222 research outputs found
Galaxy Collisions - Dawn of a New Era
The study of colliding galaxies has progressed rapidly in the last few years,
driven by observations with powerful new ground and space-based instruments.
These instruments have used for detailed studies of specific nearby systems,
statistical studies of large samples of relatively nearby systems, and
increasingly large samples of high redshift systems. Following a brief summary
of the historical context, this review attempts to integrate these studies to
address the following key issues. What role do collisions play in galaxy
evolution, and how can recently discovered processes like downsizing resolve
some apparently contradictory results of high redshift studies? What is the
role of environment in galaxy collisions? How is star formation and nuclear
activity orchestrated by the large scale dynamics, before and during merger?
Are novel modes of star formation involved? What are we to make of the
association of ultraluminous X-ray sources with colliding galaxies? To what do
degree do mergers and feedback trigger long-term secular effects? How far can
we push the archaeology of individual systems to determine the nature of
precursor systems and the precise effect of the interaction? Tentative answers
to many of these questions have been suggested, and the prospects for answering
most of them in the next few decades are good.Comment: 44 pages, 9 figures, review article in press for Astrophysics Update
Vol.
Robust Digital Holography For Ultracold Atom Trapping
We have formulated and experimentally demonstrated an improved algorithm for
design of arbitrary two-dimensional holographic traps for ultracold atoms. Our
method builds on the best previously available algorithm, MRAF, and improves on
it in two ways. First, it allows for creation of holographic atom traps with a
well defined background potential. Second, we experimentally show that for
creating trapping potentials free of fringing artifacts it is important to go
beyond the Fourier approximation in modelling light propagation. To this end,
we incorporate full Helmholtz propagation into our calculations.Comment: 7 pages, 4 figure
Proximity effect, quasiparticle transport, and local magnetic moment in ferromagnet-d-wave superconductor junctions
The proximity effect, quasiparticle transport, and local magnetic moment in
ferromagnet--d-wave superconductor junctions with {110}-oriented interface are
studied by solving self-consistently the Bogoliubov-de Gennes equations within
an extended Hubbard model. It is found that the proximity induced order
parameter oscillates in the ferromagnetic region. The modulation period is
shortened with the increased exchange field while the oscillation amplitude is
depressed by the interfacial scattering. With the determined superconducting
energy gap, a transfer matrix method is proposed to compute the subgap
conductance within a scattering approach. Many novel features including the
zero-bias conductance dip and splitting are exhibited with appropriate values
of the exchange field and interfacial scattering strength. The conductance
spectrum can be influenced seriously by the spin-flip interfacial scattering.
In addition, a sizable local magnetic moment near the {110}-oriented surface of
the d-wave superconductor is discussed.Comment: 10 pages, 16 ps-figures, to appear in Phys. Rev.
Photon and neutral pion production in Au+Au collisions at sqrt(s_{NN}) = 130 GeV
We report the first inclusive photon measurements about mid-rapidity
(|y|<0.5) from Au+Au collisions at sqrt(s_{NN}) = 130 GeV at RHIC. Photon pair
conversions were reconstructed from electron and positron tracks measured with
the Time Projection Chamber (TPC) of the STAR experiment. With this method, an
energy resolution of Delta(E)/E = 2% at 0.5 GeV has been achieved.
Reconstructed photons have also been used to measure the transverse momentum
(pt) spectra of pi0 mesons about mid-rapidity (|y| photon
photon decay channel. The fractional contribution of the pi0 -> photon photon
decay to the inclusive photon spectrum decreases by 20% +/- 5% between pt =
1.65 GeV/c and pt = 2.4 GeV/c in the most central events, indicating that
relative to pi0 -> photon photon decay the contribution of other photon sources
is substantially increasing.Comment: 13 pages, 14 figures, submitted to Phys. Rev.
