7,583 research outputs found
NASA ground terminal communication equipment automated fault isolation expert systems
The prototype expert systems are described that diagnose the Distribution and Switching System I and II (DSS1 and DSS2), Statistical Multiplexers (SM), and Multiplexer and Demultiplexer systems (MDM) at the NASA Ground Terminal (NGT). A system level fault isolation expert system monitors the activities of a selected data stream, verifies that the fault exists in the NGT and identifies the faulty equipment. Equipment level fault isolation expert systems are invoked to isolate the fault to a Line Replaceable Unit (LRU) level. Input and sometimes output data stream activities for the equipment are available. The system level fault isolation expert system compares the equipment input and output status for a data stream and performs loopback tests (if necessary) to isolate the faulty equipment. The equipment level fault isolation system utilizes the process of elimination and/or the maintenance personnel's fault isolation experience stored in its knowledge base. The DSS1, DSS2 and SM fault isolation systems, using the knowledge of the current equipment configuration and the equipment circuitry issues a set of test connections according to the predefined rules. The faulty component or board can be identified by the expert system by analyzing the test results. The MDM fault isolation system correlates the failure symptoms with the faulty component based on maintenance personnel experience. The faulty component can be determined by knowing the failure symptoms. The DSS1, DSS2, SM, and MDM equipment simulators are implemented in PASCAL. The DSS1 fault isolation expert system was converted to C language from VP-Expert and integrated into the NGT automation software for offline switch diagnoses. Potentially, the NGT fault isolation algorithms can be used for the DSS1, SM, amd MDM located at Goddard Space Flight Center (GSFC)
Galaxy evolution in groups and clusters: satellite star formation histories and quenching timescales in a hierarchical Universe
Satellite galaxies in groups and clusters are more likely to have low star
formation rates (SFR) and lie on the red-sequence than central (field)
galaxies. Using galaxy group/cluster catalogs from SDSS DR7, together with a
cosmological N-body simulation to track satellite orbits, we examine the star
formation histories and quenching timescales of satellites of M_star > 5 x 10^9
M_sun at z=0. We first explore satellite infall histories: group preprocessing
and ejected orbits are critical aspects of satellite evolution, and properly
accounting for these, satellite infall typically occurred at z~0.5, or ~5 Gyr
ago. To obtain accurate initial conditions for the SFRs of satellites at their
time of first infall, we construct an empirical parametrization for the
evolution of central galaxy SFRs and quiescent fractions. With this, we
constrain the importance and efficiency of satellite quenching as a function of
satellite and host halo mass, finding that satellite quenching is the dominant
process for building up all quiescent galaxies at M_star < 10^10 M_sun. We then
constrain satellite star formation histories, finding a 'delayed-then-rapid'
quenching scenario: satellite SFRs evolve unaffected for 2-4 Gyr after infall,
after which star formation quenches rapidly, with an e-folding time of < 0.8
Gyr. These quenching timescales are shorter for more massive satellites but do
not depend on host halo mass: the observed increase in satellite quiescent
fraction with halo mass arises simply because of satellites quenching in a
lower mass group prior to infall (group preprocessing), which is responsible
for up to half of quenched satellites in massive clusters. Because of the long
time delay before quenching starts, satellites experience significant stellar
mass growth after infall, nearly identical to central galaxies. This fact
provides key physical insight into the subhalo abundance matching method.Comment: 25 pages, 13 figures. Accepted for publication in MNRAS, matches
published versio
The Clustering of Massive Halos
The clustering properties of dark matter halos are a firm prediction of
modern theories of structure formation. We use two large volume,
high-resolution N-body simulations to study how the correlation function of
massive dark matter halos depends upon their mass and formation history. We
find that halos with the lowest concentrations are presently more clustered
than those of higher concentration, the size of the effect increasing with halo
mass; this agrees with trends found in studies of lower mass halos. The
clustering dependence on other characterizations of the full mass accretion
history appears weaker than the effect with concentration. Using the integrated
correlation function, marked correlation functions, and a power-law fit to the
correlation function, we find evidence that halos which have recently undergone
a major merger or a large mass gain have slightly enhanced clustering relative
to a randomly chosen population with the same mass distribution.Comment: 10 pages, 8 figures; text improved, references and one figure added;
accepted for publication in Ap
Close Pairs as Proxies for Galaxy Cluster Mergers
Galaxy cluster merger statistics are an important component in understanding
the formation of large-scale structure. Unfortunately, it is difficult to study
merger properties and evolution directly because the identification of cluster
mergers in observations is problematic. We use large N-body simulations to
study the statistical properties of massive halo mergers, specifically
investigating the utility of close halo pairs as proxies for mergers. We
examine the relationship between pairs and mergers for a wide range of merger
timescales, halo masses, and redshifts (0<z<1). We also quantify the utility of
pairs in measuring merger bias. While pairs at very small separations will
reliably merge, these constitute a small fraction of the total merger
population. Thus, pairs do not provide a reliable direct proxy to the total
merger population. We do find an intriguing universality in the relation
between close pairs and mergers, which in principle could allow for an estimate
of the statistical merger rate from the pair fraction within a scaled
separation, but including the effects of redshift space distortions strongly
degrades this relation. We find similar behavior for galaxy-mass halos, making
our results applicable to field galaxy mergers at high redshift. We investigate
how the halo merger rate can be statistically described by the halo mass
function via the merger kernel (coagulation), finding an interesting
environmental dependence of merging: halos within the mass resolution of our
simulations merge less efficiently in overdense environments. Specifically,
