9,724 research outputs found
The cross-correlation between galaxies of different luminosities and Colors
We study the cross-correlation between galaxies of different luminosities and
colors, using a sample selected from the SDSS Dr 4. Galaxies are divided into 6
samples according to luminosity, and each of these samples is divided into red
and blue subsamples. Projected auto-correlation and cross-correlation is
estimated for these subsample. At projected separations r_p > 1\mpch, all
correlation functions are roughly parallel, although the correlation amplitude
depends systematically on luminosity and color. On r_p < 1\mpch, the auto- and
cross-correlation functions of red galaxies are significantly enhanced relative
to the corresponding power laws obtained on larger scales. Such enhancement is
absent for blue galaxies and in the cross-correlation between red and blue
galaxies. We esimate the relative bias factor on scales r > 1\mpch for each
subsample using its auto-correlation function and cross-correlation functions.
The relative bias factors obtained from different methods are similar. For blue
galaxies the luminosity-dependence of the relative bias is strong over the
luminosity range probed (-23.0<M_r < -18.0),but for red galaxies the dependence
is weaker and becomes insignificant for luminosities below L^*. To examine
whether a significant stochastic/nonlinear component exists in the bias
relation, we study the ratio R_ij= W_{ii}W_{jj}/W_{ij}^2, where W_{ij} is the
projected correlation between subsample i and j. We find that the values of
R_ij are all consistent with 1 for all-all, red-red and blue-blue samples,
however significantly larger than 1 for red-blue samples. For faint red - faint
blue samples the values of R_{ij} are as high as ~ 2 on small scales r_p < 1
\mpch and decrease with increasing r_p.Comment: 25 pages, 18 figures, Accepted for publication in Ap
Chemistry of chlorinated species in the Antarctic stratosphere
The chemistry of Cl sub 2 O sub 2, the chlorine monoxide dimer, has been further investigated in order to better asses its potential role in catalytic ozone destruction cycles. The dimer has been generated in a flow system, in the 200 to 250 K temperature range, by using ozone and chlorine atoms as ClO precursors. The Cl-atoms are produced by a microwave discharge of either Cl sub 2, or of F sub 2 with subsequent addition of HCl. With this later scheme the dimer can be generated in the absence of Cl sub 2. The Fourier transform infrared spectra of the products clearly indicates the presence of two isomers, in agreement with earlier results (J. Phys. Shen., 91, 433, 1987). None of the observed IR bands can be attributed to a ClO-OClO adduct, since they all appear in the absence of any detectable amount of OCl. It is likely that the particles in the polar stratospheric clouds will have a relatively dilute nitric acid outer layer, even if the core is the nitric acid monohydrate, since the particles are in equilibrium with the ambient water vapor, which is present at levels of a few parts per million
Three Different Types of Galaxy Alignment within Dark Matter Halos
Using a large galaxy group catalogue based on the Sloan Digital Sky Survey
Data Release 4 we measure three different types of intrinsic galaxy alignment
within groups: halo alignment between the orientation of the brightest group
galaxies (BGG) and the distribution of its satellite galaxies, radial alignment
between the orientation of a satellite galaxy and the direction towards its
BGG, and direct alignment between the orientation of the BGG and that of its
satellites. In agreement with previous studies we find that satellite galaxies
are preferentially located along the major axis. In addition, on scales r < 0.7
Rvir we find that red satellites are preferentially aligned radially with the
direction to the BGG. The orientations of blue satellites, however, are
perfectly consistent with being isotropic. Finally, on scales r < 0.1 \Rvir, we
find a weak but significant indication for direct alignment between satellites
and BGGs. We briefly discuss the implications for weak lensing measurements.Comment: 4 pages, 4 figures, ApJL accepte
The Magnetohydrodynamic Kelvin-Helmholtz Instability: A Three-Dimensional Study of Nonlinear Evolution
We investigate through high resolution 3D simulations the nonlinear evolution
of compressible magnetohydrodynamic flows subject to the Kelvin-Helmholtz
instability. We confirm in 3D flows the conclusion from our 2D work that even
apparently weak magnetic fields embedded in Kelvin-Helmholtz unstable plasma
flows can be fundamentally important to nonlinear evolution of the instability.
