII]/Halpha. In either case,
the [SII]/[NII] ratio is not well fit by the models. Composite models for UGC
10288 are successful at reproducing the run of [SII]/[NII] for all but the the
highest values of [NII]/Halpha; however, the run of [OIII]/Halpha vs.
[NII]/Halpha does not show any discernible trend, making it difficult to
determine whether or not shocks make a contribution.
We also examine whether the data can be explained simply by an increase in
temperature with z in a pure photo-ionization model. Runs of [SII]/Halpha,
[NII]/Halpha, and [SII]/[NII] in each of the four galaxies are consistent with
such an increase. However, the runs of [OIII]/Halpha vs. z in NGC 5775 and UGC
10288 require unusually high ionization fractions of O^{++} that can not be
explained without invoking a secondary ionization source or at the very least a
much higher temperature for the [OIII]-emitting component than for the [SII]-
and [NII]-emitting component. An increase in temperature with z is generally
more successful at explaining the [OIII]/Halpha run in NGC 891.Comment: 42 pages in aaspp4.sty format. This includes the 19 figures.
Reference added. Accepted for publication in Ap
P01.56. Does practicing Reiki alter the electromagnetic field of heart and hands of practitioners?
Benevolent characteristics promote cooperative behaviour among humans
Cooperation is fundamental to the evolution of human society. We regularly
observe cooperative behaviour in everyday life and in controlled experiments
with anonymous people, even though standard economic models predict that they
should deviate from the collective interest and act so as to maximise their own
individual payoff. However, there is typically heterogeneity across subjects:
some may cooperate, while others may not. Since individual factors promoting
cooperation could be used by institutions to indirectly prime cooperation, this
heterogeneity raises the important question of who these cooperators are. We
have conducted a series of experiments to study whether benevolence, defined as
a unilateral act of paying a cost to increase the welfare of someone else
beyond one's own, is related to cooperation in a subsequent one-shot anonymous
Prisoner's dilemma. Contrary to the predictions of the widely used inequity
aversion models, we find that benevolence does exist and a large majority of
people behave this way. We also find benevolence to be correlated with
cooperative behaviour. Finally, we show a causal link between benevolence and
cooperation: priming people to think positively about benevolent behaviour
makes them significantly more cooperative than priming them to think
malevolently. Thus benevolent people exist and cooperate more
HALOGAS observations of NGC 5023 and UGC 2082: Modeling of non-cylindrically symmetric gas distributions in edge-on galaxies
In recent years it has become clear that the vertical structure of disk
galaxies is a key ingredient for understanding galaxy evolution. In particular,
the presence and structure of extra-planar gas has been a focus of research.
The Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey aims to provide a
census on the rate of cold neutral gas accretion in nearby galaxies as well as
a statistically significant set of galaxies that can be investigated for their
extra-planar gas properties.
In order to better understand the the vertical structure of the neutral
hydrogen in the two edge-on HALOGAS galaxies NGC 5023 and UGC 2082 we construct
detailed tilted ring models. The addition of distortions resembling arcs or
spiral arms significantly improves the fit of the models to these galaxies. In
the case of UGC 2082 no vertical gradient in rotational velocity is required in
either symmetric models nor non-symmetric models to match the observations. The
best fitting model features two arcs of large vertical extent that may be due
to accretion. In the case of NGC 5023 a vertical gradient is required in
symmetric models (dV/dz = km s kpc) and its
magnitude is significantly lowered when non-symmetric models are considered
(dV/dz = km s kpc). Additionally it is shown that the
underlying disk of NGC 5023 can be made symmetric, in all parameters except the
warp, in non-symmetric models. In comparison to the "classical" modeling these
models fit the data significantly better with a limited addition of free
parameters.Comment: 27 Pages, 22 Figures. Accepted for publication in MNRA
The environment power system analysis tool development program
The Environment Power System Analysis Tool (EPSAT) is being developed to provide space power system design engineers with an analysis tool for determining system performance of power systems in both naturally occurring and self-induced environments. The program is producing an easy to use computer aided engineering (CAE) tool general enough to provide a vehicle for technology transfer from space scientists and engineers to power system design engineers. The results of the project after two years of a three year development program are given. The EPSAT approach separates the CAE tool into three distinct functional units: a modern user interface to present information, a data dictionary interpreter to coordinate analysis; and a data base for storing system designs and results of analysis
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