halo pairs with separations less than a few Mpc/h are more likely to merge in
underdense environments; at larger separations, pairs are more likely to merge
in overdense environments.Comment: 12 pages, 9 figures; Accepted for publication in ApJ. Significant
additions to text and two figures changed. Added new findings on the
universality of pair mergers and added analysis of the effect of FoF linking
length on halo merger
Nitrogen and phosphorus loads to temperate seepage lakes associated with allochthonous dissolved organic carbon loads
Terrestrial loads of dissolved organic matter (DOM) have increased in recent years in many north temperate lakes. While much of the focus on the âbrowningâ phenomena has been on its consequences for carbon cycling, much less is known about how it influences nutrient loading to lakes. We characterize potential loads of nitrogen and phosphorus to seepage lakes in northern Wisconsin, USA, based on a laboratory soil leaching experiment and a model that includes landscape cover and watershed area. In these seepage lakes, nutrient concentrations are positively correlated with dissolved organic carbon concentrations (nitrogen: r = 0.68, phosphorus: r = 0.54). Using longâterm records of browning, we found that dissolved organic matterâassociated nutrient loadings may have resulted in substantial increases in nitrogen and phosphorus in seepage lakes and could account for currently observed nutrient concentrations in the lake. âSilentâ nutrient loadings to brownâwater lakes may lead to future waterâquality concerns. PLAIN LANGUAGE SUMMARY: The color of many temperate lakes is changing; some lakes are becoming more darkly stained brown. The teaâcolored stain is due to dissolved organic matter from the surrounding landscape. Much of the research related to the causes and consequences of increased staining, or âbrownification,â relate to its connection to the carbon cycle. However, by examining longâterm lake chemical records, analyzing the properties of the organic compounds, and modeling potential flows of the compounds, we find that carbon is not the only element that is influenced by browning. Nitrogen and phosphorus, two nutrients important to growth of organisms at the base of the food web, may also be increasing in lakes due to brownification."Funding for this research was supported by the Northern Research Station and the Chequamegon-Nicolet National Forest (CNNF) of the United States Department of Agriculture (USDA) Forest Service and a National Science Foundation (NSF) grant to the University of Wisconsin-Madison to support the North Temperate Lakes Long-Term Ecological Research (NTLLTER) Site (DEB-#1440297)."https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL07721
Heterocyst placement strategies to maximize growth of cyanobacterial filaments
Under conditions of limited fixed-nitrogen, some filamentous cyanobacteria
develop a regular pattern of heterocyst cells that fix nitrogen for the
remaining vegetative cells. We examine three different heterocyst placement
strategies by quantitatively modelling filament growth while varying both
external fixed-nitrogen and leakage from the filament. We find that there is an
optimum heterocyst frequency which maximizes the growth rate of the filament;
the optimum frequency decreases as the external fixed-nitrogen concentration
increases but increases as the leakage increases. In the presence of leakage,
filaments implementing a local heterocyst placement strategy grow significantly
faster than filaments implementing random heterocyst placement strategies. With
no extracellular fixed-nitrogen, consistent with recent experimental studies of
Anabaena sp. PCC 7120, the modelled heterocyst spacing distribution using our
local heterocyst placement strategy is qualitatively similar to experimentally
observed patterns. As external fixed-nitrogen is increased, the spacing
distribution for our local placement strategy retains the same shape while the
average spacing between heterocysts continuously increases.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Physical Biology. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The definitive publisher-authenticated version
will be available onlin
Galaxy evolution near groups and clusters: ejected satellites and the spatial extent of environmental quenching
Galaxies that are several virial radii beyond groups/clusters show
preferentially quiescent star formation rates. Using a galaxy group/cluster
catalog from the Sloan Digital Sky Survey, together with a cosmological N-body
simulation, we examine the origin of this environmental quenching beyond the
virial radius. Accounting for the clustering of groups/clusters, we show that
central galaxies show enhanced SFR quenching out to 2.5 virial radii beyond
groups/clusters, and we demonstrate that this extended environmental
enhancement can be explained simply by 'ejected' satellite galaxies that orbit
beyond their host halo's virial radius. We show that ejected satellites
typically orbit for several Gyr beyond the virial radius before falling back
in, and thus they compose up to 40% of all central galaxies near
groups/clusters. We show that a model in which ejected satellites experience
the same SFR quenching as satellites within a host halo can explain essentially
all environmental dependence of galaxy quenching. Furthermore, ejected
satellites (continue to) lose significant halo mass, an effect that is
potentially observable via gravitational lensing. The SFRs/colors and
stellar-to-halo masses of ejected satellites highlight the importance of
environmental history and present challenges to models of galaxy occupation
that ignore such history.Comment: 15 pages, 9 figures. Accepted for publication in MNRAS, matches
published versio
Science center to go: a mixed reality learning environment of miniature exhibits
Modern science museums deliver exhibits that involve visitors in sessions of active playful learning. Mixed Reality may additionally be used as a catalyst for science centre exhibits to provide new perspectives and reveal otherwise hidden phenomena. Such sophisticated exhibits might well be qualified for improved learning at school â except they are hardly available outside of science centres. Our approach addresses this challenge by bringing miniaturized Augmented Reality exhibits out of the science centre into schools. The presented miniature exhibits uncouple science centre exhibits from their traditional venue and deliver natural ways of learning whenever and wherever it is desired. Therefore a set of miniature exhibits has been designed with the ambitious goal to meet the requirements of nowadays school curriculums. In this paper a science centre in a suitcase will be presented and discussed offering tailor-made learning experiences augmented through normal computers
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