In fact, that statement is strengthened in 3D by this work, because it shows
how field line bundles can be stretched and twisted in 3D as the quasi-2D Cat's
Eye vortex forms out of the hydrodynamical motions. In our simulations twisting
of the field may increase the maximum field strength by more than a factor of
two over the 2D effect. If, by these developments, the Alfv\'en Mach number of
flows around the Cat's Eye drops to unity or less, our simulations suggest
magnetic stresses will eventually destroy the Cat's Eye and cause the plasma
flow to self-organize into a relatively smooth and apparently stable flow that
retains memory of the original shear. For our flow configurations the regime in
3D for such reorganization is , expressed in
terms of the Alfv\'en Mach number of the original velocity transition and the
initial Alfv\'en speed projected to the flow plan. For weaker fields the
instability remains essentially hydrodynamic in early stages, and the Cat's Eye
is destroyed by the hydrodynamic secondary instabilities of a 3D nature. Then,
the flows evolve into chaotic structures that approach decaying isotropic
turbulence. In this stage, there is considerable enhancement to the magnetic
energy due to stretching, twisting, and turbulent amplification, which is
retained long afterwards. The magnetic energy eventually catches up to the
kinetic energy, and the nature of flows become magnetohydrodynamic.Comment: 11 pages, 12 figures in degraded jpg format (2 in color), paper with
original quality figures available via ftp at
ftp://ftp.msi.umn.edu/pub/users/twj/mhdkh3dd.ps.gz or
ftp://canopus.chungnam.ac.kr/ryu/mhdkh3dd.ps.gz, to appear in The
Astrophysical Journa
Theoretical and Experimental Studies of Schottky Diodes That Use Aligned Arrays of Single Walled Carbon Nanotubes
We present theoretical and experimental studies of Schottky diodes that use
aligned arrays of single walled carbon nanotubes. A simple physical model,
taking into account the basic physics of current rectification, can adequately
describe the single-tube and array devices. We show that for as grown array
diodes, the rectification ratio, defined by the
maximum-to-minimum-current-ratio, is low due to the presence of m-SWNT shunts.
These tubes can be eliminated in a single voltage sweep resulting in a high
rectification array device. Further analysis also shows that the channel
resistance, and not the intrinsic nanotube diode properties, limits the
rectification in devices with channel length up to ten micrometer.Comment: Nano Research, 2010, accepte
Metagenomes of Red Sea Subpopulations Challenge the Use of Marker Genes and Morphology to Assess Trichodesmium Diversity
Trichodesmium are filamentous cyanobacteria of key interest due to their ability to fix carbon and nitrogen within an oligotrophic marine environment. Their blooms consist of a dynamic assemblage of subpopulations and colony morphologies that are hypothesized to occupy unique niches. Here, we assessed the poorly studied diversity of Trichodesmium in the Red Sea, based on metagenome-assembled genomes (MAGs) and hetR gene-based phylotyping. We assembled four non-redundant MAGs from morphologically distinct Trichodesmium colonies (tufts, dense and thin puffs). Trichodesmium thiebautii (puffs) and Trichodesmium erythraeum (tufts) were the dominant species within these morphotypes. While subspecies diversity is present for both T. thiebautii and T. erythraeum, a single T. thiebautii genotype comprised both thin and dense puff morphotypes, and we hypothesize that this phenotypic variation is likely attributed to gene regulation. Additionally, we found the rare non-diazotrophic clade IV and V genotypes, related to Trichodesmium nobis and Trichodesmium miru, respectively that likely occurred as single filaments. The hetR gene phylogeny further indicated that the genotype in clade IV could represent the species Trichodesmium contortum. Importantly, we show the presence of hetR paralogs in Trichodesmium, where two copies of the hetR gene were present within T. thiebautii genomes. This may lead to the overestimation of Trichodesmium diversity as one of the copies misidentified T. thiebautii as Trichodesmium aureum. Taken together, our results highlight the importance of re-assessing Trichodesmium taxonomy while showing the ability of genomics to capture the complex diversity and distribution of Trichodesmium populations
Bacterial Community Structure of an IFAS-MBRs Wastewater Treatment Plant
In this work, the bacterial community putatively involved in BNR events of a UCT-MBMBR pilot plant was elucidated by both culture-dependent and metagenomics DNA analyses. The presence of bacterial isolates belonging to Bacillus (in the anoxic compartment) and to Acinetobacter, Stenotrophomonas, Rhodococcus, Escherichia and Aeromonas (in the aerobic compartment) is in agreement with the nitrification/denitrification processes observed in the plant. Moreover, the study of bacterial community structure by NGS revealed a microbial diversity suggesting a biochemical complexity which can be further explored and exploited to improve UCT-MBMBR plant performance