Demonstration of the temporal matter-wave Talbot effect for trapped matter waves
We demonstrate the temporal Talbot effect for trapped matter waves using
ultracold atoms in an optical lattice. We investigate the phase evolution of an
array of essentially non-interacting matter waves and observe matter-wave
collapse and revival in the form of a Talbot interference pattern. By using
long expansion times, we image momentum space with sub-recoil resolution,
allowing us to observe fractional Talbot fringes up to 10th order.Comment: 17 pages, 7 figure
Pion, kaon, proton and anti-proton transverse momentum distributions from p+p and d+Au collisions at GeV
Identified mid-rapidity particle spectra of , , and
from 200 GeV p+p and d+Au collisions are reported. A
time-of-flight detector based on multi-gap resistive plate chamber technology
is used for particle identification. The particle-species dependence of the
Cronin effect is observed to be significantly smaller than that at lower
energies. The ratio of the nuclear modification factor () between
protons and charged hadrons () in the transverse momentum
range GeV/c is measured to be
(stat)(syst) in minimum-bias collisions and shows little
centrality dependence. The yield ratio of in minimum-bias d+Au
collisions is found to be a factor of 2 lower than that in Au+Au collisions,
indicating that the Cronin effect alone is not enough to account for the
relative baryon enhancement observed in heavy ion collisions at RHIC.Comment: 6 pages, 4 figures, 1 table. We extended the pion spectra from
transverse momentum 1.8 GeV/c to 3. GeV/
Azimuthal anisotropy at RHIC: the first and fourth harmonics
We report the first observations of the first harmonic (directed flow, v_1),
and the fourth harmonic (v_4), in the azimuthal distribution of particles with
respect to the reaction plane in Au+Au collisions at the Relativistic Heavy Ion
Collider (RHIC). Both measurements were done taking advantage of the large
elliptic flow (v_2) generated at RHIC. From the correlation of v_2 with v_1 it
is determined that v_2 is positive, or {\it in-plane}. The integrated v_4 is
about a factor of 10 smaller than v_2. For the sixth (v_6) and eighth (v_8)
harmonics upper limits on the magnitudes are reported.Comment: 6 pages with 3 figures, as accepted for Phys. Rev. Letters The data
tables are at
http://www.star.bnl.gov/central/publications/pubDetail.php?id=3
Persistence of single species of symbionts across multiple closelyrelated host species
Some symbiont species are highly host-specific, inhabiting only one or a very few host species, and
typically have limited dispersal abilities. When they do occur on multiple host species, populations of
such symbionts are expected to become genetically structured across these different host species,
and this may eventually lead to new symbiont species over evolutionary timescales. However, a low
number of dispersal events of symbionts between host species across time might be enough to prevent
population structure and species divergence. Overall, processes of evolutionary divergence and the
species status of most putative multi-host symbiont systems are yet to be investigated. Here, we used
DNA metabarcoding data of 6,023 feather mites (a total of 2,225 OTU representative sequences) from
147 infracommunities (i.e., the assemblage consisting of all mites of different species collected from
the same bird host individual) to investigate patterns of population genetic structure and species status
of three different putative multi-host feather mite species Proctophyllodes macedo Vitzthum, 1922,
Proctophyllodes motacillae Gaud, 1953, and Trouessartia jedliczkai (Zimmerman, 1894), each of which
inhabits a variable number of different closely related wagtail host species (genus Motacilla). We show
that mite populations from different host species represent a single species. This pattern was found in
all the mite species, suggesting that each of these species is a multi-host species in which dispersal of
mites among host species prevents species divergence. Also, we found evidence of limited evolutionary
divergence manifested by a low but significant level of population genetic structure among symbiont
populations inhabiting different host species. Our study agrees with previous studies showing a higher
than expected colonization opportunities in host-specific symbionts. Indeed, our results support
that these dispersal events would allow the persistence of multi-host species even in symbionts with
limited dispersal capabilities, though additional factors such as the geographical structure of some bird
populations may also play a role.This work was supported by the MINECO CGL2011-24466 to RJ and
CGL2015-69650-P to RJ and DS
Candida albicans Isolates from the Gut of Critically Ill Patients Respond to Phosphate Limitation by Expressing Filaments and a Lethal Phenotype
Candida albicans is an opportunistic pathogen that proliferates in the intestinal tract of critically ill patients where it continues to be a major cause of infectious-related mortality. The precise cues that shift intestinal C. albicans from its ubiquitous indolent colonizing yeast form to an invasive and lethal filamentous form remain unknown. We have previously shown that severe phosphate depletion develops in the intestinal tract during extreme physiologic stress and plays a major role in shifting intestinal Pseudomonas aeruginosa to express a lethal phenotype via conserved phosphosensory-phosphoregulatory systems. Here we studied whether phosphate dependent virulence expression could be similarly demonstrated for C. albicans. C. albicans isolates from the stool of critically ill patients and laboratory prototype strains (SC5314, BWP17, SN152) were evaluated for morphotype transformation and lethality against C. elegans and mice during exposure to phosphate limitation. Isolates ICU1 and ICU12 were able to filament and kill C. elegans in a phosphate dependent manner. In a mouse model of intestinal phosphate depletion (30% hepatectomy), direct intestinal inoculation of C. albicans caused mortality that was prevented by oral phosphate supplementation. Prototype strains displayed limited responses to phosphate limitation; however, the pho4Δ mutant displayed extensive filamentation during low phosphate conditions compared to its isogenic parent strain SN152, suggesting that mutation in the transcriptional factor Pho4p may sensitize C. albicans to phosphate limitation. Extensive filamentation was also observed in strain ICU12 suggesting that this strain is also sensitized to phosphate limitation. Analysis of the sequence of PHO4 in strain ICU12, its transcriptional response to phosphate limitation, and phosphatase assays confirmed that ICU12 demonstrates a profound response to phosphate limitation. The emergence of strains of C. albicans with marked responsiveness to phosphate limitation may represent a fitness adaptation to the complex and nutrient scarce environment typical of the gut of a critically ill patient
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