Staphylococcus epidermidis Strategies to Avoid Killing by Human Neutrophils
Staphylococcus epidermidis is a leading nosocomial pathogen. In contrast to its more aggressive relative S. aureus, it causes chronic rather than acute infections. In highly virulent S. aureus, phenol-soluble modulins (PSMs) contribute significantly to immune evasion and aggressive virulence by their strong ability to lyse human neutrophils. Members of the PSM family are also produced by S. epidermidis, but their role in immune evasion is not known. Notably, strong cytolytic capacity of S. epidermidis PSMs would be at odds with the notion that S. epidermidis is a less aggressive pathogen than S. aureus, prompting us to examine the biological activities of S. epidermidis PSMs. Surprisingly, we found that S. epidermidis has the capacity to produce PSMδ, a potent leukocyte toxin, representing the first potent cytolysin to be identified in that pathogen. However, production of strongly cytolytic PSMs was low in S. epidermidis, explaining its low cytolytic potency. Interestingly, the different approaches of S. epidermidis and S. aureus to causing human disease are thus reflected by the adaptation of biological activities within one family of virulence determinants, the PSMs. Nevertheless, S. epidermidis has the capacity to evade neutrophil killing, a phenomenon we found is partly mediated by resistance mechanisms to antimicrobial peptides (AMPs), including the protease SepA, which degrades AMPs, and the AMP sensor/resistance regulator, Aps (GraRS). These findings establish a significant function of SepA and Aps in S. epidermidis immune evasion and explain in part why S. epidermidis may evade elimination by innate host defense despite the lack of cytolytic toxin expression. Our study shows that the strategy of S. epidermidis to evade elimination by human neutrophils is characterized by a passive defense approach and provides molecular evidence to support the notion that S. epidermidis is a less aggressive pathogen than S. aureus
Bacterial Community Structure of an IFAS-MBRs Wastewater Treatment Plant
In this work, the bacterial community putatively involved in BNR events of a UCT-MBMBR pilot plant was elucidated by both culture-dependent and metagenomics DNA analyses. The presence of bacterial isolates belonging to Bacillus (in the anoxic compartment) and to Acinetobacter, Stenotrophomonas, Rhodococcus, Escherichia and Aeromonas (in the aerobic compartment) is in agreement with the nitrification/denitrification processes observed in the plant. Moreover, the study of bacterial community structure by NGS revealed a microbial diversity suggesting a biochemical complexity which can be further explored and exploited to improve UCT-MBMBR plant performance
Spatial and kinematic alignments between central and satellite halos
Based on a cosmological N-body simulation we analyze spatial and kinematic
alignments of satellite halos within six times the virial radius of group size
host halos (Rvir). We measure three different types of spatial alignment: halo
alignment between the orientation of the group central substructure (GCS) and
the distribution of its satellites, radial alignment between the orientation of
a satellite and the direction towards its GCS, and direct alignment between the
orientation of the GCS and that of its satellites. In analogy we use the
directions of satellite velocities and probe three further types of alignment:
the radial velocity alignment between the satellite velocity and connecting
line between satellite and GCS, the halo velocity alignment between the
orientation of the GCS and satellite velocities and the auto velocity alignment
between the satellites orientations and their velocities. We find that
satellites are preferentially located along the major axis of the GCS within at
least 6 Rvir (the range probed here). Furthermore, satellites preferentially
point towards the GCS. The most pronounced signal is detected on small scales
but a detectable signal extends out to 6 Rvir. The direct alignment signal is
weaker, however a systematic trend is visible at distances < 2 Rvir. All
velocity alignments are highly significant on small scales. Our results suggest
that the halo alignment reflects the filamentary large scale structure which
extends far beyond the virial radii of the groups. In contrast, the main
contribution to the radial alignment arises from the adjustment of the
satellite orientations in the group tidal field. The projected data reveal good
agreement with recent results derived from large galaxy surveys. (abridged)Comment: accepted for publication in Ap
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